# What percentage of human capillaries are located in the skin?

As in the subject line, what percentage of capillaries in a human (expressed in terms of total length, I suppose) are located in the skin, as opposed to internal organs? Google scholar has not turned up anything obvious, but I'm an outsider so I may have missed the appropriate term.

Another followup question would be: if we're considering the amount of energy expended by the heart in pumping blood throughout the body, what's the percentage of that energy that is devoted only to (1) transporting blood to the skin and (ii) circulating blood through the skin via capillaries?

The first part of your second question permits a rough calculation.

The second question ( i ): For humans the skin has been defined as ending at the hypoderm, which is 2-3mm deep. The average human skin has been estimated to cover an area of 1.5-2 m\$^2.\$ We can use 2.5 mm and 1.7 m\$^2\$, respectively. This gives a volume of 0.00425 m\$^3\$ or 4250 mL in fluid volume. The information for this calculation is at the Wiki site on human skin.

One estimate of the average total volume of fluid in a human body is 40L. Of that, 5L is blood. So roughly 1/8 of the body's fluid is blood. To a good approximation, the body is about 60% water by weight. This is presumably a little less than the percentage of fluids generally but since these are rough numbers we can use it. So 60% of the skin is fluid/water, and 1/8\$th\$ of this is blood.

Of the 4.250 L of skin, 60% is fluid, of which 1/8 \$^{th}\$ is blood, so \$(4.25) cdot (0.60) cdot frac{1}{8} = 0.3188\$ liters of blood at any given moment in the skin, subject to considerable variation no doubt. This sounds a little low but the definition of skin sort of forces this--it is only skin-deep.

One source gives an estimate of the energy expended to pump blood in a normal adult heart as 0.5-1.0 Joule per beat which at 70 beats/minute amounts to 100.8 kJ/day using 1 Joule.

So the heart is handling about 5 L of blood of which 0.3188 L is blood in skin, so very roughly we could estimate that \$0.3188/5 = 0.0638\$ or 6.38% of the work above goes toward supplying skin, i.e. 6.43 kJ/day of energy.

As a sanity check, if someone burns 1200 [nutritional] calories a day, which is about 5 MJ, we can see that keeping skin supplied with blood represents a small fraction of daily energy expenditure.

The title question: Like you, I found no study on point but that does not mean one does not exist. I can't see why anyone would do/fund such a study as the answer is unlikely to inform further work, answer any current questions, or solve any problems.

The last part of your second question (ii) Transporting blood to the capillaries and circulating the blood through capillaries are aspects of the same job. Dividing the work of the heart into skin/non-skin was already artificial. To further divide the work into segments this way requires some motivation because any answer would be a bad approximation.

## What percentage of human capillaries are located in the skin? - Biology

Skin is the soft outer tissue which covers vertebrates. In humans, it is the body’s largest organ, covering a total area of about 20 square feet. It protects our internal organs from the environment using a multi-layered system of cushioning, a cellular barrier, and protective oils.

Skin is more than just a protective barrier between our insides and the environment – it also plays an active role in maintaining our health, such as regulating body temperature by sweating and flushing when we’re hot, and raising goosebumps when we’re cold. It can also produce Vitamin D, which is important for the health of our bones, from sunlight.

Skin can vary greatly between species, and even between individual people! Here we will discuss the structure of human skin, the proper care of different skin types found among humans, and functions of skin throughout the animal kingdom.

We’ll even talk about some functions our own skin performs that we may not know about!

## Nails

Figure 2. The parts of a finger nail

The fingernail is an important structure made of keratin. The fingernail generally serve two purposes. It serves as a protective plate and enhances sensation of the fingertip. The protection function of the fingernail is commonly known, but the sensation function is equally important. The fingertip has many nerve endings in it allowing us to receive volumes of information about objects we touch. The nail acts as a counterforce to the fingertip providing even more sensory input when an object is touched.

### Nail Structure

The structure we know of as the nail is divided into six specific parts: the root, nail bed, nail plate, eponychium (cuticle), perionychium, and hyponychium.

Root The root of the fingernail is also known as the germinal matrix. This portion of the nail is actually beneath the skin behind the fingernail and extends several millimeters into the finger. The fingernail root produces most of the volume of the nail and the nail bed. This portion of the nail does not have any melanocytes, or melanin producing cells. The edge of the germinal matrix is seen as a white, crescent shaped structure called the lunula.

Nail Bed The nail bed is part of the nail matrix called the sterile matrix. It extends from the edge of the germinal matrix, or lunula, to the hyponychium. The nail bed contains the blood vessels, nerves, and melanocytes, or melanin-producing cells. As the nail is produced by the root, it streams down along the nail bed, which adds material to the undersurface of the nail making it thicker. It is important for normal nail growth that the nail bed be smooth. If it is not, the nail may split or develop grooves that can be cosmetically unappealing.

Nail Plate The nail plate is the actual fingernail, made of translucent keratin. The pink appearance of the nail comes from the blood vessels underneath the nail. The underneath surface of the nail plate has grooves along the length of the nail that help anchor it to the nail bed.

Eponychium The cuticle of the fingernail is also called the eponychium. The cuticle is situated between the skin of the finger and the nail plate fusing these structures together and providing a waterproof barrier.

Perionychium The perioncyhium is the skin that overlies the nail plate on its sides. It is also known as the paronychial edge. The perionychium is the site of hangnails, ingrown nails, and an infection of the skin called paronychia.

Hyponychium The hyponychium is the area between the nail plate and the fingertip. It is the junction between the free edge of the nail and the skin of the fingertip, also providing a waterproof barrier.

