Information

Is there nutrient absorption in the large intestine of hindgut fermenters?


In hindgut fermenters, plants are digested in the caecum by microbes. I want to know whether hindgut fermenters can absorb the nutrients obtained from the digestion in the large intestine because the absorption of digested materials is not the main function of the large intestine in other animals. I know some hindgut fermenters reingest the materials created in the caecum, but I think not all hindgut fermenters perform this behavior. I want to know the main site of absorption after the caecum.


I have divided this answer into a section on horse and rabbit. While there are many other hindgut fermenters, these species are good illustrations of different ways hindgut fermenters can digest and absorb nutrients from their food.

Horse

As you know, absorption of "nutrients" occurs along the intestines, but at varying degrees at different sites. Most compounds are primary absorbed in the small intestine of the horse. This includes protein, glucose (soluble carbohydrates), fats, calcium, and magnesium.

Any carbohydrates that are not absorbed in the small intestine reach the hind gut. Here they are fermented by bacteria to short-chain volatile fatty acids (VFAs) and lactic acid. Horses can use the VFAs (propionate, butyrate, and acetate) as an energy source, but a diet too high in grain can also lead to excessive lactic acid production in the hindgut, and as a result the horse may experience colic. As your question pertains to the hind gut, a nutrient of main concern is therefore volatile fatty acids.

The hindgut of the horse is very long and developed. To review with the anatomy of the horse hindgut, it consists of the caecum, ventral colon, dorsal colon, transverse colon, descending colon (or small colon), and rectum.

In a study in 1974, RA Argenzio et al. determined that among the parts of the horse hindgut, most VFAs were produced and absorbed in the ventral colon, 8-12 hours after feeding. This is depicted by the graph below, which shows a large negative net production of VFAs after 8-12 hours (i.e. more VFAs absorbed than produced). Note that the caecum and dorsal colon are also important sites of VFA production and absorption.

The hindgut is also an important site for phosphorus absorption in the horse, unlike many other species that primarily absorb phosphorus in the small intestine. This is thought to be because most of the dietary phosphorus in the horse in phytin phosphate, which must be exposed to the enzyme phytase produced by the microbial flora in order to be absorbed. Studies such as Schryver et al. (1972) and T. Matsui et al. (1999) have shown that most phosphorus is excreted by the small intestine and caecum, and most is absorbed in the dorsal colon and small colon. The figure below from Schryver shows the net absorption of phosphorus on the various experimental diets. Note that this is oriented opposite to the VFA figure above; absorption is positive rather than negative on this graph.


Rabbit

To begin with the rabbit anatomy, you will note that the colon of the rabbit is much less developed than that of the horse. However, both are hindgut fermenters, and what they do have in common is a very large caecum.

As in the horse, much of the digestion and absorption of nutrients occurs in the small intestine. This includes protein, fats, and simple carbohydrates (glucose), as well as some vitamins.

Also like the horse, the digestable material not absorbed by the small intestine passes to the caecum, where it undergoes fermentation by the microbial flora. The large fibrous material passes directly to the colon, where water is absorbed and forms hard faecal pellets, while the small digestible material passes through reverse peristalsis into the caecum. What is different in a rabbit to a horse, however, is that the about 8 hours after feeding, the caecal material is packaged into a small pellet called a caecotrophe. As or shortly after the caecotrophe is excreted, the rabbit re-ingests the caecotrophe. As this food has been digested once already, the small intestine is better able to absorb nutrients from it.

As an example, important nutrients that are able to be absorbed due to caecotrophy in the rabbit are the B vitamins such as niacin, riboflavin, folate, and cobalamin (vitamin B12). One of the early studies on this topic is Kulwich et al. (1953).

Summary

The reason for the difference between the horse and rabbit is fundamentally an anatomical difference. Both species have a large caecum which enables fermentation of plant matter. However, the horse has a long, voluminous colon in which it can absorb many nutrients, but they will lose protein and other nutrients in this process because the colon is less adapted to absorb them. In coprophagic (rat) or caecotrophic species (rabbit), they do not have as long a colon, but can absorb vitamins and proteins lost in the faeces during the second pass through the gastrointestinal tract.

An excellent and very detailed review of the gastrointestinal tracts and nutrient absorption in a variety of species is this freely accesible paper by Stevens and Hume, "Contributions of Microbes in Vertebrate Gastrointestinal Tract to Production and Conservation of Nutrients."

References

Argenzio, R.A., Southworth, M., and Stevens, C.E. (1974) Sites of organic acid production and absorption in the equine gastrointestinal tract. Am. J. Physiol. 226:1043-1050.

Bugaut, M. (1987) Occurence, absorption and metabolism of short chain fatty acids in the digestive tract of mammals. Comp. Biochem. Physiol. 86B(3):439-472.

Hornincke H., and Bjornhag, G. (1980) Coprophagy and related strategies for digesta utilization. In Digestive Physiology and Metabolism in Ruminants, 707-730. Avi Publishing, Westport, CT.

Kulwich, R., Struglia, L. and Pearson, P.B. (1953) The effect of coprophagy on the excretion of B vitamins by the rabbit. J. Nutr. 49:639-645.

Matsui, T., Murakami, Y., Yano, H., et al. (1999) Phytate and phosphorus movements in the digestive tract of horses. Equine Vet. J. Suppl. 30:505-7.

Schryver, H.F., Hintz, H.F., Craig, P.H., Hougue, D.E. and Lowe, J.E. (1972) Site of phosphorus absorption from the intestine of the horse. J. Nutr. 102: 143-148.

Stevens, C.E., and Hume, I.D. (1998) Contributions of Microbes in Vertebrate Gastrointestinal Tract to Production and Conservation of Nutrients. Physiological Reviews, 78(2):393-427.


BIO 140 - Human Biology I - Textbook

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Chapter 18

The Small and Large Intestines

  • Compare and contrast the location and gross anatomy of the small and large intestines
  • Identify three main adaptations of the small intestine wall that increase its absorptive capacity
  • Describe the mechanical and chemical digestion of chyme upon its release into the small intestine
  • List three features unique to the wall of the large intestine and identify their contributions to its function
  • Identify the beneficial roles of the bacterial flora in digestive system functioning
  • Trace the pathway of food waste from its point of entry into the large intestine through its exit from the body as feces

The word intestine is derived from a Latin root meaning &ldquointernal,&rdquo and indeed, the two organs together nearly fill the interior of the abdominal cavity. In addition, called the small and large bowel, or colloquially the &ldquoguts,&rdquo they constitute the greatest mass and length of the alimentary canal and, with the exception of ingestion, perform all digestive system functions.

The Small Intestine

Chyme released from the stomach enters the small intestine , which is the primary digestive organ in the body. Not only is this where most digestion occurs, it is also where practically all absorption occurs. The longest part of the alimentary canal, the small intestine is about 3.05 meters (10 feet) long in a living person (but about twice as long in a cadaver due to the loss of muscle tone). Since this makes it about five times longer than the large intestine, you might wonder why it is called &ldquosmall.&rdquo In fact, its name derives from its relatively smaller diameter of only about 2.54 cm (1 in), compared with 7.62 cm (3 in) for the large intestine. As we&rsquoll see shortly, in addition to its length, the folds and projections of the lining of the small intestine work to give it an enormous surface area, which is approximately 200 m 2 , more than 100 times the surface area of your skin. This large surface area is necessary for complex processes of digestion and absorption that occur within it.

Structure

The coiled tube of the small intestine is subdivided into three regions. From proximal (at the stomach) to distal, these are the duodenum, jejunum, and ileum (Figure 1).

The shortest region is the 25.4-cm (10-in) duodenum , which begins at the pyloric sphincter. Just past the pyloric sphincter, it bends posteriorly behind the peritoneum, becoming retroperitoneal, and then makes a C-shaped curve around the head of the pancreas before ascending anteriorly again to return to the peritoneal cavity and join the jejunum. The duodenum can therefore be subdivided into four segments: the superior, descending, horizontal, and ascending duodenum.

Of particular interest is the hepatopancreatic ampulla (ampulla of Vater). Located in the duodenal wall, the ampulla marks the transition from the anterior portion of the alimentary canal to the mid-region, and is where the bile duct (through which bile passes from the liver) and the main pancreatic duct (through which pancreatic juice passes from the pancreas) join. This ampulla opens into the duodenum at a tiny volcano-shaped structure called the major duodenal papilla . The hepatopancreatic sphincter (sphincter of Oddi) regulates the flow of both bile and pancreatic juice from the ampulla into the duodenum.

Figure 1: The three regions of the small intestine are the duodenum, jejunum, and ileum.

The jejunum is about 0.9 meters (3 feet) long (in life) and runs from the duodenum to the ileum. Jejunum means &ldquoempty&rdquo in Latin and supposedly was so named by the ancient Greeks who noticed it was always empty at death. No clear demarcation exists between the jejunum and the final segment of the small intestine, the ileum.

The ileum is the longest part of the small intestine, measuring about 1.8 meters (6 feet) in length. It is thicker, more vascular, and has more developed mucosal folds than the jejunum. The ileum joins the cecum, the first portion of the large intestine, at the ileocecal sphincter (or valve). The jejunum and ileum are tethered to the posterior abdominal wall by the mesentery. The large intestine frames these three parts of the small intestine.

