Information

Excretion of monovalent and divalent ions in sharks


I have heard that sharks excrete $ce{Na+}$ and $ce{Cl-}$ by their gill surfaces but divalent ions like magnesium are excreted through feces. What could be the reason behind this?


The main reason seems to be a mechanism to save water and allow the excretion of higher concentrations of the ions. See the image (from here):

The urine enters the glomeruli at a relatively low rate and with a low magnesium concentration. The paper cited below mentions a concentration of 1.5mM. First organic compounds as glucose are recovered actively, water follows the molecules passively. Then more ions as Ca$^{2+}$ enter in the proximal segment II. In the distal segment finally sodium and chloride are actively transported out while water passively leaves the segment. The divalent ions (magnesium, calcium) are left in high concentrations (the paper below says that the Mg concentration is now around 130mM). This helps concentrating these ions for excretion while loosing as little water as possible at the same time. Sodium and chloride are actively regulated in the gills.

For a short overview, see the link at the image above, for a detailed view, have a look at the following publication: "Kidneys sans glomeruli".


Osmoconformer

Osmoconformers are marine organisms that maintain an internal environment which is isotonic to their external environment. [1] This means that the osmotic pressure of the organism's cells is equal to the osmotic pressure of their surrounding environment. By minimizing the osmotic gradient, this subsequently minimizes the net influx and efflux of water into and out of cells. Even though osmoconformers have an internal environment that is isosmotic to their external environment, the types of ions in the two environments differ greatly in order to allow critical biological functions to occur. [2]

An advantage of osmoconformation is that such organisms don’t need to expend as much energy as osmoregulators in order to regulate ion gradients. However, to ensure that the correct types of ions are in the desired location, a small amount of energy is expended on ion transport. A disadvantage to osmoconformation is that the organisms are subject to changes in the osmolarity of their environment. [3]


2 The Kidney

This chapter analyzes the role of kidney in some marine creatures. In freshwater forms, the kidney functions largely as a water excretory device. With rare exceptions, this is accomplished by filtration at the renal glomerulus and implies the presence of suitable cellular components to conserve filtered ions and excrete dilute urine. In marine teleost forms, the kidney functions chiefly as an excretory device for magnesium and sulfate ions. In glomerular marine forms, associated machinery must be present to conserve water, monovalent ions, and other filtered plasma constituents. The cartilaginous sharks, skates, and rays, because they are hyperosmotic in a marine environment, have combined the principal functions of the kidneys of both freshwater and marine bony fishes. The chapter assembles all available information concerning the structure and function of the fish kidney and provides a unifying synthesis for the understanding of this organ's role in body fluid regulation, of specific nephron function in fishes, and of the evolutionary significance of the regions of the nephron in fishes and higher vertebrates.


Excretory Products and their Elimination Important Extra Questions Very Short Answer Type

Question 1.
What is a nephron?
Answer:
The functional unit of the kidney.

Question 2.
What is a flame cell?
Answer:
The excretory unit in planaria, tapeworm, and liver fluke.

Question 3.
What is micturition?
Answer:
It is the act of void of the urinary bladder, the activity under nervous and voluntary control.

Question 4.
What are ammonotelic animals?
Answer:
The animals which excrete nitrogenous wastes as ammonia are ed ammonotelic animals, e.g., certain fishes.

Question 5.
What is a green gland and in which animal it is found?
Answer:
It is an excretory structure found in prawns.

Question 6.
What is an antidiuretic hormone?
Answer:
It is the hormone that helps in the reabsorption of water in the nephron, also called vasopressin (Secreted by post pituitary gland).

Question 7.
Define excretion.
Answer:
Excretion is the process of elimination of metabolic wastes from the body.

Question 8.
What is the color rendering substance found in urine?
Answer:
Urochrome.

Question 9.
What are diuretics?
Answer:
The substances which increase the volume of water, to be excreted as urine, are called diuretics, e.g., tea, coffee, alcoholic beverages.

Question 10.
What is osmoregulation?
Answer:
It is the maintenance of water and osmotic concentration of blood.

Question 11.
Name the organ of the excretory system, which stores urine before its removal from the body.
Answer:
Urinary bladder.

Question 12.
In which part of the nephron does filtration occur?
Answer:
Glomerulus.

Question 13.
What happens to the useful substances that get filtered into the renal tubule?
Answer:
They are reabsorbed into the blood.

Question 14.
Point out the main excretory organ?
Answer:
Kidney.

Question 15.
Write down the products excreted by the following organs.
(a) Lung
Answer:
Lung: Carbon dioxide and water vapor

(b) Skin,
Answer:
Skin: Urea, water, and some salts

(c) Intestine.
Answer:
Intestine: Some salts like calcium and iron.

Question 16.
What is excreted by the kidney in urine?
Answer:
Urea.

Question 17.
In which part of the nephron does filtration occur?
Answer:
Glomemle.

Question 18.
Who filters the blood?
Answer:
The kidney filters the blood, which takes place between the glomerulus and Bowman’s capsule.

Question 19.
Why it is necessary to remove waste products by excretion?
Answer:
It is essential and necessary because all waste products are toxic and harmful.

Excretory Products and their Elimination Important Extra Questions Short Answer Type

Question 1.
Differentiate between sweat and sebum.
Answer:

Sweat Sebum
1. It is a liquid state excretion of tin. 1. It is semisolid excretion
2. NaCl. urea, amino-acids are excreted. 2. Waxes, fatty acids, and sterol are excreted.
3. Excreted in large amounts 3. Excreted in small amounts
4. Also thermoregulatory role. 4. No thermoregulatory role.

Question 2.
What consequences will follow with the failure of tubular reab¬sorption in nephrons?
Answer:
Nephrons are the structural and functional units of each kidney. With the failure of reabsorption in nephrons, much-needed substances like glucose, amino acids, water, salts, etc. will be excreted along with urine.

The biological functioning of organs and body will be impaired, ultimately death will occur.

Question 3.
How the net filtration pressure is obtained?
Answer:
The pressure of blood in afferent arterioles is (+ mm Hg 75). This is opposed by the osmotic pressure of plasma proteins by (-) 30 mm Hg and intertubular pressure of (-) 20 mm Hg. The net filtration pressure is (+) 25 mm Hg that acts in glomerular filtration as a driving force. About 172 liters of glomerular filtrate are produced in 24 hrs. which is nearly 4-1/ 2 times the total fluid in the human body.