## Results

### Origin of Prevalent Claims in the Literature on the Number of Bacterial Cells in Humans

Microbes are found throughout the human body, mainly on the external and internal surfaces, including the gastrointestinal tract, skin, saliva, oral mucosa, and conjunctiva. Bacteria overwhelmingly outnumber eukaryotes and archaea in the human microbiome by 2𠄳 orders of magnitude [7,8]. We therefore sometimes operationally refer to the microbial cells in the human body as bacteria. The diversity in locations where microbes reside in the body makes estimating their overall number daunting. Yet, once their quantitative distribution shows the dominance of the colon as discussed below, the problem becomes much simpler. The vast majority of the bacteria reside in the colon, with previous estimates of about 10 14 bacteria [2], followed by the skin, which is estimated to harbor

As we showed recently [4], all papers regarding the number of bacteria in the human gastrointestinal tract that gave reference to the value stated could be traced to a single back-of-the-envelope estimate [3]. That order of magnitude estimate was made by assuming 10 11 bacteria per gram of gut content and multiplying it by 1 liter (or about 1 kg) of alimentary tract capacity. To get a revised estimate for the overall number of bacteria in the human body, we first discuss the quantitative distribution of bacteria in the human body. After showing the dominance of gut bacteria, we revisit estimates of the total number of bacteria in the human body.

### Distribution of Bacteria in Different Human Organs

Table 1 shows typical order of magnitude estimates for the number of bacteria that reside in different organs in the human body. The estimates are based on multiplying measured concentrations of bacteria by the volume of each organ [9,10]. Values are rounded up to give an order of magnitude upper bound.

### Table 1

LocationTypical concentration of bacteria (1) (number/mL content)Volume (mL)Order of magnitude bound for bacteria number
Colon (large intestine)10 11 400 (2) 10 14
Dental plaque10 11 10 12
Ileum (lower small intestine)10 8 400 (5) 10 11
Saliva10 9 𼄀10 11
Skin㰐 11 per m 2 (3) 1.8 m 2 (4) 10 11
Stomach10 3 � 4 250 (5) � (6) 10 7
Duodenum and Jejunum (upper small intestine)10 3 � 4 400 (5) 10 7

(1) Except for skin, concentrations are according to [9]. For the skin, we used bacterial areal density and total skin surface to reach an upper bound.

(2) See derivation in section below.

(3) Skin surface bacteria density is taken from [11].

(4) Skin area calculated as inferred from standard formula by DuBois for the body surface area [12].

(5) Volume of the organs of the gastrointestinal tract is derived from weights taken from [13] by assuming content density of 1.04 g/mL [6].

(6) Higher value is given in [14].

Although the bacterial concentrations in the saliva and dental plaque are high, because of their small volume the overall numbers of bacteria in the mouth are less than 1% of the colon bacteria number. The concentration of bacteria in the stomach and the upper 2/3 of the small intestine (duodenum and jejunum) is only 10 3 � 4 bacteria/mL, owing to the relatively low pH of the stomach and the fast flow of the content through the stomach and the small intestine [10]. Table 1 reveals that the bacterial content of the colon exceeds all other organs by at least two orders of magnitude. Importantly, within the alimentary tract, the colon is the only substantial contributor to the total bacterial population, while the stomach and small intestine make negligible contributions.

### Revisiting the Original Back-of-the-Envelope Estimate for the Number of Bacteria in the Colon

The primary source for the often cited value of

10 14 bacteria in the body dates back to the 1970s [3] and only consists of a sentence-long �rivation,” which assumes the volume of the alimentary tract to be 1 liter, and multiplies this volume by the number density of bacteria, known to be about 10 11 bacteria per gram of wet content. Such estimates are often very illuminating, yet it is useful to revisit them as more empirical data accumulates. This pioneering estimate of 10 14 bacteria in the intestine is based on assuming a constant bacterial density over the 1 liter of alimentary tract volume (converting from volume to mass via a density of 1 g/mL). Yet, the parts of the alimentary tract proximal to the colon contain a negligible number of bacteria in comparison to the colon content, as can be appreciated from Table 1 . We thus conclude that the relevant volume for the high bacteria density of 10 11 bacteria/g is only that of the colon. As discussed in Box 1, we integrated data sources on the volume of the colon to arrive at 0.4 L.

### Box 1. The Volume of the Human Colon Content

This is a critical parameter in our calculation. We used a value of 0.4 L based on the following studies (see also S1 Data, tab ColonContent). The volume of the colon content of the reference adult man was previously estimated as 340 mL (355 g at density of 1.04 g/mL [6]), based on various indirect methods including flow measurements, barium meal X-ray measurements and postmortem examination [13]. A recent study [15] gives more detailed data about the volume of undisturbed colon that was gathered by MRI scans. The authors report a height-standardized colonic inner volume for males of 97 ± 24 mL/m 3 (where the best fit was found when dividing the colonic volume by the cube of the height). Taking a height of 1.70 m for the reference man [6], we arrive at a colon volume of 480 ± 120 mL (where unless noted otherwise ± refers to the standard deviation [SD]). This volume includes an unreported volume of gas and did not include the rectum. Most recently, studies analyzing MRI images of the colon provided the most detailed and complete data. The inner colon volume in that cohort was 760 mL in total [16,17]. This cohort was, however, significantly taller than the reference man. Normalizing for height, we arrive at 600 mL total volume for a standard man. In order to deduct the volume occupied by gas, stool fraction in this report was estimated at �% of colon volume leading to 430 mL of standardized wet colon content. Therefore, this most reliable analysis together with earlier studies support an average value of about 0.4 L.