Parasympathetic nerve fibers from the vagus nerve and sympathetic nerve fibers from the thoracic splanchnic nerve provide extrinsic innervation to the small intestine. The superior mesenteric artery is its main arterial supply. Veins run parallel to the arteries and drain into the superior mesenteric vein. Nutrient-rich blood from the small intestine is then carried to the liver via the hepatic portal vein.

Histology

The wall of the small intestine is composed of the same four layers typically present in the alimentary system. However, three features of the mucosa and submucosa are unique. These features, which increase the absorptive surface area of the small intestine more than 600-fold, include circular folds, villi, and microvilli (Figure 2). These adaptations are most abundant in the proximal two-thirds of the small intestine, where the majority of absorption occurs.

Figure 2: (a) The absorptive surface of the small intestine is vastly enlarged by the presence of circular folds, villi, and microvilli. (b) Micrograph of the circular folds. (c) Micrograph of the villi. (d) Electron micrograph of the microvilli. From left to right, LM x 56, LM x 508, EM x 196,000. (credit b-d: Micrograph provided by the Regents of University of Michigan Medical School © 2012)

Circular folds

Also called a plica circulare, a circular fold is a deep ridge in the mucosa and submucosa. Beginning near the proximal part of the duodenum and ending near the middle of the ileum, these folds facilitate absorption. Their shape causes the chyme to spiral, rather than move in a straight line, through the small intestine. Spiraling slows the movement of chyme and provides the time needed for nutrients to be fully absorbed.

Within the circular folds are small (0.5&ndash1 mm long) hairlike vascularized projections called villi (singular = villus) that give the mucosa a furry texture. There are about 20 to 40 villi per square millimeter, increasing the surface area of the epithelium tremendously. The mucosal epithelium, primarily composed of absorptive cells, covers the villi. In addition to muscle and connective tissue to support its structure, each villus contains a capillary bed composed of one arteriole and one venule, as well as a lymphatic capillary called a lacteal. The breakdown products of carbohydrates and proteins (sugars and amino acids) can enter the bloodstream directly, but lipid breakdown products are absorbed by the lacteals and transported to the bloodstream via the lymphatic system.

As their name suggests, microvilli (singular = microvillus) are much smaller (1 µm) than villi. They are cylindrical apical surface extensions of the plasma membrane of the mucosa&rsquos epithelial cells, and are supported by microfilaments within those cells. Although their small size makes it difficult to see each microvillus, their combined microscopic appearance suggests a mass of bristles, which is termed the brush border. Fixed to the surface of the microvilli membranes are enzymes that finish digesting carbohydrates and proteins. There are an estimated 200 million microvilli per square millimeter of small intestine, greatly expanding the surface area of the plasma membrane and thus greatly enhancing absorption.

Intestinal Glands

In addition to the three specialized absorptive features just discussed, the mucosa between the villi is dotted with deep crevices that each lead into a tubular intestinal gland (crypt of Lieberkühn), which is formed by cells that line the crevices (see Figure 2). These produce intestinal juice, a slightly alkaline (pH 7.4 to 7.8) mixture of water and mucus. Each day, about 0.95 to 1.9 liters (1 to 2 quarts) are secreted in response to the distention of the small intestine or the irritating effects of chyme on the intestinal mucosa.

The submucosa of the duodenum is the only site of the complex mucus-secreting duodenal glands (Brunner&rsquos glands), which produce a bicarbonate-rich alkaline mucus that buffers the acidic chyme as it enters from the stomach.

The roles of the cells in the small intestinal mucosa are detailed in Table 1.

Table 1: Cells of the Small Intestinal Mucosa

Cell type Location in the mucosa Function
Absorptive Epithelium/intestinal glands Digestion and absorption of nutrients in chyme
Goblet Epithelium/intestinal glands Secretion of mucus
Paneth Intestinal glands Secretion of the bactericidal enzyme lysozyme phagocytosis
G cells Intestinal glands of duodenum Secretion of the hormone intestinal gastrin
I cells Intestinal glands of duodenum Secretion of the hormone cholecystokinin, which stimulates release of pancreatic juices and bile
K cells Intestinal glands Secretion of the hormone glucose-dependent insulinotropic peptide, which stimulates the release of insulin
M cells Intestinal glands of duodenum and jejunum Secretion of the hormone motilin, which accelerates gastric emptying, stimulates intestinal peristalsis, and stimulates the production of pepsin
S cells Intestinal glands Secretion of the hormone secretin

Intestinal MALT

The lamina propria of the small intestine mucosa is studded with quite a bit of MALT. In addition to solitary lymphatic nodules, aggregations of intestinal MALT, which are typically referred to as Peyer&rsquos patches, are concentrated in the distal ileum, and serve to keep bacteria from entering the bloodstream. Peyer&rsquos patches are most prominent in young people and become less distinct as you age, which coincides with the general activity of our immune system.

Watch the video linked below to see the structure of the small intestine, and, in particular, the villi. Epithelial cells continue the digestion and absorption of nutrients and transport these nutrients to the lymphatic and circulatory systems. In the small intestine, the products of food digestion are absorbed by different structures in the villi. Which structure absorbs and transports fats?

The movement of intestinal smooth muscles includes both segmentation and a form of peristalsis called migrating motility complexes. The kind of peristaltic mixing waves seen in the stomach are not observed here.

If you could see into the small intestine when it was going through segmentation, it would look as if the contents were being shoved incrementally back and forth, as the rings of smooth muscle repeatedly contract and then relax. Segmentation in the small intestine does not force chyme through the tract. Instead, it combines the chyme with digestive juices and pushes food particles against the mucosa to be absorbed. The duodenum is where the most rapid segmentation occurs, at a rate of about 12 times per minute. In the ileum, segmentations are only about eight times per minute (Figure 3).

Segmentation

Figure 3: Segmentation separates chyme and then pushes it back together, mixing it and providing time for digestion and absorption.

When most of the chyme has been absorbed, the small intestinal wall becomes less distended. At this point, the localized segmentation process is replaced by transport movements. The duodenal mucosa secretes the hormone motilin, which initiates peristalsis in the form of a migrating motility complex. These complexes, which begin in the duodenum, force chyme through a short section of the small intestine and then stop. The next contraction begins a little bit farther down than the first, forces chyme a bit farther through the small intestine, then stops. These complexes move slowly down the small intestine, forcing chyme on the way, taking around 90 to 120 minutes to finally reach the end of the ileum. At this point, the process is repeated, starting in the duodenum.

The ileocecal valve, a sphincter, is usually in a constricted state, but when motility in the ileum increases, this sphincter relaxes, allowing food residue to enter the first portion of the large intestine, the cecum. Relaxation of the ileocecal sphincter is controlled by both nerves and hormones. First, digestive activity in the stomach provokes the astroileal reflex, which increases the force of ileal segmentation. Second, the stomach releases the hormone gastrin, which enhances ileal motility, thus relaxing the ileocecal sphincter. After chyme passes through, backward pressure helps close the sphincter, preventing backflow into the ileum. Because of this reflex, your lunch is completely emptied from your stomach and small intestine by the time you eat your dinner. It takes about 3 to 5 hours for all chyme to leave the small intestine.

Chemical Digestion in the Small Intestine

The digestion of proteins and carbohydrates, which partially occurs in the stomach, is completed in the small intestine with the aid of intestinal and pancreatic juices. Lipids arrive in the intestine largely undigested, so much of the focus here is on lipid digestion, which is facilitated by bile and the enzyme pancreatic lipase.

Moreover, intestinal juice combines with pancreatic juice to provide a liquid medium that facilitates absorption. The intestine is also where most water is absorbed, via osmosis. The small intestine&rsquos absorptive cells also synthesize digestive enzymes and then place them in the plasma membranes of the microvilli. This distinguishes the small intestine from the stomach that is, enzymatic digestion occurs not only in the lumen, but also on the luminal surfaces of the mucosal cells.

For optimal chemical digestion, chyme must be delivered from the stomach slowly and in small amounts. This is because chyme from the stomach is typically hypertonic, and if large quantities were forced all at once into the small intestine, the resulting osmotic water loss from the blood into the intestinal lumen would result in potentially life-threatening low blood volume. In addition, continued digestion requires an upward adjustment of the low pH of stomach chyme, along with rigorous mixing of the chyme with bile and pancreatic juices. Both processes take time, so the pumping action of the pylorus must be carefully controlled to prevent the duodenum from being overwhelmed with chyme.

DISORDERS OF THE&hellip

Small Intestine: Lactose Intolerance

Lactose intolerance is a condition characterized by indigestion caused by dairy products. It occurs when the absorptive cells of the small intestine do not produce enough lactase, the enzyme that digests the milk sugar lactose. In most mammals, lactose intolerance increases with age. In contrast, some human populations, most notably Caucasians, are able to maintain the ability to produce lactase as adults.

In people with lactose intolerance, the lactose in chyme is not digested. Bacteria in the large intestine ferment the undigested lactose, a process that produces gas. In addition to gas, symptoms include abdominal cramps, bloating, and diarrhea. Symptom severity ranges from mild discomfort to severe pain however, symptoms resolve once the lactose is eliminated in feces.

The hydrogen breath test is used to help diagnose lactose intolerance. Lactose-tolerant people have very little hydrogen in their breath. Those with lactose intolerance exhale hydrogen, which is one of the gases produced by the bacterial fermentation of lactose in the colon. After the hydrogen is absorbed from the intestine, it is transported through blood vessels into the lungs. There are a number of lactose-free dairy products available in grocery stores. In addition, dietary supplements are available. Taken with food, they provide lactase to help digest lactose.