Question 4.
List some important functions of kidneys?
Answer:
Kidneys play a vital role as follows:
(a) It removes nitrogenous wastes from the blood.
(b) It regulates fluid balance, between intake and fluid loss.
(c) It removes drugs, penicillin, poisons, etc. from blood.
(d) It maintains acid-base (pH) balance
(e) It regulates electrolyte balance.

Question 5.
Differentiate between ureter and urethra?
Answer:

Ureter Urethra
1. It is a muscular tube. 1. It is a membranous tube.
2. It is long. 2. It is short.
3. It arises from the renal pelvis of the kidney. 3. It arises from the urinary bladder.
4. It carries urine to the urinary bladder. 4. It eliminates stored urine of the exterior.
5. No muscular splincter. 5. Muscular splinter keeps urethra-closed except for micturition.

Question 6.
How does the excretion of uric acid take place in birds and reptiles?
Answer:
In birds and reptiles, uric acid is formed mostly in the liver, transported to the kidney through blood. It is separated by renal tubules and temporarily stored in cloacae. Water is absorbed by cloacal walls, needing only a minimum amount of water for excretion. In birds, urine is eliminated in a paste-like form along with feces.

Question 7.
Name and state in brief the processes involved in the formation of urine.
Answer:
The urine is formed by the combined processes as follows:
(a) Glomerular filtration: Metabolism wastes and other substances are filtered out by glomerulus due to the generation of net filtration pressure.
(b) Re-absorption: Water and other required substances are selectively reabsorbed from the filtrate, so that urine becomes concentrated.
(c) Tubular secretion: Tubules secrete certain ions (like K + in exchange for Na + ), urea, creatinine, uric acid, ammonia, etc. This process is of more significance in marine fishes and desert amphibians than mammals.

Question 8.
Differentiate between ureotelism and Uricotelism.
Answer:

Ureotelism Uricotelism
1. The process of elimination of main urea. 1. The process of elimination of mainly uric acid.
2. Water moderately required for excretion. 2. Much less water required for excretion.
3. Synthesis of urea requires less energy expenditure. 3. Synthesis of uric acid needs more energy expenditure.

Question 9.
What is Polynephritis? What is uremia?
Answer:
It is a bacterial infection that causes inflammation of the renal pelvis, nephrons, and medullary tissues of the kidney. It affects the counter-current mechanism. Its main symptoms are frequent and painful urination, fever, and pain in the lumbar region.

A high concentration of urea, uric acid, creatinine, etc. in the blood due to some bacteria infection or some obstruction in the passage of the urinary system is called uremia.

Question 10.
Indicate whether the following statements are True or False
(a) Micturition is carried out by a reflex.
Answer:
True

(b) ADH helps in water elimination, making the urine hypotonic.
Answer:
False

(c) Protein-free fluid is filtered from blood plasma into the Bowman’s capsule
Answer:
True

(d) Henle’s loop plays an important role in concentrating the urine.
Answer:
True
(e) Glucose is actively reabsorbed in the proximal convoluted tubule.
Answer:
True

Question 11.
Match the items of Column I with these of Column II.

Column I Column-II
(a) Ammonotelism (i) Birds
(b) Bowman’s Capsule (ii) Hypertonic urine
(c) Micturition (iii) Counter-current system
(d) Uricotelism (iv) Bony fish
(e) Vasa recta (v) Urinary bladder
(f) Sebum (vi) Glucose
(g) ADH (vii) Glomerular Alteration
(h) Tubular reabsorption (viii) Skin

Column I Column-II
(a) Ammonotelism (iv) Bony fish
(b) Bowman’s Capsule (vii) Glomerular Alteration
(c) Micturition (v) Urinary bladder
(d) Uricotelism (i) Birds
(e) Vasa recta (iii) Counter-current system
(f) Sebum (viii) Skin
(g) ADH (ii) Hypertonic urine
(h) Tubular reabsorption (vi) Glucose

Question 12
Fill in the blanks with appropriate words:
(a) During micturition, the urinary bladder, and the urethral sphincters contract , and relax
(b) Flame cells and malpighian tubules are found in and Bowman’s capsule and glomerulus respectively.
(c) Blood enters the glomerulus through the renal arteriole and leaves via the afferent arteriole.
(d) Two counter-current systems formed in the kidney are the Renal medulla and the renal cortex
(e) Sweat serves to eliminate mainly water and salt

Question 13.
Compare and contrast the osmoregulatory problems and adaptations of a marine bony fish with a freshwater bony fish.
Answer:
Osmoregulation in freshwater Marine bony fish, do not drink water to reduce the need to expel excess water. In this case, water uptake and salt loss are minimized by a specialized body covering. Freshwater animals have the ability to take up salts from the environment. The active transport of ions takes place against the concentration gradient, specialized cells called monocytes or chloride cells in the gill membrane of freshwater fish. These can import Na + and CI – from the surrounding water containing less than 1 mm NaCl when their plasma concentration of NaCl exceeds 100 mm.

Osmoregulation in marine environment Seawater has an osmolarity of about 1000m Osm L The osmoregulatory problems in marine water are opposite to those in a freshwater environment. Marine bony fish have the body fluids hypotonic to seawater and thereby, they tend to lose water from the body through permeable surfaces.

To compensate for the water loss, marine bony fish drink seawater, which results in a gain of excess salts. The monocytes or chloride cells of the gill membrane of marine bony fish help to eliminate excess monovalent ions from the body fluid to the seawater. Divalent cations are generally eliminated with feces.

Question 14.
State the importance of counter-current systems in renal functioning.
Answer:
Vasa rectal is responsible for the concentration of urine. The vase rectal is in the form of loops. Therefore, the blood flows in the opposite directions in two limbs of each vasa Fecta the blood entering its descending limb comes into close contact with the outgoing blood in the ascending limb. This is called a Counter-Current System. The two limbs of the loops of Henle form another Counter-Current System.

Importance: The counter-current system significantly contributes to concentrating urine in mammals.