We can sanity-check this volume estimate by looking at the volume of stool that flows through the colon. An adult human is reported to produce on average 100� grams of wet stool per day [18]. The colonic transit time is negatively correlated with the daily fecal output, and its normal values are about 25� hours [18,19]. By multiplying the daily output and the colon transit time, we thus get a volume estimate of 150� mL, which is somewhat lower than but consistent with the values above, given the uncertainties and very crude estimate that did not account for water in the colon that is absorbed before defecation. To summarize, the volume of colon content as evaluated by recent analyses of MRI images is in keeping with previous estimates and fecal transit dynamics. Values for a reference adult man averaged 0.4 L (standard error of the mean [SEM] 17%, coefficient of variation [CV] 25%), which will be used in calculations below. Following a typical meal, the volume changes by about 10% [15], while each defecation event reduces the content by a quarter to a third [18].

### The Total Number of Bacteria in the Body

We are now able to repeat the original calculation for the number of bacteria in the colon [3]. Given 0.9뜐 11 bacteria/g wet stool as derived in Box 2 and 0.4 L of colon, we find 3.8뜐 13 bacteria in the colon with a standard error uncertainty of 25% and a variation of 52% SD over a population of 70 kg males. Considering that the contribution to the total number of bacteria from other organs is at most 10 12 , we use 3.8뜐 13 as our estimate for the number of bacteria across the whole body of the "reference man."

### Box 2. Concentration of Bacteria in the Colon

The most widely used approach for measuring the bacterial cell density in the colon is by examining bacteria content in stool samples. This assumes that stool samples give adequate representation of colon content. We return to this assumption in the discussion. The first such experiments date back to the 1960s and 1970s [20,21]. In those early studies, counting was based on direct microscopic clump counts from diluted stool samples. Later experiments [22,23] used DAPI nucleic acid staining and fluorescent in situ hybridization [FISH] to bacterial 16S RNA. Values are usually reported as bacteria per gram of dry stool. For our calculation, we are interested in the bacteria content for the wet rather than dry content of the colon. To move from bacteria /g dry stool to bacteria /g wet stool we use the fraction of dry matter as reported in each article. Table 2 reports the values we extracted from 14 studies in the literature and translated them to a common basis enabling comparison.

### Table 2

Articlebac. #/g dry stool (x10 11 )dry matter as % of stoolbac. #/g wet stool (x10 11 )CV(%)
AuthorYear
Houte & Gibbons1966--3.253%
Moore & Holdeman1974522%1.178%
Holdeman, Good & Moore19764.131%1.366%
Stephen & Cummings1980429% (1) 1.225%
Langendijk et al.1995--2.726%
Franks et al.19982.9-0.74 (2) 39%
Simmering & Kleessen19994.8-1.3 (2) 44%
Tannock et al.2000--0.9540%
Harmsen, Raangs, He, Degener & Welling20022.130%0.6238%
Zoetendal et al20022.9-0.77 (2) 24%
Zhong et al.20041.523%0.3573%
Thiel & Blaut20053.525%0.8753%
He et al.20081.5-0.39 (2) 43%
Uyeno, Sekiguchi & Kamagata2008--0.4434%
Mean -27% ± 2%0.92 ± 19%46%

Full references are provided in Table A in S1 Appendix. Mean bacteria number is calculated using the geometric mean to give robustness towards outlier values. Values quoted directly from the articles are written in bold, values derived by us are written in italic. Values reported with more than two significant digits are rounded to two significant digits as the uncertainty makes such overspecification nonsensible. ± standard error of the mean.

(1) Value for [21] derived from their Table 1 .

(2) From derivation, assuming the averaged dry matter fraction of 27%.

From the measurements collected in Table 2 , we calculated the representative bacteria concentration in the colon by two methods, yielding very close values: the geometric mean is 0.92뜐 11 (SEM 19%) bacteria per gram of wet stool, while the median of the values is 0.91뜐 11 (SEM 19% by bootstrapping, see methods in S1 Appendix). The variation across the population, given by the average CV, is 46%.

We note that the uncertainty estimate value takes into account known variation in the colon volume, bacteria density, etc., but cannot account for unquantified systematic biases. One prominent such bias is the knowledge gap on differences between the actual bacteria density in the colon, with all its spatial heterogeneity, and the measurements of concentration in feces, which serve as the proxy for estimating bacteria number.

What is the total mass of bacteria in the body? From the total colon content of about 0.4 kg and a bacteria mass fraction of about one-half [21,24], we get a contribution of about 0.2 kg (wet weight) from bacteria to the overall mass of the colon content. Given the dominance of bacteria in the colon over all other microbiota populations in the body, we conclude that there is about 0.2 kg of bacteria in the body overall. Given the water content of bacteria, the total dry weight of bacteria in the body is about 50�g. This value is consistent with a parallel alternative estimate for the total mass of bacteria that multiplies the average mass of a gut bacterium of about 5 pg (wet weight, corresponding to a dry weight of 1𠄲 pg, see S1 Appendix) with the updated total number of bacteria. We note that this empirically observed average gut bacterium is several times bigger than the conveniently chosen “standard” 1 μm 3 volume and 1 pg wet mass bacterium often referred to in textbooks. The total bacteria mass we find represents about 0.3% of the overall body weight, significantly updating previous statements that 1%𠄳% of the body mass is composed of bacteria or that a normal human hosts 1𠄳 kg of bacteria [25].

### The Number of Human Cells in a “Standard” Adult Male

Many literature sources make general statements on the number of cells in the human body ranging between 10 12 to 10 14 cells [26,27]. An order of magnitude back-of-the-envelope argument behind such values is shown in Box 3.

### Box 3. Order of Magnitude, Naïve Estimate for the Number of Human Cells in the Body

Assume a 10 2 kg man, consisting of “representative” mammalian cells. Each mammalian cell, using a cell volume of 1,000�,000 μm 3 , and a cell density similar to that of water, will weigh 10 � � � kg. We thus arrive at 10 13 � 14 human cells in total in the body, as shown in Fig 1 . For these kind of estimates, where cell mass is estimated to within an order of magnitude, factors contributing to less than 2-fold difference are neglected. Thus, we use 100 kg as the mass of a reference man instead of 70 kg and similarly ignore the contribution of extracellular mass to the total mass. These simplifications are useful in making the estimate concise and transparent.