The Large Intestine

The large intestine is the terminal part of the alimentary canal. The primary function of this organ is to finish absorption of nutrients and water, synthesize certain vitamins, form feces, and eliminate feces from the body.

Structure

The large intestine runs from the appendix to the anus. It frames the small intestine on three sides. Despite its being about one-half as long as the small intestine, it is called large because it is more than twice the diameter of the small intestine, about 3 inches.

Subdivisions

The large intestine is subdivided into four main regions: the cecum, the colon, the rectum, and the anus. The ileocecal valve, located at the opening between the ileum and the large intestine, controls the flow of chyme from the small intestine to the large intestine.

The first part of the large intestine is the cecum, a sac-like structure that is suspended inferior to the ileocecal valve. It is about 6 cm (2.4 in) long, receives the contents of the ileum, and continues the absorption of water and salts. The appendix (or vermiform appendix) is a winding tube that attaches to the cecum. Although the 7.6-cm (3-in) long appendix contains lymphoid tissue, suggesting an immunologic function, this organ is generally considered vestigial. However, at least one recent report postulates a survival advantage conferred by the appendix: In diarrheal illness, the appendix may serve as a bacterial reservoir to repopulate the enteric bacteria for those surviving the initial phases of the illness. Moreover, its twisted anatomy provides a haven for the accumulation and multiplication of enteric bacteria. The mesoappendix, the mesentery of the appendix, tethers it to the mesentery of the ileum.

The cecum blends seamlessly with the colon. Upon entering the colon, the food residue first travels up the ascending colon on the right side of the abdomen. At the inferior surface of the liver, the colon bends to form the right colic flexure (hepatic flexure) and becomes the transverse colon. The region defined as hindgut begins with the last third of the transverse colon and continues on. Food residue passing through the transverse colon travels across to the left side of the abdomen, where the colon angles sharply immediately inferior to the spleen, at the left colic flexure (splenic flexure). From there, food residue passes through the descending colon, which runs down the left side of the posterior abdominal wall. After entering the pelvis inferiorly, it becomes the s-shaped sigmoid colon, which extends medially to the midline (Figure 4). The ascending and descending colon, and the rectum (discussed next) are located in the retroperitoneum. The transverse and sigmoid colon are tethered to the posterior abdominal wall by the mesocolon.

Large Intestine

Figure 4: The large intestine includes the cecum, colon, and rectum.

HOMEOSTATIC IMBALANCES

Colorectal Cancer

Each year, approximately 140,000 Americans are diagnosed with colorectal cancer, and another 49,000 die from it, making it one of the most deadly malignancies. People with a family history of colorectal cancer are at increased risk. Smoking, excessive alcohol consumption, and a diet high in animal fat and protein also increase the risk. Despite popular opinion to the contrary, studies support the conclusion that dietary fiber and calcium do not reduce the risk of colorectal cancer.

Colorectal cancer may be signaled by constipation or diarrhea, cramping, abdominal pain, and rectal bleeding. Bleeding from the rectum may be either obvious or occult (hidden in feces). Since most colon cancers arise from benign mucosal growths called polyps, cancer prevention is focused on identifying these polyps. The colonoscopy is both diagnostic and therapeutic. Colonoscopy not only allows identification of precancerous polyps, the procedure also enables them to be removed before they become malignant. Screening for fecal occult blood tests and colonoscopy is recommended for those over 50 years of age.

Food residue leaving the sigmoid colon enters the rectum in the pelvis, near the third sacral vertebra. The final 20.3 cm (8 in) of the alimentary canal, the rectum extends anterior to the sacrum and coccyx. Even though rectum is Latin for &ldquostraight,&rdquo this structure follows the curved contour of the sacrum and has three lateral bends that create a trio of internal transverse folds called the rectal valves. These valves help separate the feces from gas to prevent the simultaneous passage of feces and gas.

Finally, food residue reaches the last part of the large intestine, the anal canal, which is located in the perineum, completely outside of the abdominopelvic cavity. This 3.8&ndash5 cm (1.5&ndash2 in) long structure opens to the exterior of the body at the anus. The anal canal includes two sphincters. The internal anal sphincter is made of smooth muscle, and its contractions are involuntary. The external anal sphincter is made of skeletal muscle, which is under voluntary control. Except when defecating, both usually remain closed.

Histology

There are several notable differences between the walls of the large and small intestines (Figure 5). For example, few enzyme-secreting cells are found in the wall of the large intestine, and there are no circular folds or villi. Other than in the anal canal, the mucosa of the colon is simple columnar epithelium made mostly of enterocytes (absorptive cells) and goblet cells. In addition, the wall of the large intestine has far more intestinal glands, which contain a vast population of enterocytes and goblet cells. These goblet cells secrete mucus that eases the movement of feces and protects the intestine from the effects of the acids and gases produced by enteric bacteria. The enterocytes absorb water and salts as well as vitamins produced by your intestinal bacteria.

Histology of the large Intestine

Figure 5: (a) The histologies of the large intestine and small intestine (not shown) are adapted for the digestive functions of each organ. (b) This micrograph shows the colon&rsquos simple columnar epithelium and goblet cells. LM x 464. (credit b: Micrograph provided by the Regents of University of Michigan Medical School © 2012)

Anatomy

Three features are unique to the large intestine: teniae coli, haustra, and epiploic appendages (Figure 6). The teniae coli are three bands of smooth muscle that make up the longitudinal muscle layer of the muscularis of the large intestine, except at its terminal end. Tonic contractions of the teniae coli bunch up the colon into a succession of pouches called haustra (singular = haustrum), which are responsible for the wrinkled appearance of the colon. Attached to the teniae coli are small, fat-filled sacs of visceral peritoneum called epiploic appendages. The purpose of these is unknown. Although the rectum and anal canal have neither teniae coli nor haustra, they do have well-developed layers of muscularis that create the strong contractions needed for defecation.

Teniae Coli, Haustra, and Epiploic Appendages

The stratified squamous epithelial mucosa of the anal canal connects to the skin on the outside of the anus. This mucosa varies considerably from that of the rest of the colon to accommodate the high level of abrasion as feces pass through. The anal canal&rsquos mucous membrane is organized into longitudinal folds, each called an anal column, which house a grid of arteries and veins. Two superficial venous plexuses are found in the anal canal: one within the anal columns and one at the anus.

Depressions between the anal columns, each called an anal sinus, secrete mucus that facilitates defecation. The pectinate line (or dentate line) is a horizontal, jagged band that runs circumferentially just below the level of the anal sinuses, and represents the junction between the hindgut and external skin. The mucosa above this line is fairly insensitive, whereas the area below is very sensitive. The resulting difference in pain threshold is due to the fact that the upper region is innervated by visceral sensory fibers, and the lower region is innervated by somatic sensory fibers.

Bacterial Flora

Most bacteria that enter the alimentary canal are killed by lysozyme, defensins, HCl, or protein-digesting enzymes. However, trillions of bacteria live within the large intestine and are referred to as the bacterial flora. Most of the more than 700 species of these bacteria are nonpathogenic commensal organisms that cause no harm as long as they stay in the gut lumen. In fact, many facilitate chemical digestion and absorption, and some synthesize certain vitamins, mainly biotin, pantothenic acid, and vitamin K. Some are linked to increased immune response. A refined system prevents these bacteria from crossing the mucosal barrier. First, peptidoglycan, a component of bacterial cell walls, activates the release of chemicals by the mucosa&rsquos epithelial cells, which draft immune cells, especially dendritic cells, into the mucosa. Dendritic cells open the tight junctions between epithelial cells and extend probes into the lumen to evaluate the microbial antigens. The dendritic cells with antigens then travel to neighboring lymphoid follicles in the mucosa where T cells inspect for antigens. This process triggers an IgA-mediated response, if warranted, in the lumen that blocks the commensal organisms from infiltrating the mucosa and setting off a far greater, widespread systematic reaction.

Digestive Functions of the Large Intestine

The residue of chyme that enters the large intestine contains few nutrients except water, which is reabsorbed as the residue lingers in the large intestine, typically for 12 to 24 hours. Thus, it may not surprise you that the large intestine can be completely removed without significantly affecting digestive functioning. For example, in severe cases of inflammatory bowel disease, the large intestine can be removed by a procedure known as a colectomy. Often, a new fecal pouch can be crafted from the small intestine and sutured to the anus, but if not, an ileostomy can be created by bringing the distal ileum through the abdominal wall, allowing the watery chyme to be collected in a bag-like adhesive appliance.

Mechanical Digestion

In the large intestine, mechanical digestion begins when chyme moves from the ileum into the cecum, an activity regulated by the ileocecal sphincter. Right after you eat, peristalsis in the ileum forces chyme into the cecum. When the cecum is distended with chyme, contractions of the ileocecal sphincter strengthen. Once chyme enters the cecum, colon movements begin.