Question 15.
State the position and function of the juxtaglomerular apparatus?
Answer:
This is a specialized cellular apparatus located where the distal convoluted tubule passes close to the Bowman’s capsule between the afferent and efferent arterioles. JGA cells secrete substance like renin that modulates blood pressure and renal blood flow and thus, GFR is regulated.

Question 16.
Describe the hormonal feedback circuits in controlling renal functions.
Answer:
Two important hormonal control of the kidney function by negative feedback circuits can be identified:
1. Control by Antidiuretic Hormone ADH: ADH produced in the hypothalamus of the brain and released into the blood from the pituitary gland, enhances fluid retention by making the kidneys reabsorb more water. The release of ADH is triggered when osmoreceptors in the hypothalamus detect an increase in the osmolarity of the blood.

The osmoreceptors cells also promote thirst. Drinking reduces the osmolarity of the blood which inhibits the secretion of ADB, thereby completing the feedback circuit.

2. Control by Juxtaglomerular Apparatus (JGH): It operates a multihormonal Renin-Angiotensin-Aldosterone System (RAAS). JGA responds to decrease the blood pressure and release enzyme renin into the blood. In the blood, the enzyme initiates chemical reactions that convert a plasma protein called angiotensinogen to a peptide called angiotensin II which works as a hormone.

Angiotensin II increases blood pressure and stimulates the adrenal gland to release aldosterone, a hormone. This leads to an increase in blood volume and pressure completing the feed¬back circuit by supporting the release of renin.

Still another hormone, a peptide called Atrial Natriuretic Factor ANF), opposes the regulation by RAAS.

Thus, ADH, the RAAS, and ANF provide an elaborate system of checks and balance that regulate the kidney functioning to control body fluid, osmolarity, salt concentration, blood pressure, and blood volume.

Question 17.
State the normal and abnormal constituents of human urine.
Answer:
Urine is a pale yellow colored slightly acidic watery fluid.

  • Abnormal Urine: Various metabolic errors of kidney malfunctioning changes the composition of urine.
  • Proteinuria: Excess of protein level.
  • Albuminuria: The presence of albumin, usually occurs in nephritis.
  • Glycosuria: Presence of glucose in urea as in case of diabetes mellitus.
  • Ketonuria: Presence of abnormally high ketone bodies.
  • Hematuria: Presence of blood or blood cells in urine.
  • hemoglobinuria: Presence of hemoglobin in urine.
  • Uremia: Presence of excess urea.
  • Normal Urine: Normal urine is slightly heavier than water. It gives an aromatic odor due to the presence of volatile, bad-smelling organic substances, the ruined water, organic and inorganic materials are the main constituents of normal urine.

The other nitrogenous constituents of normal urine are ammonia, uric acid, hippuric acid, and creatinine.

Non-nitrogenous substances are vitamin C, oxalic acid, phenolic substances. In inorganic substances, sodium chloride is the principal mineral salt in the urine.

Question 18.
State the role of skin and lungs in excretion.
Answer:
Role of Skin: Human skin possesses glands for secreting sweat and sebum (from the sebaceous gland). Sweat contains NaCl, lactic acid, urea, amino acids, and glucose. The volume of sweat various negligible to 14 L a day. The principal function of sweat is the evaporative cooling of the body surface.

Sebum is a waxy protective secretion to keep the skin oily and this secretion eliminates some lipids, such as waxes, sterols, other hydrocarbons, and fatty acids. Integument in many animals is excreting ammonia into the surrounding by diffusion.

Role of lungs in excretion: Human lungs eliminate around 18L of CO2 per day and about 400 ml of water in normal resting conditions. Water loss via lungs is small in hot humid climates and large in cold dry climates. The rate of ventilation and ventilation pattern also affects the water loss through the lungs. Different volatile materials are also readily eliminated through the lungs.

Excretory Products and their Elimination Important Extra Questions Long Answer Type

Question 1.
Briefly state the mechanism of urine formation in the human kidney.
Answer:
Three main processes are involved in urine formation
1. Glomerular filtration: Kidneys filter the equivalent of blood volume every 4 – 5 minutes. Filtration slits are formed by the assemblages of fine cellular processes of podocytes (foot cells). The process of ultra-filtration depends upon two main factors, first the net hydrostatic pressure difference between the lumen of the capillary and the lumen of the Bowman’s capsule favor filtration.

The glomerular ultrafiltrate contains essentially all the constituents of the blood except for blood corpuscles and plasma proteins. Nearly 15% – 25% of the water and salutes are removed from the plasma that flows through the glomerulus. The glomerular filtration rate is about 125 ml min1 or about 180 L day -1 in human kidneys.

2. Two important intrinsic mechanisms provide autoregulation of glomerular filtration rate.
(a) Myogenic mechanism: Increase in blood pressure will tend to stretch the efferent arteriole, which would increase the blood flow to the glomerulus. The diameter of the arteriole is reduced, increasing the resistance to flow. This myogenic mechanism thus reduced variations inflow to the glomerulus in case of fluctuations in blood pressure.

(b) Juxtaglomerular apparatus (JGA): This specialized cellular apparatus is located where the distal convoluted tubule passes close to the Bowman’s capsule between the afferent and efferent arterioles. JGA cells secrete substances like renin that modulate blood pressure and renal blood flow and GFR are regulated.

Myogenic and juxtaglomerular mechanisms work together to autoregulate the GFR over a wide range of blood pressure. In addition to these extrinsic neural control also regulates the filtration rate.

3. Tubular re-absorption: The selective transport of substances across the epithelium of the excretory tubule from the ultrafiltrate to the interstitial fluid is called re-absorption. Nearly all the sugar, vitamins, organic substances (nutrients), and most of the water are reabsorbed.

4. Tubular secretion: It is a very selective process involving both passive and active transport. The filtrate travel through the nephron, substances that are transported across the epithelium from the surrounding interstitial fluid and join it. The net effect of renal secretion is the addition of plasma solutes to the filtrate within the tubule.

Question 2.
Explain the following:
(a) Skin functions as an accessory excretory organ.
Answer:
The skin retains some excretory role in many animals. Human skin possesses two glands for secreting fluid on its surface. These are sweat from sweat glands and sebum from sebaceous glands.