An alternative method that does not require considering a representative "average" cell systematically counts cells by type. Such an approach was taken in a recent detailed analysis [1]. The number of human cells in the body of each different category (by either cell type or organ system) was estimated. For each category, the cell count was obtained from a literature reference or by a calculation based on direct counts in histological cross sections. Summing over a total of 56 cell categories [1] resulted in an overall estimate of 3.7뜐 13 human cells in the body (SD 0.8뜐 13 , i.e., CV of 22%).

### Updated Inventory of Human Cells in the Body

In our effort to revisit the measurements cited, we employed an approach that tries to combine the detailed, census approach with the benefits of a heuristic calculation used as a sanity check. We focused on the six cell types that were recently identified [1] to comprise 97% of the human cell count: red blood cells (accounting for 70%), glial cells (8%), endothelial cells (7%), dermal fibroblasts (5%), platelets (4%), and bone marrow cells (2%). The other 50 cell types account for the remaining 3%. In four cases (red blood cells, glial cells, endothelial cells, and dermal fibroblasts), we arrived at revised calculations as detailed in Box 4.

### Box 4. Revised Estimates for the Number of Red Blood Cells, Glial Cells, Endothelial Cells, and Dermal Fibroblasts

The largest contributor to the overall number of human cells are red blood cells. Calculation of the number of red blood cells was made by taking an average blood volume of 4.9 L (SEM 1.6%, CV 9%) multiplied by a mean RBC count of 5.0뜐 12 cells/L (SEM 1.2%, CV 7%) (see Table 3 and S1 Data). The latter could be verified by looking at your routine complete blood count, normal values range from 4.6𠄶.1뜐 12 cells/L for males and 4.2𠄵.4뜐 12 cells/L for females. This led to a total of 2.5뜐 13 red blood cells (SEM 2%, CV 12%). This is similar to the earlier report of 2.6뜐 13 cells [1].

### Table 3

See Table B in S1 Appendix for full references.

population segmentbody weight [kg]age [y]blood volume [L]RBC count [10 12 /L]colon content [g]bac. conc. [10 11 /g wet] (1) total human cells [10 12 ] (2) total bacteria [10 12 ]B:H
ref. man7020�4.95.04200.9230381.3
ref. woman63 3.94.54800.9221442.2
young infant4.44 weeks0.43.8480.921.94.42.3
infant9.610.84.5800.92471.7
elder70663.8 (3) 4.84200.9222381.8
obese140 6.75.0 (4) 610 (5) 0.9240561.4

(1) No significant change in bacteria concentrations in relation to high variation for the reference man [40,43].

(2) Assuming RBCs account for 84% of the total host cells as observed for the reference man.

(3) Decrease of 24% in the blood volume, according to [44].

(4) No significant change in the hematocrit in obesity [45].

(5) We could not find any direct measurements of the colonic volume for obese individuals in the literature, yet from an indirect analysis the volume increases with weight and plateaus at about 600 mL [46].

The number of glial cells was previously reported as 3뜐 12 [1]. This estimate is based on a 10:1 ratio between glial cells and neurons in the brain. This ratio of glia:neurons was held as a broadly accepted convention across the literature. However, a recent analysis [28] revisits this value and, after analyzing the variation across brain regions, concludes that the ratio is close to 1:1. The study concludes that there are 8.5뜐 10 glial cells (CV 11%) in the brain and a similar number of neurons and so we use these updated values here.

The number of endothelial cells in the body was earlier estimated at 2.5뜐 12 cells (CV 40%), based on the mean surface area of one endothelial cell [1] and the total surface area of the blood vessels, based on a total capillary length of 8뜐 9 cm. We could not find a primary source for the total length of the capillary bed and thus decided to revisit this estimate. We used data regarding the percentage of the blood volume in each type of blood vessels [29]. Using mean diameters for different blood vessels [30], we were able to derive (S1 Data) the total length of each type of vessel (arteries, veins, capillaries, etc.) and its corresponding surface area. Dividing by the mean surface area of one endothelial cell [31], we derive a reduced total estimate of 6뜐 11 cells.

The number of dermal fibroblasts was previously estimated to be 1.85뜐 12 [1], based on multiplying the total surface area (SA) of the human body (1.85 m 2 [32]) by the areal density of dermal fibroblasts [33]. We wished to incorporate the dermal thickness (d) into the calculation. Dermal thickness was directly measured at 17 locations throughout the body [34], with the mean of these measurements yielding 0.11ଐ.04 cm. The dermis is composed of two main layers: papillary dermis (about 10% of the dermis thickness) and reticular dermis (the other 90%) [35]. The fibroblast density is greater in the papillary dermis, with a reported areal density, σpap. of 10 6 cells/cm 2 (with 100 μm thickness of papillary, giving 10 8 cells/cm 3 ) [33]. The fibroblast density in the middle of the dermis was reported to be about 3뜐 6 cells/cm 3 [36], giving an areal density of σret. = 3뜐 5 cells/cm 2 . Combining these we find: Nder.fib. = SA·(σpap. + σret.) = 1.85뜐 4 cm 2 (10 6 + 3뜐 5 ) cells/cm 2 = 2.6뜐 10 cells. After this 100-fold decrease in number, dermal fibroblasts are estimated to account for only 𢒀.05% of the human cell count.