Mechanical digestion in the large intestine includes a combination of three types of movements. The presence of food residues in the colon stimulates a slow-moving haustral contraction. This type of movement involves sluggish segmentation, primarily in the transverse and descending colons. When a haustrum is distended with chyme, its muscle contracts, pushing the residue into the next haustrum. These contractions occur about every 30 minutes, and each last about 1 minute. These movements also mix the food residue, which helps the large intestine absorb water. The second type of movement is peristalsis, which, in the large intestine, is slower than in the more proximal portions of the alimentary canal. The third type is a mass movement. These strong waves start midway through the transverse colon and quickly force the contents toward the rectum. Mass movements usually occur three or four times per day, either while you eat or immediately afterward. Distension in the stomach and the breakdown products of digestion in the small intestine provoke the gastrocolic reflex, which increases motility, including mass movements, in the colon. Fiber in the diet both softens the stool and increases the power of colonic contractions, optimizing the activities of the colon.

Chemical Digestion

Although the glands of the large intestine secrete mucus, they do not secrete digestive enzymes. Therefore, chemical digestion in the large intestine occurs exclusively because of bacteria in the lumen of the colon. Through the process of saccharolytic fermentation, bacteria break down some of the remaining carbohydrates. This results in the discharge of hydrogen, carbon dioxide, and methane gases that create flatus (gas) in the colon flatulence is excessive flatus. Each day, up to 1500 mL of flatus is produced in the colon. More is produced when you eat foods such as beans, which are rich in otherwise indigestible sugars and complex carbohydrates like soluble dietary fiber.

Absorption, Feces Formation, and Defecation

The small intestine absorbs about 90 percent of the water you ingest (either as liquid or within solid food). The large intestine absorbs most of the remaining water, a process that converts the liquid chyme residue into semisolid feces (&ldquostool&rdquo). Feces is composed of undigested food residues, unabsorbed digested substances, millions of bacteria, old epithelial cells from the GI mucosa, inorganic salts, and enough water to let it pass smoothly out of the body. Of every 500 mL (17 ounces) of food residue that enters the cecum each day, about 150 mL (5 ounces) become feces.

Feces are eliminated through contractions of the rectal muscles. You help this process by a voluntary procedure called Valsalva&rsquos maneuver, in which you increase intra-abdominal pressure by contracting your diaphragm and abdominal wall muscles, and closing your glottis.

The process of defecation begins when mass movements force feces from the colon into the rectum, stretching the rectal wall and provoking the defecation reflex, which eliminates feces from the rectum. This parasympathetic reflex is mediated by the spinal cord. It contracts the sigmoid colon and rectum, relaxes the internal anal sphincter, and initially contracts the external anal sphincter. The presence of feces in the anal canal sends a signal to the brain, which gives you the choice of voluntarily opening the external anal sphincter (defecating) or keeping it temporarily closed. If you decide to delay defecation, it takes a few seconds for the reflex contractions to stop and the rectal walls to relax. The next mass movement will trigger additional defecation reflexes until you defecate.

If defecation is delayed for an extended time, additional water is absorbed, making the feces firmer and potentially leading to constipation. On the other hand, if the waste matter moves too quickly through the intestines, not enough water is absorbed, and diarrhea can result. This can be caused by the ingestion of foodborne pathogens. In general, diet, health, and stress determine the frequency of bowel movements. The number of bowel movements varies greatly between individuals, ranging from two or three per day to three or four per week.

Watch the video linked below to see that for the various food groups&mdashproteins, fats, and carbohydrates&mdashdigestion begins in different parts of the digestion system, though all end in the same place. Of the three major food classes (carbohydrates, fats, and proteins), which is digested in the mouth, the stomach, and the small intestine?

Chapter Review

The three main regions of the small intestine are the duodenum, the jejunum, and the ileum. The small intestine is where digestion is completed and virtually all absorption occurs. These two activities are facilitated by structural adaptations that increase the mucosal surface area by 600-fold, including circular folds, villi, and microvilli. There are around 200 million microvilli per square millimeter of small intestine, which contain brush border enzymes that complete the digestion of carbohydrates and proteins. Combined with pancreatic juice, intestinal juice provides the liquid medium needed to further digest and absorb substances from chyme. The small intestine is also the site of unique mechanical digestive movements. Segmentation moves the chyme back and forth, increasing mixing and opportunities for absorption. Migrating motility complexes propel the residual chyme toward the large intestine.

The main regions of the large intestine are the cecum, the colon, and the rectum. The large intestine absorbs water and forms feces, and is responsible for defecation. Bacterial flora break down additional carbohydrate residue, and synthesize certain vitamins. The mucosa of the large intestinal wall is generously endowed with goblet cells, which secrete mucus that eases the passage of feces. The entry of feces into the rectum activates the defecation reflex.


Digestive strategies of small hindgut fermenters

Small mammalian herbivores have a limitation in their supply system of nutrients to their energy and protein demands because they need much more energy and protein per unit body mass than larger herbivorous animals. Therefore, small herbivores need to have characteristic strategies in their digestive systems to overcome the limitation of their small body mass compared with larger animals. Although small herbivorous mammals commonly have an enlarged cecum, the pattern of flow and mixing of digesta in the large intestine varies among them. Distinct separation of the larger fiber particles from smaller and liquid contents which are retained in the cecum can be recognized in some species. Coprophagy, practiced by many small herbivores, has nutritional significance providing a source of vitamins, amino acids, and other nutrients which are excreted with feces. Among coprophagous mammals, several species produce two types of feces: soft feces, which are eaten and hard, which are not eaten. Soft feces contain more water than hard feces and dry matter includes more protein and less fiber. Coprophagic behavior must be supported by the colonic separation mechanism, which operates retrograde transport of fluid and fine particle digesta or bacteria trapped in the mucus, resulting in high density bacteria in the cecum contents, which is successively consumed as cecotroph. These mechanisms must be necessary for small herbivores to survive on the feed in their habitat.


The GI tract of an adult horse (

500kg) is about 30 meters long and has a total volume of approximately 180 litres. The entire tract can be divided into two functional parts the foregut and the hindgut (see fig 1). In part one and two (March and April issues) we described the digestive process in the foregut of the horse. In this, the last part of the series, we discuss the final stage of food digestion – the large intestine and fermentation process.

The large intestine

The large intestine (hindgut) of the horse has three parts: caecum, colon and rectum (figure 1). Horses have an enlarged caecum, a blind sac at the junction of the small and large intestine and an enlarged and sacculated (large) colon (see fig 1). In the adult horse (500kg) the caecum is about 1 m long and has a capacity of about 30-34 liters. Nearly all of the non-starch polysaccharides (NSP) and undigested soluble carbohydrates in feed passes from the small intestine into the caecum. Together with the colon (large intestine) it contains micro-organisms that hydrolyse (break down with water) much of the fiber and soluble carbohydrates that are ingested. After digestion the nutrients (volatile fatty acids (VFA)), are absorbed from the caecum and colon.

Digestion in the caecum and colon depends almost entirely on the activity of micro organisms. In contrast with the small intestine, the walls of the large intestine contain only mucus-secreting glands, and does not produce digestive enzymes. However, high alkaline phosphatase activity is found in the large intestine. This is not been seen in other species like cats, dogs and man and is known to be related to a high digestive and absorption action.

The colon consists of three parts ascending, transverse and descending. The first part of the colon has the greatest capacity and is known as the large colon. In contrast, the descending part of the colon is known as the small colon. The large colon is 3 to 3.7 m long and has a capacity of 50 to 60 litres.

The large colon can be divided into four compartments the right and left segments of the ventral colon and the left and right segments of the dorsal colon (see figure 1). The four parts of the large colon are connected by three flexures (bends) . The diameter of the different segments of the large colon varies abruptly (20 to 25 cm), but reaches a maximum in the right dorsal colon where it forms the large sacculation (sack) with a diameter of up to 50 cm. The small colon is about 3 m long with an average diameter of 7.5 to 10 cm and has a capacity of 18 to 19 liters. Together with the large colon it has a total capacity of 70 to 80 litres. The rectum is located at the end of the large colon and is about 0.3 m long and opens to the exterior at the anus.

Microbial population and fermentation

There appears to be little difference in the biochemisty of fermentation in all hoofed animals studied, whatever the affinities of the animal or the site of fermentation chamber. The taxonomic composition of micro-organisms in the digestive system of all animals is also apparently broadly similar.

Microorganisms (e.g. bacteria, protozoa and fungi) can live in most segments of the animal gut. But the rumen and hindgut provide a unique environment for microorganisms. The anaerobic (without oxygen) system, constant pH conditions and nutrient supply are ideal for the growth of microbes. The pH (6-7) remains relatively constant because fermentation acids are absorbed rapidly across the rumen and hindgut wall or neutralized by saliva.

The flora of the caecum and colon of horses and rumen of cattle consist mainly of bacteria. Protozoa and fungi are present in much lower numbers, because of the lower rates of turnover.

Bacteria make the greatest contribution to metabolic work during fermentation compared to protozoa and fungi, especially the small bacteria. Bacteria that colonize the hindgut or rumen are diverse.

Bacterial species can be divided based on the type of energy source used, type of fermentation products produced, thickness, structure and composition of the cell walls. However, there are only two major types of bacterial cell wall whether a given cell has one or the other type of wall can be determined by the cell’s reaction to certain dyes (Hans Christian Gram method with violet and iodine staining). Bacteria which retain the dye are called Gram-positive bacteria those which can be decolorized are called Gram-negative bacteria.

The majority of the bacteria in the rumen and hindgut are Gram-negative. The number of Gram-positive bacteria tends to increase if animals are fed a high-energy diet containing abundant carbohydrates. This is an important concept that is studied a lot in relation to digestive disorders such as hindgut acidosis and laminitis (see the full series on laminitis in the lameness section of the website.)