(b) Mammals can eliminate hypotonic and hypertonic urine according to body needs.
Answer:
When the animal takes a large quantity of water the kidneys excrete a very high amount of hypotonic urine. At the same time when the animal takes a small number of water kidneys to excrete a very high amount of hypertonic urine.

At the same time when the animal takes a small number of water kidneys to excrete a small amount of hypertonic urine, as kidneys need to conserve water. In this way, the osmotic concentration of blood is maintained by the kidneys. This flexibility of kidney nephrons is highly observed in mammals.

Hypotonic urine removes excess water from the body in order to raise the osmotic concentration of the blood to normal. Excess of water in body fluids generally lowers the osmotic pressure of blood and increases the volume of blood. This increase in the volume of blood raises the blood pressure and hydrostatic pressure which increases the rate of ultrafiltration. In this way, a large amount of hypotonic urine is produced in order to bring the volume of fluids to normal.

(c) Micturition is a reflex process but is under some voluntary control.
Answer:
It is the process of passing out urine. Nephrons produce urine and drain. When enough urine collects in the bladder the distension of its walls raises enough pressure which generates a spontaneous nervous activity under the stimulation of the sympathetic and parasympathetic nervous system. This nervous stimulation causes the smooth muscles on the urinary bladder to rise too high to control.

Similarly, micturition can voluntarily be initiated even before enough urine has accumulated in the bladder. Backflow of the urine into the ureters from the urinary bladder is prevented because the terminal part of each ureter passes through the bladder and gets closed as soon as the contraction of the bladder occurs.

(d) Mammals are ureotelic, but birds are uricotelic.
Answer:
Mammals are ureotelic animals as they eliminate nitrogen mainly urea. It is very soluble in water and needs a considerable amount of water for its elimination. Mammals can thus form hypertonic urine which they excrete. While the birds cannot excrete urine as hypertonic since nitrogen occurs mainly in the form of uric acid. The uric acid is insoluble in water and does not require much water for its elimination.

Question 3.
Describe the functional anatomy of a human nephron.
Answer:
Nephrons are structural and functional units of each kidney to form the urine. Each nephron is fine microscopic highly coiled tubular structure differentiated into malpighian body and the renal tubule. The malpighian body comprises a large double-walled cup-shaped structure the Bowman’s capsule present in the renal cortex. It is lined by thin, semipermeable epithelial cells, the podocytes. Bowman’s capsule receives the blood supply through a branch of the renal artery.

The afferent arteriole forms a fine capillary network in the form of glomerules with high hydrostatic pressure. The lumen between two layers of Bowman’s capsule is continuous with the lumen of the tubule. The Bowman’s capsule and the glomerulus together form a globular body, the Malpighian body or the renal capsules.

The capillaries forming the glomerulus at the exit of Bowman’s capsule unite to form a narrow efferent arteriole which breaks up into a peritubular network of capillaries with low hydrostatic pressure.

The renal tubule is a long highly coiled tubular structure differentiated into proximal convoluted tubule (PCT) Henle’s loop, distal convoluted tubule (DCT). The U-shaped loop-like structure, descending and ascending from the renal tubule is called Henle’s loop.

Collecting tubules of several nephrons open into a wider duct called the collecting duct. A number of collecting ducts unite with each other in the medulla to form the ducts of Bellini, which drains down the urine into the ureter from each kidney to be stored in the urinary bladder.

The efferent arteriole emerges out from the glomerules breaks up into a peritubular capillary network around the renal tubule in the cortex. These capillaries also form a thin-walled, straight capillary the vasa recta. The vasa recta help in retaining the reabsorbed ions and urea in medullary interstitial fluid to maintain high osmotic pressure in kidneys.

Glomerular filtrate undergoes tubular reabsorption and tubular secretion for the formation of urine. (See diagram opposite page)

Uriniferous tubules Or nephron of the kidney

Question 4.
Describe the gross anatomical features of the human kidney with a suitable diagram.
Answer:
Kidney: Kidney is chocolate brown, bean-shaped, large-sized about 10 cm long and 5 – 7 cm broad, 3 – 4cm thick flattened, metamorphic. The weight of each kidney is 150 to 170 gm. They are situated against the back wall of the abdominal cavity, just below the diaphragm, between the 12th thoracic and 3rd lumbar vertebrae.

The outer margin is convex. The inner concave presents a longitudinal opening called the hilum. The renal artery and renal vein respectively enter and leave the kidney through its hilum.

The two kidneys are slightly asymmetrical in position because the right kidney is slightly at a lower level than the left. Kidneys are held in position by a mass of adipose tissue called Renal fat. These rest against the abdominal muscles. Each kidney is covered on the ventral side by the peritoneum and is thus retroperitoneal in nature.

Surrounding the kidneys and the renal fat is a sheath of fibro elastic tissue known as renal fascia or capsule. They protect the kidney. The renal fat forms a shock-absorbing cushion. The renal fascia fixes the kidney to the abdominal wall.

Longitudinal section (Diagrammatic of Kidney)

Question 5.
(a)What is the role of the liver in excretion in mammals?
Answer:
Role of liver in excretion: The liver changes ammonia into urea which is less toxic than ammonia. Urea is eliminated from the body by the kidneys through urine.

The liver is the principal organ of excretion of cholesterol, bile pigments (bilirubin and biliverdin) some vitamins, drugs, and inactivated products of steroid hormones. The liver excretes these substances in the bile which carries them to the small intestine. Ultimately, these substances get eliminated along with feces.

(b) What are the diseases associated with the urinary system?
Answer:
Diseases associated with the urinary system:
1. Polynephritis: It is a bacterial infection, which causes inflammation of renal pelvic nephrons and medullary tissues of the kidney. It affects the counter-current mechanism. Its main symptoms are frequent and painful urination, fever, and pain in the lumbar region.

2. Uremia: It causes the presence of a high concentration of urea, uric acid, creatinine, etc, in the blood due to some bacterial infection or some obstruction in the passage of the urinary system. Urea poisons the cells. It is not passed in the urine and accumulates in the blood.

3. Renal stones: When uric acid precipitates and accumulates in the nephrons of kidneys in the form of renal stones or when calcium phosphates and oxalates accumulate in the nephrons of the kidneys in the form of renal stones. It causes blockage or frequent painful urination along with blood in the urine. Renal stone causes severe colic pain starting in the back and radiating down to the front of the thigh or vulva or testicle on that side.