Our revised calculations of the number of glial cells, endothelial cells, and dermal fibroblast yield only 0.9뜐 12 cells, in contrast to 7.5뜐 12 cells in the previous estimate. This leaves us with 3.0뜐 13 human cells in the 70 kg “reference man,” with a calculated 2% uncertainty and 14% CV. We note that the uncertainty and CV estimates might be too optimistically low, as they are dominated by the reported high confidence of studies dealing with red blood cells but may underestimate systematic errors, omissions of some cell types, and similar factors that are hard to quantify. In Fig 2 , we summarize the revised results for the contribution of the different cell types to the total number of human cells. Categories contributing Ϡ.4% in cell count are presented. All the other categories sum up to about 2% together. We find that the body includes only 3뜐 12 non-blood human cells, merely 10% of the total updated human cell count. The visualization in Fig 2 highlights that almost 90% of the cells are estimated to be enucleated cells (26뜐 12 cells), mostly red blood cells and platelets, while the other �% consist of 𢒃뜐 12 nucleated cells. The striking dominance of the hematopoietic lineage in the cell count (90% of the total) is counterintuitive given the composition of the body by mass. This is the subject of the following analysis.

Representation as a Voronoi tree map where polygon area is proportional to the number of cells. Visualization performed using the online tool at http://bionic-vis.biologie.uni-greifswald.de/.

### Mass-Centered Approach as Sanity Check for Cell Count

It is prudent in making such estimates to approach the analysis from different angles. In that spirit, we now ask does the cumulative mass of the cells counted fall within the expected range for a reference adult? To properly tackle that question, we first need to state what the anticipated result is, i.e., total body cell mass. For a reference man mass of 70 kg, 25% is extracellular fluid [37], another 7% is extracellular solids [37], thus we need to account for � kg of cell mass (including fat).

A comprehensive systematic source for the composition of total cell mass (rather than total cell count) is the Report of the Task Group on Reference Man [6], which gives values for the mass of the main tissues of the human body. This mass per tissue analysis includes both intra- and extracellular components. To distinguish between intra- and extracellular portions of each tissue, we can leverage total body potassium measurements [38] as detailed in S1 Appendix. Fig 3 compares the main tissues that contribute to the human body, in terms of cell number and masses.

The upper bar displays the number of cells, while the lower bar displays the contribution from each of the main cell types comprising the overall cellular body mass (not including extracellular mass that adds another � kg). For comparison, the contribution of bacteria is shown on the right, amounting to only 0.2 kg, which is about 0.3% of the body weight.

A striking outcome of this juxtaposition is the evident discordance between contributors to total cell mass and to cell number. The cell count is dominated by red blood cells (84%), among the smallest cell types in the human body with a volume of about 100 μm 3 . In contrast, 75% of total cell mass is composed of two cell types, fat cells (adipocytes) and muscle cells (myocytes), both large cells (usually 㸐,000 μm 3 by volume) that constitute only a minute fraction (𢒀.2%) of total cell number. At the other extreme, bacteria have a minor contribution in terms of mass, but a cell count comparable to all human cells combined, as discussed above. The mass balance accounts well for all expected body mass, giving support to our analysis. The option of overlooking a collection of very small cells numerous enough to alter the total cell count is further discussed in the S1 Appendix.

### The Ratio of Bacteria to Human Cells in the Adult Body

With the revised estimates for the number of human (3.0� 13 ) and bacterial cells (3.8� 13 ) in the body (the numerator and denominator of the B/H ratio), we can give an updated estimate of B/H = 1.3, with an uncertainty of 25% and a variation of 53% over the population of standard 70 kg males. This B/H value of about 1:1 (with the associated uncertainty range) should replace the 10:1 or 100:1 values that are stated in the literature until more accurate measurements become available.

We note that if one chooses to compare the number of bacteria in the human body (3.8뜐 13 ) to the number of nucleated human cells (𢒀.3뜐 13 ), the ratio will be about 10:1. This is because the dominant population of non-nucleated red blood cells is not included in the calculation. We note that this ratio is the result of both the number of bacteria and the number of nucleated human cells in the body being several times lower than in the original 1970s estimate (that did not restrict the analysis to nucleated cells). The issue of whether cells without a nucleus should be included or discarded in the calculation of the number of human cells, and thus the B/H ratio, seems to be a question of definition. We view red blood cells as bona fide cells, as their name suggests. But it is also plausible to choose not to include them as some may think of them as �gs full of hemoglobin.” Inclusion of platelets in the count, which corresponds to their inclusion in previous counts, is also potentially disputable but has only a minor quantitative effect. Indeed, this opens an interesting tangential discussion on what should be defined as a cell.

### Variations in the Ratio of Bacteria to Human Cells across Population Segments

After reviewing the B/H ratio for the “reference man,” we now generalize our results by addressing other segments of the population. Looking at our estimate, we identify four main parameters that dominate the calculation:

bacterial density in the colon

hematocrit (i.e., red blood cells per unit volume).

These are the governing parameters due to the dominating contribution of the colonic bacteria and RBC count to the total bacterial and human cell counts, respectively. In order to evaluate the effect of gender, age, and obesity on the B/H ratio, we examine the change in these parameters across those groups.

Table 3 collects the changes to each of the previously mentioned parameters for individuals that represent different segments of the human population: reference adult woman (1.63 m, 60 kg [39]), young infant (age 4 weeks), infant (age 1 year), elder (66 years), and obese (140 kg).

Review of the literature shows no significant effect on the colonic bacterial concentrations over age from the one month old infant to the elderly [40,41]. The colonization of the neonatal GI tract from negligible colon bacterial concentrations of � 5 bacteria/mL to concentrations equivalent to those of adults occurs in just under one month [42]. For this dynamic period that is yet to be charted in high resolution, we do not supply a B/H ratio estimate. As with age, extremes of weight have low impact on bacterial cell counts. [43]. The reported values for infants and obese are in the range of variation of “the reference man.” In addition, we could not find any report in the literature on gender-specific differences in bacteria density in the colon. As can be appreciated from Table 3 , the B/H ratio varies by up to 2-fold across those different population groups from a low of 1.3 to a high of 2.3.