Herbivores, because of the symbiotic relationship with microorganisms, are able to gain energy indirectly from fibrous materials i.e. non-starch polysaccharides (NSP).The microorganisms are able to break down the plant polymers to monomers and oligomers primarily by exogenous (generated outside a system) microbial enzymes. The products of this hydrolysis process are engulfed by microbes and converted to pyruvate in intracellular metabolism. Pyruvate is converted into volatile fatty acids (VFA’s: propionate, butyrate and acetate), CO2 and methane. The microbes cannot fully utilize these products. However the host animals, are able to absorb and gain energy from VFAs. The absorption of VFAs takes place in the enlarged colon of the horse and in the rumen, reticulum and omasum of cattle. However as hindgut fermenters, horses have the advantage to first digest and absorb the simple nutritional compounds, such as starches to glucose in the small intestine (which we described in detail in part 2 of this series). This system is more metabolically efficient for energy utilisation than fermentation to VFAs, which is obligatory in the ruminant.

Today most horses are used for sports and a grass/plant-only diet is usually not adequate to cover energy demands. Many commercial manufactured pelleted feeds are added to equine diets to supply the horse with energy, protein and micronutrients, but are based on cereal grains containing abundant starch. Starch plays a minor role in the natural diet of the horse as they are grazing/browing herbivores and receive most carbohydrates in the form of fructans and NSP.

Grain supplements may cause starch overload and can affect the physiology of the horse due to the lack of amylase to hydrolyse all the ingested starch. If starch is not well digested in the small intestine of the horse a proportion of the ingested starch will reach the hindgut and be rapidly fermented. The microbial degradation rate of starches is much faster than that of NPS. A change in the ratio of starch to fiber in the diet has a rapid impact on VFA yields.

With increasing starch in the diet (e.g. grains) more propionate and lactate are produced within a short time compared to that of other NSP’s, which leads to reduced digesta pH. Several studies associate excess feeding of grain concentrates with a number of digestive and metabolic disorders, including, acidosis, laminitis, gastric ulcers, developmental orthopedic disease and some forms of exertional rhabdomyolysis. However, it should also be stated that excess intake of lush pastures with high levels of fructans can also cause these digestive and metabolic disorders.

Digestive and metabolic disorders are very common in the domestic horse population all over the world and as you now understand the majority of the cases it can be traced back to the way we manage and feed our horses! The horse is designed to eat large quantities of fibre on a continuous basis. The concentrate diet should be mixed with a fibre source such as (low non-structural carbohydrate NSC) chaff or super fibres. You can also offer roughage before the feeding of the concentrate diet to slow down the passage rate and facilitate fiber digestion. The main aim is that we maximise the fiber intake and minimise the NSC intake so that we promote healthy functioning of the digestive system of our horses.


Utilization of caecal digesta by caecotrophy (soft faeces ingestion) in the rabbit Utilisation des produits de la fermentation caecale par la caecotrophie (ingestion des crottes molles) chez le lapin Verwertung der blinddarmfermetationsprodukte durch coecotrophie (weichkotaufnahme) beim kaninchen

As a hindgut fermenter, the rabbit obtains the vitamins and proteins synthesized in the large intestine by caecotrophy, i.e. the production and preferential ingestion of special soft faeces. This is a higher degree of specialization than coprophagy (reingestion of normal faeces) which is practised by many rodents. Some characteristics of caecotrophy in rabbits are described.


Digestive system

To fuel endothermy, mammals require more calories per ounce (or gram) of tissue than do ectothermic vertebrates such as reptiles. This is accomplished by more efficient digestion of food stuffs and more efficient absorption of nutrients. This efficiency begins with specialization of the teeth. Mammals have four different kinds of teeth (heterodonty) that are ideally shaped to cut, slice, grind, and crush food. An exception is the toothed whales in which all the teeth are similar (homodonty). The four types of teeth are incisors for slicing, canines for pierc ing, premolars for crushing or slicing, and molars for crushing. They are commonly represented by a notation called a dental formula, e.g., I2/2 C1/1 P3/3 M2/3, the dental formula for the Egyptian fruit bat (Rousettus aegyptiacus). The first in each group of two numbers represents the teeth in the upper jaw and the second is the number of teeth in the lower jaw. Multiplying the dental formula by 2 gives the total number of teeth, 34. The Egyptian fruit bat's dental formula indicates that the full set of teeth for the upper jaw is four upper incisors, two upper canines, six upper premolars, and four molars. There is a great deal of variation in the number and type of teeth present. For example, the prosimian primate, the aye-aye (Daubentonia madagascariensis), has a dental formula of I1/1 C0/0 P1/0 M3/3, which illustrates a reduction in number of some teeth and the complete loss of others. In the case of some herbivorous mammals the upper incisors are either reduced in number or completely replaced by a hard dental or gummy pad that functions as a cutting board for the lower incisors. In some gnawing mammals, such as rodents and rabbits, the upper and lower incisors grow throughout the entire life span and the canines have been lost. Modification of teeth may be extreme, such as complete loss in most anteaters, or the formation of large tusks, derived from the second upper incisors in elephants, or from the canines in walruses (Odobenus rosmarus).

The relationship between dental structure and function is so precise that the diets of long-extinct mammals can be de

duced from their teeth. The teeth are often the only fossil remains recovered from paleontological sites. Teeth perform mechanical (or physical) digestion by breaking down a food morsel into smaller pieces, providing additional surface area for action by digestive enzymes. Premolars in herbivorous mammals usually have ridges for grinding. In some carnivores such as wolves, the last upper premolar has a blade that shears against the first lower molar. Fruit-eating mammals such as flying foxes often have flattened premolars and molars.

Other modifications for efficient digestion occur in the stomach, a portion of the gastrointestinal tract. The stomach serves as a storage receptacle in most mammals and as a site of protein breakdown. A simple stomach is found in most mammal species, including some that consume fibrous plant material. In other mammals that consume a high fiber diet the stomach has become enlarged and modified to handle more difficult digestion. These modifications comprise a foregut digestive strategy, for which the stomach contains compartments where symbiotic microbes break down cellulose and produce volatile fatty acids (VFA) that can be utilized by the mammal. Foregut fermentation has been developed to the greatest degree among the mammal order Artiodactyla, which includes pigs, peccaries, camels, llamas, giraffes, deer, cattle, goats, and sheep. Rumination, reprocessing of partially digested food, is accomplished by the four-compartment stomachs of giraffes, deer, cattle, and sheep. Less complex tubular and sacculated stomachs are found in kangaroos, colobus monkeys, and sloths. Stomachs in foregut fermenting species are neutral or only slightly acidic, around pH 6.7, to provide a favorable environment for symbionts.

Food moves from the stomach to the intestines which consist of the small intestine, where most digestion and absorption occurs, and the large intestine. The wall of the small intestine contains epithelial tissue with small finger-like projections called villi. In turn, each villus has smaller extensions called microvilli. The villi secrete enzymes for further carbohydrate and protein digestion. The microvilli absorb the digested nutrients. The presence of the villi and microvilli in the mammal small intestine increases the absorptive surface area by at least 600 times that of a straight smooth tube. The

Black rhinoceros (Diceros bicornis) profile showing its horns used for protection and fighting for supremacy and social heirarchy. (Photo by Leonard Lee Rue III. Bruce Coleman, Inc. Reproduced by permission.) An elephant can reach a long way up for a meal using its trunk. (Photo by K & K Ammann. Bruce Coleman, Inc. Reproduced by permission.)

villi of the human small intestine, for example, provide 3,230 ft2 (300 m2) of surface area whereas the surface area of a smooth tube of the same size as the small intestine is about 5.4 ft2 (0.5 m2). Nutrient absorption occurs through the membranes of the microvilli of each intestinal epithelial cell. Also distributed throughout the small intestine are glands that secrete special enzymes for further digestion of proteins, carbohydrates, and lipids.

For mammals, diet and the length of the small intestine are closely correlated. Mammals that consume a diet that is either digested in the stomach (such as animal protein consumed by faunivores) or easily absorbed (such as nectar consumed by nectarivores) have a shorter small intestine than

A free-ranging yak (Bos grunniens) exhibits the rare golden color phase. (Photo by Harald Schütz. Reproduced by permission.) An aardvark (Orycteropus afer) unearths the subterrranean nests of ants and termites through active digging using powerful and well-adapted claws. (Photo by Rudi van Aarde. Reproduced by permission.)

other mammals. Herbivores that eat very fibrous plant matter tend to have the longest small intestine. The small intestines of fruit-eaters tend to be intermediate in length.

The foregut fermentation strategy of herbivores requires a medium or large body size to accommodate the necessarily large stomach. A strategy generally used by smaller herbivores is hindgut fermentation (although there are large hindgut fermenters such as horses, elephants, and howler monkeys). The hindgut, also called the large intestine, consists of the cecum and the colon. The cecum is a blind pouch that serves as the principal fermentation chamber in the hindgut strategy. As in the stomach of foregut herbivores, colonies of symbionts in the cecum of hindgut fermenters break down cellulose and excrete products advantageous to the mammal. Nutrients appear to be absorbed through the wall of both the cecum and colon, especially in the larger mammals.