4. Glomerulonephritis: It is characterized by the inflammation of Glomeruliduct, some injury to the kidney, abnormal allergic reaction, or by some streptococci bacteria infection. Proteins and red blood corpuscles become filtered into the glomerular filtrate. It may lead to kidney failure in severe infection.

5. Oedema: It is characterized by the increased volume of interstitial fluid mainly caused by retention of excess Na+ ions which in turn causes water retention. Blood pressure increases dining edema.

Question 6.
Write a short account on hemodialysis.
Answer:
In case of renal failure, an artificial kidney is used for removing excess urea from the blood of the patient by a process called hemodialysis. Blood is taken out from the artery of the patient, cooled to 0°C, mixed with an anticoagulant such as heparin, and then pumped into the apparatus called artificial kidney. In this apparatus, blood flows through channels

Working of artificial kidneys for hemodialysis

bounded by cellophane membrane. The membrane is impermeable to macromolecules but permeable to small solutes. The membrane separates the blood flowing inside the channels from a dialyzing fluid flowing outside the membrane. The wastes like urea, uric acid, and creatinine diffuse from the blood to the dialyzing fluid across the cellophane membrane.

Thus the blood is considerably cleared of nitrogenous waste products without losing plasma proteins. Such a processor separation of macromolecules from small solute particles with the help of a permeable membrane is called dialysis. The blood coming out of the artificial kidney is warmed to body temperature, mixed with an Antiheparin to restore its normal coagulability, and returned to a vein of the patient.


Mono- and Divalent Electrolyte Patterns, pCO2 and pH in Relation to Flow Rate in Normal Human Parotid Saliva *

***Max-Planck-Institute for Biophysics, Frankfurt a.M.

***Department of Internal Medicine, Freiburg i.Br.

Department of Internal Medicine, University of Würzburg

Department of Internal Medicine, University of Würzburg

Medizinische Universitätsklinik D-8700 Würzburg Federal Republic of Germany.Search for more papers by this author

Department of Internal Medicine, University of Würzburg

**Department of Otolaryngology, University of Würzburg

***Max-Planck-Institute for Biophysics, Frankfurt a.M.

***Department of Internal Medicine, Freiburg i.Br.

**Department of Otolaryngology, Univ. of Erlangen-Nürnberg.

*Supported by the Deutsche Forschungsgemeinsehaft, Bad Godesberg

Abstract

Abstract In parotid saliva of normal subjects the pH, pCO2 and concentrations of sodium, potassium, calcium, magnesium, bicarbonate and inorganic phosphate were determined continuously after stimulation of salivary secretion by pilocarpine. The electrolyte concentrations showed a marked dependence on salivary flow rate. Sodium, calcium and bicarbonate concentrations and pH increased with increasing flow rate but the concentrations of potassium, magnesium and inorganic phosphate decreased with increasing flow rate. In general salivary electrolyte concentrations showed a tendency to approach plasma concentrations with increasing flow rate with the exception of the salivary magnesium concentration, which fell below its plasma level and bicarbonate which exceeded the plasma concentration. The results will be considered as a basis of further investigations on electrolyte excretion patterns in patients with hormonal and metabolic disturbances.


Characteristics Of Osmoconformers

Osmoconformers are well adapted to seawater environments and cannot tolerate freshwater habitats. The organisms have permeable bodies which facilitate the in and out movement of water and, therefore, do not have to ingest surrounding water. Osmoconformers such as sharks hold high concentrations of waste chemicals in their bodies such as urea to create the diffusion gradient necessary to absorb water. Sharks remain one of the most adapted creatures to their habitat due to such mechanisms. However, Osmoconformers are not ionoconformers, meaning that they have different ions than those in seawater. This factor enables important biological processes to occur in their bodies. The organisms have adapted to their saline habitats by utilizing the ions in the surrounding habitat. Sodium ions for example, when paired with the potassium ions in the organisms’ bodies, aids in neuronal signaling and muscle contraction. Some osmoconformers are also classified as stenohaline, which means that they are unable to adapt to a huge variation in water salinity. The word stenohaline is broken down into steno to mean narrow and haline which translates to salt. If a stenohaline organism is transferred to an environment less or more concentrated than marine water, its cell membranes and organelles end up getting damaged. A euryhaline on the other hand thrives in variations of salinity by use of a variety of adaptations.


Excretion of monovalent and divalent ions in sharks - Biology

Unit Six. Animal Life

26. Maintaining the Internal Environment

Although the same basic design has been retained in all vertebrate kidneys, there have been a few modifications. Because the original glomerular filtrate is isotonic to blood, all vertebrates can produce a urine that is isotonic to (by reabsorbing ions) or hypotonic to (more dilute than) blood. Only birds and mammals can reabsorb water from their glomerular filtrate to produce a urine that is hypertonic to (more concentrated than) blood.

Kidneys are thought to have evolved first among the freshwater teleosts, or bony fish. Because the body fluids of a freshwater fish have a greater osmotic concentration than the surrounding water, these animals face two serious problems because of osmosis and diffusion: (1) Water tends to enter the body from the environment, and (2) solutes tend to leave the body and enter the environment. Freshwater fish address the first problem by not drinking water (water enters the mouth but passes out through the gills—it is not swallowed) and by excreting a large volume of dilute urine, which is hypotonic to their body fluids (as shown in the freshwater fish above). They address the second problem by reabsorbing ions (NaCl) across the nephron tubules, from the glomerular filtrate back into the blood. In addition, they actively transport ions (NaCl) across their gills from the surrounding water into the blood.

Although most groups of animals seem to have evolved first in the sea, marine bony fish (teleosts) probably evolved from freshwater ancestors. They faced significant new problems in making the transition to the sea because their body fluids are hypotonic to the surrounding seawater. Consequently, water tends to leave their bodies by osmosis across their gills, and they also lose water in their urine. To compensate for this continuous water loss, marine fish drink large amounts of seawater.