We note that additional factors such as race and ethnicity may influence the B:H ratio. It has been shown that the bacterial population in the colon is strongly affected by geography [47], but current data is not enough to allow robust inference for the colonic concentrations and represents a data gap.

## The Five Layers of the Epidermis

• The stratum basale is the deepest layer of the epidermis. It consists of a single layer of cells. The cells divide to replace the skin cells that are shed.
• The cells of the stratum spinosum are linked to each other by structures called desmosomes. Desmosomes enable cells to adhere strongly to one another. Filaments made of keratin extend from a desmosome and produce a spiny or prickly appearance. The stratum basale and the stratum spinosum are sometimes grouped together and known as the stratum germinativum.
• The cells of the stratum granulosum contain granules made of a substance called keratohyalin. The granules produces a grainy appearance.
• The stratum lucidum is a clear layer that contains dead cells. It’s found in the thick skin of the palms and on the soles of the feet.
• The stratum corneum forms the surface of the skin and contains multiple layers of flattened cells. The cells have no organelles and are gradually shed from the body. Researchers have discovered that the stratum corneum has important barrier functions.

## 5.1 Layers of the Skin

Although you may not typically think of the skin as an organ, it is in fact made of tissues that work together as a single structure to perform unique and critical functions. The skin and its accessory structures make up the integumentary system , which provides the body with overall protection. The skin is made of multiple layers of cells and tissues, which are held to underlying structures by connective tissue (Figure 5.2). The deeper layer of skin is well vascularized (has numerous blood vessels). It also has numerous sensory, and autonomic and sympathetic nerve fibers ensuring communication to and from the brain.

The skin consists of two main layers and a closely associated layer. View this animation to learn more about layers of the skin. What are the basic functions of each of these layers?

### The Epidermis

The epidermis is composed of keratinized, stratified squamous epithelium. It is made of four or five layers of epithelial cells, depending on its location in the body. It does not have any blood vessels within it (i.e., it is avascular). Skin that has four layers of cells is referred to as “thin skin.” From deep to superficial, these layers are the stratum basale, stratum spinosum, stratum granulosum, and stratum corneum. Most of the skin can be classified as thin skin. “Thick skin” is found only on the palms of the hands and the soles of the feet. It has a fifth layer, called the stratum lucidum, located between the stratum corneum and the stratum granulosum (Figure 5.3).

The cells in all of the layers except the stratum basale are called keratinocytes. A keratinocyte is a cell that manufactures and stores the protein keratin. Keratin is an intracellular fibrous protein that gives hair, nails, and skin their hardness and water-resistant properties. The keratinocytes in the stratum corneum are dead and regularly slough away, being replaced by cells from the deeper layers (Figure 5.4).

View the University of Michigan WebScope to explore the tissue sample in greater detail. If you zoom on the cells at the outermost layer of this section of skin, what do you notice about the cells?

#### Stratum Basale

The stratum basale (also called the stratum germinativum) is the deepest epidermal layer and attaches the epidermis to the basal lamina, below which lie the layers of the dermis. The cells in the stratum basale bond to the dermis via intertwining collagen fibers, referred to as the basement membrane. A finger-like projection, or fold, known as the dermal papilla (plural = dermal papillae) is found in the superficial portion of the dermis. Dermal papillae increase the strength of the connection between the epidermis and dermis the greater the folding, the stronger the connections made (Figure 5.5).

The stratum basale is a single layer of cells primarily made of basal cells. A basal cell is a cuboidal-shaped stem cell that is a precursor of the keratinocytes of the epidermis. All of the keratinocytes are produced from this single layer of cells, which are constantly going through mitosis to produce new cells. As new cells are formed, the existing cells are pushed superficially away from the stratum basale. Two other cell types are found dispersed among the basal cells in the stratum basale. The first is a Merkel cell , which functions as a receptor and is responsible for stimulating sensory nerves that the brain perceives as touch. These cells are especially abundant on the surfaces of the hands and feet. The second is a melanocyte , a cell that produces the pigment melanin. Melanin gives hair and skin its color, and also helps protect the living cells of the epidermis from ultraviolet (UV) radiation damage.

In a growing fetus, fingerprints form where the cells of the stratum basale meet the papillae of the underlying dermal layer (papillary layer), resulting in the formation of the ridges on your fingers that you recognize as fingerprints. Fingerprints are unique to each individual and are used for forensic analyses because the patterns do not change with the growth and aging processes.

#### Stratum Spinosum

As the name suggests, the stratum spinosum is spiny in appearance due to the protruding cell processes that join the cells via a structure called a desmosome . The desmosomes interlock with each other and strengthen the bond between the cells. It is interesting to note that the “spiny” nature of this layer is an artifact of the staining process. Unstained epidermis samples do not exhibit this characteristic appearance. The stratum spinosum is composed of eight to 10 layers of keratinocytes, formed as a result of cell division in the stratum basale (Figure 5.6). Interspersed among the keratinocytes of this layer is a type of dendritic cell called the Langerhans cell , which functions as a macrophage by engulfing bacteria, foreign particles, and damaged cells that occur in this layer.

View the University of Michigan WebScope to explore the tissue sample in greater detail. If you zoom on the cells at the outermost layer of this section of skin, what do you notice about the cells?

The keratinocytes in the stratum spinosum begin the synthesis of keratin and release a water-repelling glycolipid that helps prevent water loss from the body, making the skin relatively waterproof. As new keratinocytes are produced atop the stratum basale, the keratinocytes of the stratum spinosum are pushed into the stratum granulosum.