Small hindgut fermenters, such as many rodents and rabbits, have the problem that food can only be retained in the gut for a short time. As it leaves the hindgut, digestion is incomplete and many valuable nutrients may be left unabsorbed. This problem is solved by a behavioral adaptation: a soft pellet is produced in the cecum, defecated, and immediately

A barbary ape (Macaca sylvanus) juvenile in a female's arms. (Photo by Animals Animals ©J. & P. Wegner. Reproduced by permission.)

picked up by the animal and reingested. This reingestation of feces is called coprophagy. The soft pellet then goes through the digestive process a second time and the end product is a hard fecal pellet devoid of nutrients. Many owners of pet rabbits are familiar with the hard pellet, often called a "raisin." The softer pellet is usually consumed at night (when coprophagy goes unobserved by the pet owner) and is called the "midnight pellet." Coprophagy is efficient voles are able to extract 67-75% of the energy contained in their food.


Gastrointestinal Physiology and Nutrition

Rabbits are described as selective feeders in feeding on natural vegetation, they select the most tender, succulent plant parts (i.e., the parts that are most nutrient-dense and lowest in available cell walls). 12 Their natural diet consists of the preferred succulent buds and young leaves of bushes, and they routinely graze on grasses, weeds, and even the bark around bushes and trees. 7 They have a relatively high metabolic rate and a fast feed-transit time (19 hours) 32 moreover, the practice of selective feeding (often termed “concentrate selection”) allows them to meet their dietary requirements while minimizing the volume of food that must be eaten. Rabbits eat approximately 30 times per day (2-8 g of food per time) over 4- to 6-minute periods. 48 The selection of food is based on olfactory cues and tactile information obtained via the sensitive vibrissae around the nose and lips. 27 The incisors have evolved to cut through vegetation with a vertical slicing motion, while the cheek teeth are responsible for grinding the food before it is swallowed. Jaw movements during mastication are reported to be up to 120 per minute 5 and amylase-containing saliva is secreted continuously during the process. Jaw movements feature a lateral motion, which helps keep the constantly growing teeth worn down to maintain effective occlusal surfaces. Hunger is stimulated by a dry mouth and gastric contractions or a decrease in blood levels of metabolites such as glucose, amino acids, volatile fatty acids, or lactic acid. 18

The gastric pH of the rabbit varies diurnally but is generally very acidic compared with that of other species postprandially it can drop to as low as 1.0 to 2.0, whereas following the ingestion of cecotrophs it rises to 3.0. 4 This low pH effectively sterilizes ingesta. Juvenile (preweaned) rabbits have a much higher gastric pH (5.0-6.5) (Fig. 14-1), which promotes the survival and passage of ingested bacteria, facilitating the establishment of the vitally important large intestinal flora. 27




The cecum is the largest organ in the abdominal cavity and has 10 times the capacity of the stomach. It usually contains approximately 40% of the intestinal contents. It is very thin-walled and coiled, ending in the blind-ended vermiform appendix. 41 The appendix is rich in lymphoid tissue and also has a secretory function (water and bicarbonate). The cecum receives the short particles and fluid selectively retained by the proximal colon therefore its contents are generally semifluid in consistency. Microbial fermentation is the primary mechanism by which nutrients are released from ingested food, and the retained particles directed from the proximal colon to the cecum provide the substrate for the indigenous population of cecal microorganisms. Some of the products of fermentation are absorbed directly through the cecal wall, while many others are expelled and reingested as cecotrophs. 27 The nutrients present within cecotrophs are made available to the rabbit when they pass through the stomach into the small intestine.

To facilitate the digestion of selectively retained materials, the rabbit’s cecum has a well-established autochthonous population of microorganisms. These microorganisms produce the volatile fatty acids acetate, butyrate, and propionate, 23 which provide up to 40% of the rabbit’s maintenance energy requirement. 38 The proportion of the three volatile fatty acids varies according to the time of day, the diet, and the rabbit’s developmental stage. The microbiology and ecology of the rabbit gastrointestinal tract has not been as extensively studied as that of ruminant species or humans, 52 but it is widely agreed that across all mammalian species a healthy intestinal flora is vital for systemic health. The indigenous intestinal microflora is essential for digestion but also serves a protective role in relation to potential pathogens. Studies to date have identified an extensive list of organisms that may be present, including a variety of anaerobes, 52 gram-positive and gram-negative facultative anaerobes, 53 large ciliated protozoa, and a rabbit-specific ascosporogenous yeast (Cyniclomyces guttulatulus). 20 It is generally agreed that the strict anaerobes of the genus Bacteroides are the predominant organisms within the cecum, accounting for 10 9 to 10 10 per gram out of a total bacterial load of 10 10 to 10 12 per gram of cecal contents. 45 Coliform bacteria and Clostridium species are occasionally isolated from normobiotic rabbits if they are present, however, they represent a very small percentage of the total bacterial population. 52 In contrast with other mammals, rabbits have very rarely been found to harbor lactobacilli. 35, 55 Energy is the limiting factor for the cecal microbial population 36 and, as is the case in other species, the composition of the microflora in any one individual does not remain constant. The cecal production of carbohydrates and nitrogenous substances, along with cecal goblet cell mucin production, supports abundant microbial synthesis. 7 The nitrogen sources for cecal bacteria are predominantly ammonia, urea, and biuret.

Disruption of the normal balance of microflora in the gut, usually with overgrowth of known or potential pathogens, is termed dysbiosis and is a common and serious clinical problem in rabbit medicine. 11, 53 Such alterations in normal homeostatic mechanisms within the gut may occur secondary to inappropriate therapeutic antibiotic administration, exposure to pathogenic organisms or toxins, increased glucocorticoid levels (iatrogenic or secondary to stress), gastrointestinal hypomotility, and poor dietary composition (low fiber, high carbohydrate, and high protein levels) (see Chapter 15).

Suckling rabbits are unique among nursing neonates in that they feed for only 3 to 4 minutes in every 24 hour period 30 and in that they produce an antimicrobial fatty acid referred to as “milk oil.” This oil results from an enzymatic reaction that occurs in the stomach following the ingestion of doe’s milk, and it appears to be a factor in the control of the gastrointestinal microbial content of young rabbits. 16 Rabbits fed milk from other species do not develop this antimicrobial factor and are more susceptible to infection. 27 There is virtually no microflora in the rabbit gastrointestinal tract at 3 days of age, and although some bacteria can be found in the small and large intestines over the following 3 weeks, the stomach remains largely devoid of microflora during this time. The production of milk oil wanes as the weaning process progresses and solids replace milk. 16 In this transitional period, as the production of stomach oil ceases but before gastric pH has decreased to adult levels, populations of bacteria pass from the stomach to colonize the small intestine, cecum, and colon.

Rabbits produce two types of feces over the course of a day: hard feces and cecotrophs (also termed “soft feces” or “night feces”) (Fig. 14-4). 53 These two excretory products differ markedly in composition, 29 and in a healthy rabbit the excretion of only one type occurs at any one time. Hard fecal pellets are composed of compressed indigestible fiber (dry matter, 52.7%) that is separated from the remainder of the ingesta in the proximal colon, whereas cecotrophs (dry matter, 38.6%) contain semiliquid cecal contents and are rich in essential amino acids, volatile fatty acids, enzymes such as amylase and lysozyme, vitamins B and K, and microorganisms including bacteria, yeasts, and protozoa. 7, 27, 50 The protein content of cecotrophs varies between 24.4% and 37.8%, of which 81% is in the form of bacterial cells. 25 Cecotrophs are excreted according to a complex circadian pattern, which opposes that of voluntary food intake and hard feces production. 10 Initiation of cecotroph formation occurs when segmental and haustral contractions of the large intestine are replaced by mass peristaltic activity resulting in the expulsion of material from the cecum. 15 The expelled cecal content is then packaged and covered by a mucous envelope in the colon and passed as soft pellets approximately 5 mm in diameter and arranged in clusters. The transit time for cecotrophs through the colon is 1.5 to 2.5 times faster than that for hard feces. 19 They are consumed directly from the anus, a practice referred to as cecotrophy (Fig. 14-5), and are swallowed without mastication. 11 The ingestion of cecotrophs is triggered by stimulation of rectal mechanoreceptors, the perception of the specific odor of cecotrophs, and blood concentrations of various metabolites and hormones. 18 The percentage of cecotrophs eaten varies depending on feeding regime and dietary composition. For example, fewer cecotrophs are consumed on a high-protein diet compared with a high-fiber one.


AVIAN DIGESTIVE SYSTEM

An understanding of the avian digestive system is essential for developing an effective and economical feeding program for your poultry flock and for recognizing when something is wrong and taking necessary actions to correct the problem.

The digestive system of any animal is important in converting the food the animal eats into the nutrients its body needs for growth, maintenance, and production (such as egg production). An animal’s body breaks down food through both mechanical and chemical means. In many animals, the mechanical action involves chewing however, because birds do not have teeth, their bodies use other mechanical action. The chemical action includes the release of digestive enzymes and fluids from various parts of the digestive system. After being released from food during digestion, nutrients are absorbed and distributed throughout the animal’s body.

PARTS OF A CHICKEN DIGESTIVE TRACT

The chicken has a typical avian digestive system. In chickens, the digestive tract (also referred to as the gastrointestinal tract or GI tract) begins at the mouth, includes several important organs, and ends at the cloaca. Figure 1 shows a chicken digestive tract, and Figure 2 shows the location of the digestive tract in the chicken’s body.