Many of the divalent cations in the seawater that a marine fish drinks (principally Ca ++ and Mg ++ in the form of MgSO4) remain in the digestive tract and are eliminated through the anus. Some, however, are absorbed into the blood, as are the monovalent ions K + , Na + , and Cl - . Most of the monovalent ions are actively transported out of the blood across the gills, while the divalent ions that enter the blood (represented by MgSO4 in figure) are secreted into the nephron tubules and excreted in the urine. In these two ways, marine bony fish eliminate the ions they get from the seawater they drink. The urine they excrete is isotonic to their body fluids. It is more concentrated than the urine of freshwater fish but not as concentrated as that of birds and mammals.

The elasmobranchs—sharks, skates, and rays like the one in the photo—are by far the most common subclass in the class Chondrichthyes (cartilaginous fish). Elasmobranchs have solved the osmotic problem posed by their seawater environment in a different way than have the bony fish. Instead of having body fluids that are hypotonic to seawater, so that they have to continuously drink seawater and actively pump out ions, the elasmobranchs reabsorb urea from the nephron tubules and maintain a blood urea concentration that is 100 times higher than that of mammals. This added urea makes their blood approximately isotonic to the surrounding sea. Because there is no net water movement between isotonic solutions, water loss is prevented. Hence, these fish do not need to drink seawater for osmotic balance, and their kidneys and gills do not have to remove large amounts of ions from their bodies. The enzymes and tissues of the cartilaginous fish have evolved to tolerate the high urea concentrations.

The first terrestrial vertebrates were the amphibians (pictured at the top of figure 26.7), and the amphibian kidney is identical to that of freshwater fish. This is not surprising because amphibians spend a significant portion of their time in freshwater, and when on land, they generally stay in wet places. Like their freshwater ancestors, amphibians produce a very dilute urine and they compensate for their loss of Na+ by actively transporting Na+ across their skin from the surrounding water.

Figure 26.7. Osmoregulation by some vertebrates.

Only birds and mammals can produce a hypertonic urine and thereby retain water efficiently, but marine reptiles and birds can drink seawater and excrete the excess salt through salt glands.

Reptiles, on the other hand, live in diverse habitats. Those living mainly in freshwater, like some of the crocodilians, occupy a habitat in many ways similar to that of the freshwater fish and amphibians, and thus have similar kidneys. Marine reptiles, which consist of other crocodilians, turtles (the second entry in figure 26.7), sea snakes, and one lizard, possess kidneys similar to those of their freshwater relatives but face opposite problems they tend to lose water and take in salts. Like marine teleosts (bony fish), they drink the seawater and excrete an isotonic urine. Marine teleosts eliminate the excess salt by transport across their gills, while marine reptiles eliminate excess salt through salt glands near the nose or eye.

The kidneys of terrestrial reptiles also reabsorb much of the salt and water in the nephron tubules, helping somewhat to conserve blood volume in dry environments. Like fish and amphibians, they cannot produce urine that is more concentrated than the blood plasma. However, when their urine enters their cloaca (the common exit of the digestive and urinary tracts), additional water can be reabsorbed.

Mammals and birds are the only vertebrates able to produce urine with a higher osmotic concentration than their body fluids. This allows these vertebrates to excrete their waste products in a small volume of water, so that more water can be retained in the body. Human kidneys can produce urine that is as much as 4.2 times as concentrated as blood plasma, but the kidneys of some other mammals are even more efficient at conserving water. For example, camels, gerbils, and pocket mice, Perogna- thus, can excrete urine 8, 14, and 22 times as concentrated as their blood plasma, respectively. The kidneys of the kangaroo rat shown in figure 26.8 are so efficient it never has to drink water it can obtain all the water it needs from its food and from water produced in aerobic cellular respiration!

Figure 26.8. A desert mammal.

The kangaroo rat (Dipodomys panamintensis) has very efficient kidneys that can concentrate urine to a high degree by reabsorbing water, thereby minimizing water loss from the body. This feature is extremely important to the kangaroo rat's survival in dry or desert habitats.

The production of hypertonic urine is accomplished by the looped portion of the nephron, found only in mammals and birds. A nephron with a long loop, called the loop of Henle, extends deeper into the tissue of the kidney and can produce more concentrated urine. Most mammals have some nephrons with short loops and other nephrons with loops that are much longer. Birds, however, have relatively few or no nephrons with long loops, so they cannot produce urine that is as concentrated as that of mammals. At most, they can only reabsorb enough water to produce a urine that is about twice the concentration of their blood. Marine birds solve the problem of water loss by drinking seawater and then excreting the excess salt from salt glands near the eyes, which dribbles down the beak as shown in figure 26.9.

Figure 26.9. Marine birds drink seawater and then excrete the salt through salt glands.

The moderately hypertonic urine of a bird is delivered to its cloaca, along with the fecal material from its digestive tract. If needed, additional water can be absorbed across the wall of the cloaca to produce a semisolid white paste or pellet, which is excreted.

Key Learning Outcome 26.3. The kidneys of freshwater fish must excrete copious amounts of very dilute urine, whereas marine teleosts drink seawater and excrete an isotonic urine. The basic design and function of the nephron of freshwater fish have been retained in the terrestrial vertebrates. Modifications, particularly the loop of Henle, allow mammals and birds to reabsorb water and produce a hypertonic urine.

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RESULTS

Specificity of anti-SKCaR antiserum. Chromogenic reaction product from bound antibody was present in dogfish kidney sections exposed to immune anti-SKCaR antiserum but not preimmune serum (Fig. 2, A and B). Anti-SKCaR antiserum labeled the membranes of epithelial cells and renal tubule cells of distinct nephron subsegments as well as the cytoplasm of selected interstitial cells. By contrast, glomeruli displayed no immunoreactivity. The pattern of anti-SKCaR immunoreactivity in all nephron subsegments was independent of either tissue fixation or preparation of cryosections, paraffin sections, and thin sections of LR-White blocks. For each of the nephron segments described below, labeling by immune anti-SKCaR antiserum was present, whereas no chromogenic product was present after exposure to corresponding preimmune serum. A summary of labeling by anti-SKCaR antibody is provided in Table 1.

Table 1. Immunoreactivity of SKCaR in the kidney of Squalus acanthias

For the nomenclature of the nephron portions in spiny dogfish, see Ref. 12. −, No reaction (+), faint reaction + to ++++, weak to very strong reaction.