#### Stratum Granulosum

The stratum granulosum has a grainy appearance due to further changes to the keratinocytes as they are pushed from the stratum spinosum. The cells (three to five layers deep) become flatter, their cell membranes thicken, and they generate large amounts of the proteins keratin, which is fibrous, and keratohyalin , which accumulates as lamellar granules within the cells (see Figure 5.5). These two proteins make up the bulk of the keratinocyte mass in the stratum granulosum and give the layer its grainy appearance. The nuclei and other cell organelles disintegrate as the cells die, leaving behind the keratin, keratohyalin, and cell membranes that will form the stratum lucidum, the stratum corneum, and the accessory structures of hair and nails.

#### Stratum Lucidum

The stratum lucidum is a smooth, seemingly translucent layer of the epidermis located just above the stratum granulosum and below the stratum corneum. This thin layer of cells is found only in the thick skin of the palms, soles, and digits. The keratinocytes that compose the stratum lucidum are dead and flattened (see Figure 5.5). These cells are densely packed with eleiden , a clear protein rich in lipids, derived from keratohyalin, which gives these cells their transparent (i.e., lucid) appearance and provides a barrier to water.

#### Stratum Corneum

The stratum corneum is the most superficial layer of the epidermis and is the layer exposed to the outside environment (see Figure 5.5). The increased keratinization (also called cornification) of the cells in this layer gives it its name. There are usually 15 to 30 layers of cells in the stratum corneum. This dry, dead layer helps prevent the penetration of microbes and the dehydration of underlying tissues, and provides a mechanical protection against abrasion for the more delicate, underlying layers. Cells in this layer are shed periodically and are replaced by cells pushed up from the stratum granulosum (or stratum lucidum in the case of the palms and soles of feet). The entire layer is replaced during a period of about 4 weeks. Cosmetic procedures, such as microdermabrasion, help remove some of the dry, upper layer and aim to keep the skin looking “fresh” and healthy.

### Dermis

The dermis might be considered the “core” of the integumentary system (derma- = “skin”), as distinct from the epidermis (epi- = “upon” or “over”) and hypodermis (hypo- = “below”). It contains blood and lymph vessels, nerves, and other structures, such as hair follicles and sweat glands. The dermis is made of two layers of connective tissue that compose an interconnected mesh of elastin and collagenous fibers, produced by fibroblasts (Figure 5.7).

#### Papillary Layer

The papillary layer is made of loose, areolar connective tissue, which means the collagen and elastin fibers of this layer form a loose mesh. This superficial layer of the dermis projects into the stratum basale of the epidermis to form finger-like dermal papillae (see Figure 5.7). Within the papillary layer are fibroblasts, a small number of fat cells (adipocytes), and an abundance of small blood vessels. In addition, the papillary layer contains phagocytes, defensive cells that help fight bacteria or other infections that have breached the skin. This layer also contains lymphatic capillaries, nerve fibers, and touch receptors called the Meissner corpuscles.

#### Reticular Layer

Underlying the papillary layer is the much thicker reticular layer , composed of dense, irregular connective tissue. This layer is well vascularized and has a rich sensory and sympathetic nerve supply. The reticular layer appears reticulated (net-like) due to a tight meshwork of fibers. Elastin fibers provide some elasticity to the skin, enabling movement. Collagen fibers provide structure and tensile strength, with strands of collagen extending into both the papillary layer and the hypodermis. In addition, collagen binds water to keep the skin hydrated. Collagen injections and Retin-A creams help restore skin turgor by either introducing collagen externally or stimulating blood flow and repair of the dermis, respectively.

### Hypodermis

The hypodermis (also called the subcutaneous layer or superficial fascia) is a layer directly below the dermis and serves to connect the skin to the underlying fascia (fibrous tissue) of the bones and muscles. It is not strictly a part of the skin, although the border between the hypodermis and dermis can be difficult to distinguish. The hypodermis consists of well-vascularized, loose, areolar connective tissue and adipose tissue, which functions as a mode of fat storage and provides insulation and cushioning for the integument.

### Everyday Connection

#### Lipid Storage

The hypodermis is home to most of the fat that concerns people when they are trying to keep their weight under control. Adipose tissue present in the hypodermis consists of fat-storing cells called adipocytes. This stored fat can serve as an energy reserve, insulate the body to prevent heat loss, and act as a cushion to protect underlying structures from trauma.

Where the fat is deposited and accumulates within the hypodermis depends on hormones (testosterone, estrogen, insulin, glucagon, leptin, and others), as well as genetic factors. Fat distribution changes as our bodies mature and age. Men tend to accumulate fat in different areas (neck, arms, lower back, and abdomen) than do women (breasts, hips, thighs, and buttocks). The body mass index (BMI) is often used as a measure of fat, although this measure is, in fact, derived from a mathematical formula that compares body weight (mass) to height. Therefore, its accuracy as a health indicator can be called into question in individuals who are extremely physically fit.

In many animals, there is a pattern of storing excess calories as fat to be used in times when food is not readily available. In much of the developed world, insufficient exercise coupled with the ready availability and consumption of high-calorie foods have resulted in unwanted accumulations of adipose tissue in many people. Although periodic accumulation of excess fat may have provided an evolutionary advantage to our ancestors, who experienced unpredictable bouts of famine, it is now becoming chronic and considered a major health threat. Recent studies indicate that a distressing percentage of our population is overweight and/or clinically obese. Not only is this a problem for the individuals affected, but it also has a severe impact on our healthcare system. Changes in lifestyle, specifically in diet and exercise, are the best ways to control body fat accumulation, especially when it reaches levels that increase the risk of heart disease and diabetes.

### Pigmentation

The color of skin is influenced by a number of pigments, including melanin, carotene, and hemoglobin. Recall that melanin is produced by cells called melanocytes, which are found scattered throughout the stratum basale of the epidermis. The melanin is transferred into the keratinocytes via a cellular vesicle called a melanosome (Figure 5.8).