Figure 1. Digestive tract of a female chicken (Image by Dr. Jacquie Jacob, University of Kentucky) Figure 2. Location of the digestive tract in a female chicken (Image from PoultryHub.org and used with permission)

Beak/Mouth

As with most birds, a chicken obtains feed by using its beak. Food picked up by the beak enters the mouth. Chickens do not have teeth, so they cannot chew their food. However, the mouth contains glands that secrete saliva, which wets the feed to make it easier to swallow. Also, the saliva contains enzymes, such as amylase, that start the digestion process. The chicken uses its tongue to push the feed to the back of the mouth to be swallowed.

Esophagus

The esophagus is a flexible tube that connects the mouth with the rest of the digestive tract. It carries food from the mouth to the crop and from the crop to the proventriculus.

Crop

The crop is an out-pocketing of the esophagus and is located just outside the body cavity in the neck region (see Figure 3). Swallowed feed and water are stored in the crop until they are passed to the rest of the digestive tract. When the crop is empty or nearly empty, it sends hunger signals to the brain so that the chicken will eat more.

Figure 3. Location of the crop in a female chicken (Image by Dr. Jacquie Jacob, University of Kentucky)

Although the digestive enzymes secreted in the mouth began the digestion process, very little digestion takes place in the crop—it is simply a temporary storage pouch. The crop evolved for birds that are typically hunted by other animals but need to move to the open to find feed. These birds can consume relatively large amounts of food quickly and then move to a more secure location to digest that food.

Occasionally, the crop becomes impacted, or backed up. This problem—called crop impaction, crop binding, or pendulous crop—can occur when a chicken goes a long time without feed and then eats too much too quickly when feed is available again. Crop impaction also can occur when a chicken free ranges on a pasture of tough, fibrous vegetation or eats long pieces of string. With crop impaction, even if a chicken continues to eat, the feed cannot pass the impacted crop. The swollen crop also can block the windpipe, causing the chicken to suffocate.

Proventriculus

The esophagus continues past the crop, connecting the crop to the proventriculus. The proventriculus (also known as the true stomach) is the glandular stomach where digestion primarily begins. Hydrochloric acid and digestive enzymes, such as pepsin, are added to the feed here and begin to break it down more significantly than the enzymes secreted by the salivary glands. At this point, however, the food has not yet been ground—this organ is called the proventriculus because its location in the digestive tract is before the ventriculus, where food is ground (see Figure 4).

Figure 4. Two views of the proventriculus and gizzard from the digestive tract of a chicken (Image by Dr. Jacquie Jacob, University of Kentucky)

Ventriculus (Gizzard)

The ventriculus, or gizzard, is a part of the digestive tract of birds, reptiles, earthworms, and fish. Often referred to as the mechanical stomach, the gizzard is made up of two sets of strong muscles that act as the bird’s teeth and has a thick lining that protects those muscles (see Figure 5). Consumed feed and the digestive juices from the salivary glands and proventriculus pass into the gizzard for grinding, mixing, and mashing.

Figure 5. Inside of a chicken gizzard, with the internal lining removed (Image by Dr. Jacquie Jacob, University of Kentucky).

When allowed to free-range, chickens typically eat small stones. The acidic environment in the proventriculus softens the stones, and then the strong muscles of the gizzard grind them into tiny pieces. The stones remain in the gizzard until they are ground into pieces small enough to pass to the rest of the digestive tract.

Grit, a commercial product made up of small stones, can be used as a supplement to chicken feed. Chickens fed only commercially prepared feed do not need grit. Chickens that eat whole grains or chickens kept on pasture that do not consume enough pebbles with the forage typically require a supplementation of grit. Grit should not be confused with limestone or oystershell, which are given to laying hens as sources of calcium for their eggs’ shells.

When a chicken eats a small, sharp object, such as a tack or staple, the object is likely to get stuck in the gizzard. Because of the strong grinding motion of the gizzard’s muscles, such sharp objects can put holes in the gizzard wall. Chickens with damaged gizzards grow thin and eventually die. Preventing this situation is a good reason to keep a poultry house free of nails, glass shards, bits of wire, and so on.

Small Intestine

The small intestine is made up of the duodenum (also referred to as the duodenal loop) and the lower small intestine. The remainder of the digestion occurs in the duodenum, and the released nutrients are absorbed mainly in the lower small intestine.

The duodenum receives digestive enzymes and bicarbonate (to counter the hydrochloric acid from the proventriculus) from the pancreas and bile from the liver (via the gall bladder). The digestive juices produced by the pancreas are involved primarily in protein digestion. Bile is a detergent that is important in the digestion of lipids and the absorption of fat-soluble vitamins (A, D, E, and K).

The lower small intestine is composed of two parts, the jejunum and the ileum. The Meckel’s diverticulum marks the end of the jejunum and the start of the ileum (see Figure 6). The Meckel’s diverticulum is formed during a chicken’s embryonic stage. In the egg, the yolk sac supplies the nutrients needed for the embryo to develop and grow. Right before hatch, the yolk sac is taken into the navel cavity of the embryo. The residual tiny sac is the Meckel’s diverticulum.

Figure 6. Location of the Meckel’s diverticulum in the digestive tract of a chicken (Image by Dr. Jacquie Jacob, University of Kentucky).

Ceca

The ceca (plural form of cecum ) are two blind pouches located where the small and large intestines join. Some of the water remaining in the digested material is reabsorbed here. Another important function of the ceca is the fermentation of any remaining coarse materials. During this fermentation, the ceca produce several fatty acids as well as the eight B vitamins (thiamine, riboflavin, niacin, pantothenic acid, pyridoxine, biotin, folic acid, and vitamin B12). Because the ceca are located so close to the end of the digestive tract, however, few of the produced nutrients are absorbed and available to the chicken.

Large Intestine (Colon)

Despite the name, the large intestine is actually shorter than the small intestine. The large intestine is where the last of the water reabsorption occurs.

Cloaca

In the cloaca, the digestive wastes mix with wastes from the urinary system (urates). Chickens usually void fecal material as digestive waste with uric acid crystals on the outer surface—that is, chickens do not urinate. The color and texture of chicken fecal material can indicate the health status of the chicken’s digestive tract: the white, pasty material coating chicken fecal material is uric acid, the avian form of urine, and is normal.

The reproductive tract also exits through this area. When a hen lays an egg, the vagina folds over to allow the egg to leave through the cloaca opening without coming into contact with feces or urine.

Intestinal Microflora

Both the small and large intestines normally are populated with beneficial organisms (bacteria, yeast, etc.), referred to as microflora ( micro meaning “small” and flora meaning “plants”). This microflora aid in digestion.

When chicks hatch, their digestive tracts are virtually sterile. If raised by a mother hen, a chick obtains the beneficial microflora by consuming some of its mother’s fecal material. In artificial incubation and brooding, chicks do not have this option. In such situations, producers can provide the chicks with probiotics, which are preparations containing the beneficial microflora that normally inhabit a chicken’s digestive tract. Through the probiotics, the chicks receive the beneficial bacteria they need to fight off infection by pathogenic bacteria, such as salmonella.

Intestinal disease in chickens normally occurs when the balance of normal microflora is upset—that is, the normal microflora is overrun by too many foreign organisms. The result is enteritis or inflammation of the intestines. Enteritis produces symptoms that include diarrhea, increased thirst, dehydration, loss of appetite, weakness, and weight loss or slow growth. Severe damage to the intestinal tract typically is called necrotic enteritis ( necrotic meaning “dead tissue”), which is a problem in many types of production systems.


The Gut And Digestion

Digestive systems show great variation in the Animal Kingdom. Gut structures do not always correlate with feeding habits (although large stomachs may reflect large, infrequent meals). There is a close correlation between gut structures and the nature of the ingested food: carnivores tend to have short absorptive intestines.

A five-part gut is recognizable.

(1) Reception by the mouth and pharynx. Mechanical trituration can occur here (e.g. use of tongue and teeth in mammals) enzymic digestion may commence toxic factors may paralyse prey. Fluids lubricate the food.

(2) Conduction and storage in the oesophagus which may be distended to form a crop (birds) or fermentation chambers (ruminant mammals).

(3) Trituration and early digestion in the stomach and the first part of the intestine. Mechanisms for internal trituration may be found in animals which swallow large food masses [e.g. mammalian stomach muscles muscles and gravel in bird gizzards rotation of a crystalline style in the bivalve stomach]. In crustaceans, a chitin-lined gizzard precedes the intestine: food is ground in a gastric mill with dorsal and lateral teeth moved by muscles. Enzymic digestion is commonly found in this region. Diverticula are common (e.g. teleost fishes): cilia may propel food into diverticula in invertebrates (e.g. molluscs).

(4) Absorption and further digestion occurs in the vertebrate small intestine: most of the enzymes come from the pancreas. The liver secretes bile. Some absorption occurs in the duodenum but mostly in the ileum. In most invertebrates, digestion has occurred in stomach diverticula and the intestine is absorptive. The area is increased by typhlosole (longitudinal fold, e.g. earthworm), spiral valve (elasmobranchs), folding and villi (e.g. mammals).

Area of the human intestine:

With microvilli 2 × 106 cm 2

(5) Compaction and formation of feces where much water is absorbed in terrestrial species (particularly important in insect osmoregulation).