When immunoblots containing crude membranes isolated from dogfish kidney were probed with affinity-purified anti-SKCaR antibody, prominent bands of 240, 140, and 91 kDa were present (Fig. 2C). These bands were completely ablated after preincubation with an excess of competing peptide. These bands display molecular masses similar to those reported previously for SKCaR protein expressed in human embryonic kidney cells (29) as well as anti-CaR-reactive proteins present in a variety of mammalian tissues (33, 34).

Anti-SKCaR labeling of specific nephron subsegments. Anti-SKCaR antibody labeled specific nephron subsegments in both mesial tissue and lateral bundle zones of dogfish kidney (Fig. 3). The PIa segment in the lateral bundle zone displayed apical SKCaR staining within the region of its brush border (Fig. 3 and see Fig. 6). The PIb segment, which displays multiple bands within mesial tissue near the glomeruli, showed SKCaR-specific staining only at the base of the microvilli of a few cells (Fig. 3).

Fig. 3.Part of a cross section through the kidney. Mesial tissue displays large profiles of PIIa with luminal brush border, small profiles of PIIb with luminal brush border and cilia of multiciliary cells, and small profiles of late distal tubules (LDT). The small profiles (PIIb and LDT) show immunoreactivity (brownish red) at the luminal side (apex of epithelial cells). A large glomerulus (GL) exhibits close contact between the collecting tubule (CT) and afferent arteriole (AA). The CT at this vascular field of the glomerulus was labeled by chromogenic reaction. In the lateral bundle zone (left), several early distal tubules (EDT) show immunostaining. PIa, CT, and collecting ducts (CD) in the bundle zone show marked immunostaining of the apical cell region. Intermediate segment (IS) and central vessel (CV) are only stained by counterstain methyl green.

Fig. 6.Cross section through a countercurrent bundle. The bundle is sectioned near the tip, where the CD leaves and a small CT is entering, coming from a neighbouring bundle (see also Ref. 25). The apical cell membrane of proximal tubule PIa cells (first hairpin loop) is labeled with immunostain (brownish red). Strong binding occurs at the apex of CT and CD cells. EDT of this profile reacts with antibody along the “intracellular striations,” which represent amplifications of the basolateral cell membrane. IS, bundle vein (BV), CV, and bundle artery (BA) appear negative for SKCaR.

The PIIa and PIIb segments of the proximal tubule present in mesial tissue displayed markedly different patterns of anti-SKCaR staining characteristics (Figs. 3, 4, 5): PIIa cells exhibited no SKCaR staining. In contrast, PIIb cells of all 14 animals studied displayed specific SKCaR immunoreactivity that was observed at their apical membranes (brush border). Interestingly, the intensity of SKCaR labeling of PIIb apical membrane varied greatly among various individual animals studied despite the fact that consistent SKCaR labeling was observed in most other shark nephron segments (see below).

Fig. 4.Cross sections of segments in mesial tissue. Immunostaining of LTD reveals distinct binding sites of antiserum against SKCaR at the apical cell membrane (red). A faint staining can be seen at the basolateral cell membrane forming “intracellular striations.” PIIa shows no reaction. SC, sinus capillaries of the renal portal system.

Fig. 5.Section through mesial tissue in the vicinity of glomeruli. Proximal tubule segment PIb with distinct brush border and the 2 portions of the second proximal tubule, PIIa and PIIb, are shown. These segments are in close contact with the LDT. In this animal, pronounced staining (red) for SKCaR is confined to the LDT and a few portions of the brush border of PIIb (arrow).

The EDT, which is present exclusively in the lateral bundle zone, is contiguous with the LDT, which thereafter performs numerous bands in mesial tissue. EDT cells were diffusely labeled by anti-SKCaR antiserum (Figs. 3 and 6). The LDT is present in mesial tissue, where it courses along the pathway of PIIa tubules and is frequently in close proximity to both PIIa and PIIb cells (Figs. 3, 4, 5). LDT cells in all animals examined showed sharply defined SKCaR staining at their apical cell membranes. In addition, only very weak immunostaining was observed at the LDT basolateral membrane in two animals (Fig. 4).

Electron microscopy of LDT cells revealed that they possess short, stubby microvilli with a marked asymmetry of the apical cell membrane, where the extracellular (luminally facing) side was thickened and had a fuzzy coat (glycocalyx). Immunoelectron microscopy of the apical region of LDT cells showed that anti-SKCaR immunoreactivity protein was 1) in the immediate vicinity of the cell membrane 2) in the apical cytoplasmic region, presumably at apical vesicles 3) associated with membrane-bound granules located in close proximity to the apical membrane and 4) outside the cell in the glycocalyx (Fig. 7).

Fig. 7.Electron micrograph of thin section with immunogold staining of apical region of the LDT. Numerous gold particles (10 nm) are present at the cell membrane, in the fuzzy coat, at a large granule in close proximity to the apical membrane (arrow), and at small apical vesicles, indicating a large amount of SKCaR antigen.

Significant anti-SKCaR immunoreactivity was observed in the CT as well as in the CD (Figs. 3 and 6). The CT at the vascular field of the glomerulus was labeled by the chromogenic reaction. In CT and CD cells, SKCaR antibody binding was confined to the region of the apical cell membrane and its adjacent cytoplasmic zone, where membrane-bound granules abound.

SKCaR staining was also observed in the cytoplasm of small, round cells with large spherical nuclei that were arranged in islets in the interstitium of the lateral bundle zone (Fig. 3). These cells belong to the renal lymphomyeloid tissue that is involved in hematopoiesis of elasmobranch fish (22). Although the function of these cells is presently unknown, they may correspond to hematopoietic cells that possess CaR proteins in mammals (15).

In summary, we consistently found SKCaR labeling in nephron segments PIa, PIb, PIIb, EDT, the apical membrane of the LDT, the CT/CD system, and in a subpopulation of cells of the interstitial tissue. However, we observed that SKCaR reactivity was less pronounced in PIIb of four animals, and with the exception of two animals, the basolateral membrane of LDT was not labeled.


RESULTS

Absolute flux rates of egg masses of similar age varied from batch to batch even under similar conditions. The reason for this variation is unknown but underscores the importance of performing control and treatment experiments simultaneously and on egg masses from the same batch as was done in the present and previous study (Ebanks et al., 2010). Results from our multi-pronged pharmacological approach indicated that voltage-dependent Ca 2+ channels, H + -pump activity and Na + /H + antiport function in conjunction with CA activity allow the post-metamorphic embryonic snail to complete calcification and shell formation in freshwater. The two voltage-dependent Ca 2+ channel blockers had different effectiveness against net Ca 2+ uptake with nifedipine being the more effective blocker when examined by net Ca 2+ flux measurements and SIET techniques (Figs 1 and 2). Verapamil did not significantly block net Ca 2+ transport at 10 μmol l –1 but 10 μmol l –1 nifedipine did in whole egg mass experiments (Fig. 1). Scanning Ca 2+ -selective microelectrode analysis showed that both voltage-dependent Ca 2+ -channel blockers reduced Ca 2+ transport at 10 μmol l –1 but that 100 μmol l –1 verapamil completely blocked net Ca 2+ uptake (Fig. 2). Neither drug influenced net titratable alkalinity flux (data not shown). Lanthanum blocked net Ca 2+ uptake at 100 μmol l –1 but not at 10 μmol l –1 in whole egg mass experiments (Fig. 3). There was also a significant increase in apparent net titratable alkalinity uptake/acid extrusion in the presence of 100 μmol La 3+ (data not shown). Microelectrode experiments revealed a tendency for reduced Ca 2+ uptake with 100 μmol l –1 La 3+ but just below statistical significance (Fig. 4).

Using 100 μmol l –1 ETOX to evaluate the role of CA-catalyzed hydration of CO2 on whole egg mass flux rates, we observed a significant reduction in both net Ca 2+ uptake and apparent titratable alkalinity uptake/acid extrusion (Fig. 5). This link between CO2 hydration and Ca 2+ uptake led to the testing of the possible role of the apical H + pump and Na + /H + exchange by evaluating the inhibitory effects of bafilomycin and EIPA at concentrations of 1 μmol l –1 and 100 μmol l –1 , respectively, on Ca 2+ and titratable alkalinity transport. Proton-pump inhibition resulted in inhibition of net Ca 2+ uptake (Fig. 6A) and apparent net titratable alkalinity uptake (equivalent to reduced acid secretion Fig. 6B). Scanning Ca 2+ -selective microelectrode measurements revealed that bafilomycin had a significant inhibitory effect of up to 80% on Ca 2+ transport for post-metamorphic egg masses (data not shown).

Net Ca 2+ flux rate (μmol g –1 h –1 ) under control conditions (0.1% DMSO) and during pharmacological manipulations (10 μmol l –1 verapamil or 10 μmol l –1 nifedipine). Values are means ± s.e.m. Numbers in parentheses are the numbers of egg masses examined. Ambient [Ca 2+ ]=648±7 μmol l –1 . *, significantly different from control (P<0.05).

Net Ca 2+ flux rate (μmol g –1 h –1 ) under control conditions (0.1% DMSO) and during pharmacological manipulations (10 μmol l –1 verapamil or 10 μmol l –1 nifedipine). Values are means ± s.e.m. Numbers in parentheses are the numbers of egg masses examined. Ambient [Ca 2+ ]=648±7 μmol l –1 . *, significantly different from control (P<0.05).

Results of scanning Ca 2+ -selective microelectrode analysis of day-9 isolated eggs in control conditions (0.1% DMSO) and during treatment with a voltage-dependent Ca 2+ channel blocker (nifedipine or verapamil). Values are means ± s.e.m. Numbers in parentheses are the numbers of eggs scanned five to seven measurements for each egg. *, significant reduction from control values. Ambient [Ca 2+ ]=400 μmol l –1 .

Results of scanning Ca 2+ -selective microelectrode analysis of day-9 isolated eggs in control conditions (0.1% DMSO) and during treatment with a voltage-dependent Ca 2+ channel blocker (nifedipine or verapamil). Values are means ± s.e.m. Numbers in parentheses are the numbers of eggs scanned five to seven measurements for each egg. *, significant reduction from control values. Ambient [Ca 2+ ]=400 μmol l –1 .

Furthermore, and in agreement with whole egg mass fluxes, Ca 2+ uptake was inhibited in isolated eggs containing later-stage day-11 to -13 embryos, treated with 1 μmol l –1 bafilomycin (Fig. 7A) and H + gradients were effectively reversed (Fig. 7B) as measured with ion-selective microelectrodes in the unstirred boundary layer. Similar to the bafilomycin treatment, both Ca 2+ (Fig. 8A) and titratable alkalinity (Fig. 8B) net flux were reversed in whole egg mass fluxes where flux medium was treated with 100 μmol l –1 EIPA.


Excretion of monovalent and divalent ions in sharks - Biology

Web research seems to indicate that most fish do drink plenty of water, some continuously. Some fish absorb water through their skin and/or gills, and may excrete water that way, too. Apparently, fish need to maintain a fairly high concentration of salt, so how a fish deals with this need depends on whether it is a saltwater or freshwater fish. Most saltwater fish get enough or too much salt, and so preventing water loss is their concern. Freshwater fish are short on salt, and since water dilutes sodium, they have many mechanisms for excreting water. It is interesting to note that kidneys process water for excretion, so some saltwater fish have dysfunctional kidneys or are missing kidneys to help prevent water loss. The four "water/salt" strategies are listed below:

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Admin Note: The problem with fish vs. watery environment has do with the process of osmosis , defined as 'net movement of water molecules through a semipermeable membrane from a dilute solution to a more concentrated solution. The 'membrane' is the 'skin' or any other part of the fish that separates it from the water. If the concentration of salts and other solutes in the fish is greater than the outside, watery world, the fish takes on water (natural attempt to 'dilute the fish to what it's like outside' - bad for the fish). In scientific terms the fish is hypertonic to its surroundings (freshwater fish, e.g.). If a fish is hypotonic to its environment (marine fish), then the seawater has a higher solute concentration than the fish the fish loses water to its surroundings (also bad for the fish). Kidneys function to overcome the effects of osmosis so fishX can stay either more or less concentrated than its environment.
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1. Agnathans - hagfishes

2. Elasmobranchs - sharks, skates and rays

3. Teleosts - freshwater fish

4. Marine teleosts

(Thanks to E. Lund - Rummler-Brache Corp. - Investigator - For providing this info)


Watch the video: Excretion part 1 (December 2021).