Human gut Structure

  • The rather unspecialized human gut reflects our omnivorous diet. The inner surface of the gut is continuous with the exterior of the body so the gut lumen is technically outside of the body.
  • The gut is lined by a mucosa (epithelial tissue, underlying basement membrane, connective tissue with thin smooth muscle), a submucosa (outside the mucosa with connective tissue, blood vessels, nerves), muscularis externa (with inner circular muscles and outer longitudinal muscles) and an outer serosa of connective tissue.
  • Co-ordinated muscle contractions produce ring-like contractions to churn up food and waves of peristalsis to propel food along the gut. Mouth Food enters the gut via the mouth mastication by the teeth and the tongue takes place. Saliva, which contains bicarbonate (HCO3 – ) and an α-amylase, lubricates the bolus of food.
  • Approximately 1500 cm3 saliva per day are secreted by the salivary glands secretion is under parasympathetic nervous control. Human teeth are relatively unspecialized each jaw quadrant has two biting incisors, one tearing canine, two premolars and three molars for grinding. Teeth are coated with enamel over dentine: in the middle is a vascularized, innervated pulp.
  • Teeth are embedded in jaw bones by ligaments and cement. Absorption of some drugs occurs in the mouth (e.g. morphine).

Esophagus

The triturated, lubricated food bolus passes down the esophagus by peristalsis the esophagus is lined (like the mouth) with stratified squamous epithelium. Peristalsis is movement of a muscular tube, such as the gut, by co-ordinated contractions of longitudinal and circular smooth muscle in a definite (usually anterio-posterior) direction.

Stomach

  • Food is triturated further in the stomach by bands of smooth muscle – longitudinal and circular muscles antagonize each other. This allows mixing with gastric juices.
  • Food first collects in the relaxed body of the stomach pronounced peristaltic waves from the top (fundus) to the base (antrum) occur – this generates chyme which is squirted through the pyloric sphincter in small quanta.
  • The sphincter is normally open except after a squirt into the duodenum (i.e. it acts as a valve: a large meal can take more than 4 hours to enter the duodenum).
  • The stomach mucosa is very thick, with many gastric pits. Mucus-secreting cells cover the stomach surface and line the pits.
  • Gastric juice is very acid with hydrochloric acid (HCl, pH = 1.5–2.5), which is secreted by gastric glands in the lower parts of the pits: acid kills bacteria and living cells in the food, loosens fibrous components in food and facilitates conversion of pepsinogen to pepsin.
  • Protein digestion is initiated in the stomach: gastric juice contains pepsinogen (an enzyme precursor or zymogen) which is converted to pepsin by gastric HCl.
  • Zymogens prevent digestion of the stomach by self-enzymes. Mucus also protects cells from activated enzymes the mucus forms a protective coat absorbed onto a glycocalyx on the surfaces of microvilli of lining cells.) Rennin precipitates soluble proteins in milk. Water, salts, some vitamins and some drugs (e.g. ethanol) are absorbed in the stomach.

Small intestine

  • The small bowel (intestine) comprises, in sequence, the duodenum, jejunum and ileum: it is the site of enzymatic digestion using enzymes from the pancreatic juice and in or on the surfaces of intestinal epithelial cells.
  • Pepsin is inactivated by mildly alkaline conditions in the small bowel. Protein digestion continues using pancreatic trypsin and chymotrypsin.
  • Pancreatic juice also has an amylase and a lipase. Bile salts from the liver (draining into the duodenum from bile ducts after temporary storage in the gall-bladder see below) emulsify fats: bile also acts as an excretion medium for hemoglobin degradation products and cholesterol.
  • Much of the final breakdown of proteins, fats and sugar polymers occurs within cells lining the small bowel: as intestinal epithelial cells mature and pass up to the tips of the villi, the enzyme contents increase.
  • It is now thought that digestion occurs in the cells or on their membranes – there does not seem to be a secretion of large quantities of enzymes into the small bowel lumen (in which those enzymes which are found derive from the pancreas).
  • Digestion products are absorbed in the small bowel whose area is increased by villi and microvilli (see above). Some fats, hydolyzed to fatty acids and glycerol, are resynthesized to new fats, and are packaged into chylomicrons and absorbed by lacteals (of the lymphatic system) in the villi: chylomicrons deliver fats to the adipose cells or liver or are broken down in the bloodstream.
  • Cholesterol is made in the liver and is packaged into low density lipoprotein (LDL) complexes which are excreted in bile. Stored cholesterol can be repackaged for delivery to cells for membrane or steroid hormone synthesis.

Large intestine

  • Absorption of water, sodium and other minerals occurs in the large bowel (intestine). The first part is the colon where, in humans, 7 liters of water are absorbed per day. The colon harbors bacterial flora which can further digest food and synthesize absorbable amino acids and vitamins (e.g. vitamin K).
  • The cecum, almost absent in humans, is a blind pouch with the vermiform appendix at its end it is a secondary lymphoid organ. Residues (dead cells, bacteria, cholesterol, bile pigments, undigested food, especially cellulose fibers) form the feces, stored in the colon. Feces are expelled periodically by passing them down the rectum and out through the anus.

Co-ordination of Digestion

  • Enzymes act consecutively along the gut at specific sites: this requires release of small quantities of food and a precise sequence of enzyme release. Food movement is largely under autonomic nervous control. Saliva secretion is nervous alone: it is initiated by food in the mouth, or by the smell, sight or anticipation of food.
  • Gastric juice secretion is initiated by nervous stimulation due to food in the mouth and/or the stomach. Distention of the stomach antrum results in the hormone gastrin being secreted by stomach-lining cells into the bloodstream, stimulating further gastric juice secretion (an isolated, denervated stomach pouch still secretes gastric juice when food is in the antrum).
  • Gastrin secretion is inhibited by high acid levels in the stomach (negative feedback). As stomach contents are released into the duodenum, the duodenum secretes a gastrininhibitory polypeptide (GIP) which further inhibits gastric juice secretion.
  • Acid stomach contents in the duodenum stimulate release of secretin: this enhances the flow of bicarbonate-rich (alkaline) pancreatic juice. Protein fragments stimulate cholecystekinin–pancreozymin (CCK–PZ) which leads to the release of pancreatic enzymes and stimulates gall-bladder contraction. Motilityhormones affect movement of the villi by smooth muscle contraction.

The mammalian Liver

  • The liver develops as an outgrowth of the gut most of its cells are hepatocytes arranged in cylindrical lobules intimately associated with venules, arterioles and bile canaliculi.
  • Food-enriched blood is brought from the gut to the liver by the hepatic portal vein, and oxygenated blood is transported to the liver by the hepatic artery.
  • The liver is drained by the hepatic vein. Liver cells secrete bile which passes down bile canaliculi into a hepatic duct and then up a cystic duct to the gall-bladder.
  • Relaxation of a sphincter at the neck of the gall-bladder and contraction of the bladder following the release of food into the duodenum allows bile to flow down the common bile-duct into the small intestine.
  • Bile acts as an excretion medium for cholesterol and hemoglobin degradation pigments, but its main role is to carry bile salts which emulsify fats in the bowel.
  • Other functions of the liver include glycogen, amino acid and fat storage and metabolism, detoxification of ammonia to urea, cholesterol synthesis, fetal erythropoiesis (red cell manufacture), breakdown of excess hemoglobin, red blood cell storage, vitamin storage, synthesis of many plasma proteins and heat production

The mammalian Pancreas

Like the liver, the pancreas develops as an outgrowth of the small intestine, its cells producing large quantities of digestive enzymes which travel to the gut via the pancreatic duct, which joins the common bile duct just before its junction with the bowel. The islets of Langerhans are important endocrine glands producing hormones (e.g. insulin, glucagon) associated with blood glucose control

Cellulose digestion by symbionts

  • Ruminants, for example cows and sheep, have a gut divided into sections similar to those of humans, except that the esophagus and stomach are greatly modified. They use symbiotic bacteria, yeasts and protoctistans to break the β,1–4 links in cellulose.
  • There are four chambers: the rumen, reticulum and omasum are sacculations of the esophagus the abomasum is the true stomach.
  • On being swallowed, food goes to the reticulum (tripe) where it is made into cud balls. The fermenting mass is regurgitated to the mouth for further trituration (chewing the cud). On second swallowing, food passes to the rumen.
  • The rumen is rich in anaerobic symbiotic microorganisms (especially bacteria and ciliates) which ferment cellulose to fatty acids, carbon dioxide and methane, and starches to sugars. Fatty acids are absorbed by the glandular epithelium of the rumen, the gases are eructed (‘burped’!).
  • The fermented mass then passes to the omasum (psalterium): here it is further triturated, strained and squeezed by strong muscular contractions.
  • It then passes to the abomasum where digestive juices start work: bacteria are also digested in the abomasum and are rich sources of nitrogen and vitamins of the B complex. Lagomorphs, for example rabbits, have a large cecum and appendix off the colon.
  • This contains bacteria which digest cellulose and produce B vitamins. Lagomorphs re-ingest fecal pellets (coprophagy), giving food a second passage through the gut and permitting the products of microorganism cellulose digestion to be absorbed.
  • Hindgut fermenters such as horses have a capacious cecum. Symbiotic microorganisms break down cellulose. Digestion products can pass forward, by reverse peristalsis, for absorption in the small bowel.

Mammal tooth Patterns

Basic (‘primitive’) pattern

The basic pattern is three incisors, one canine, four premolars and three molars on each side, in each of the upper and lower jaws. The dental formula is I 3/3 C 1/1 PM 4/4 M 3/3.


Watch the video: Small intestine and food absorption. Physiology. Biology. FuseSchool (January 2022).