Melanin occurs in two primary forms. Eumelanin exists as black and brown, whereas pheomelanin provides a red color. Dark-skinned individuals produce more melanin than those with pale skin. Exposure to the UV rays of the sun or a tanning salon causes melanin to be manufactured and built up in keratinocytes, as sun exposure stimulates keratinocytes to secrete chemicals that stimulate melanocytes. The accumulation of melanin in keratinocytes results in the darkening of the skin, or a tan. This increased melanin accumulation protects the DNA of epidermal cells from UV ray damage and the breakdown of folic acid, a nutrient necessary for our health and well-being. In contrast, too much melanin can interfere with the production of vitamin D, an important nutrient involved in calcium absorption. Thus, the amount of melanin present in our skin is dependent on a balance between available sunlight and folic acid destruction, and protection from UV radiation and vitamin D production.

It requires about 10 days after initial sun exposure for melanin synthesis to peak, which is why pale-skinned individuals tend to suffer sunburns of the epidermis initially. Dark-skinned individuals can also get sunburns, but are more protected than are pale-skinned individuals. Melanosomes are temporary structures that are eventually destroyed by fusion with lysosomes this fact, along with melanin-filled keratinocytes in the stratum corneum sloughing off, makes tanning impermanent.

Too much sun exposure can eventually lead to wrinkling due to the destruction of the cellular structure of the skin, and in severe cases, can cause sufficient DNA damage to result in skin cancer. When there is an irregular accumulation of melanocytes in the skin, freckles appear. Moles are larger masses of melanocytes, and although most are benign, they should be monitored for changes that might indicate the presence of cancer (Figure 5.9).

### Disorders of the.

#### Integumentary System

The first thing a clinician sees is the skin, and so the examination of the skin should be part of any thorough physical examination. Most skin disorders are relatively benign, but a few, including melanomas, can be fatal if untreated. A couple of the more noticeable disorders, albinism and vitiligo, affect the appearance of the skin and its accessory organs. Although neither is fatal, it would be hard to claim that they are benign, at least to the individuals so afflicted.

Albinism is a genetic disorder that affects (completely or partially) the coloring of skin, hair, and eyes. The defect is primarily due to the inability of melanocytes to produce melanin. Individuals with albinism tend to appear white or very pale due to the lack of melanin in their skin and hair. Recall that melanin helps protect the skin from the harmful effects of UV radiation. Individuals with albinism tend to need more protection from UV radiation, as they are more prone to sunburns and skin cancer. They also tend to be more sensitive to light and have vision problems due to the lack of pigmentation on the retinal wall. Treatment of this disorder usually involves addressing the symptoms, such as limiting UV light exposure to the skin and eyes. In vitiligo , the melanocytes in certain areas lose their ability to produce melanin, possibly due to an autoimmune reaction. This leads to a loss of color in patches (Figure 5.10). Neither albinism nor vitiligo directly affects the lifespan of an individual.

Other changes in the appearance of skin coloration can be indicative of diseases associated with other body systems. Liver disease or liver cancer can cause the accumulation of bile and the yellow pigment bilirubin, leading to the skin appearing yellow or jaundiced (jaune is the French word for “yellow”). Tumors of the pituitary gland can result in the secretion of large amounts of melanocyte-stimulating hormone (MSH), which results in a darkening of the skin. Similarly, Addison’s disease can stimulate the release of excess amounts of adrenocorticotropic hormone (ACTH), which can give the skin a deep bronze color. A sudden drop in oxygenation can affect skin color, causing the skin to initially turn ashen (white). With a prolonged reduction in oxygen levels, dark red deoxyhemoglobin becomes dominant in the blood, making the skin appear blue, a condition referred to as cyanosis (kyanos is the Greek word for “blue”). This happens when the oxygen supply is restricted, as when someone is experiencing difficulty in breathing because of asthma or a heart attack. However, in these cases the effect on skin color has nothing do with the skin’s pigmentation.

This ABC video follows the story of a pair of fraternal African-American twins, one of whom is albino. Watch this video to learn about the challenges these children and their family face. Which ethnicities do you think are exempt from the possibility of albinism?

## Epithelia Lab

Epithelia are tissues composed of closely aggregated cells that cover most body surfaces, cavities, and tubes. These include the outer surface of the body (skin), tracts traversing the body (gastrointestinal tract), dead-end tracts that have openings at the body surface (respiratory, urinary, and genital tracts), and ducts that open into these tracts (exocrine glands). The functions of epithelia are numerous, and a single epithelium may have several functions. The most important of these include physical protection and selective transport (diffusion, absorption, secretion).

Surface epithelia form continuous sheets that can have one or multiple cell layers. Several different types of cell junctions mediate physical strength and cell communication within the epithelium. A basement membrane lies beneath the epithelium and separates it from underlying tissue because blood vessels do not penetrate the basement membrane, nutrients like oxygen and metabolites reach the epithelium by diffusion. Epithelia are polarized, with an apical surface that faces the external environment and a basal surface that faces the basement membrane.

### Simple squamous epithelium

Simple squamous epithelia consist of a single layer of flattened cells. This type of epithelia lines the inner surface of all blood vessels (endothelium), forms the wall of alveolar sacs in the lung and lines the body cavities (mesothelium). The primary function of simple squamous epithelia is to facilitate diffusion of gases and small molecules.

### Simple cuboidal epithelium

Simple cuboidal epithelium consist of a single layer of cells that are approximately as tall as they are wide. This type of epithelium lines collecting ducts and tubes and is involved in absorbing or secreting material into the ducts or tubes.

### Simple columnar epithelium

Simple columnar epithelium consist of a single layer of cells that are taller than they are wide. This type of epithelia lines the small intestine where it absorbs nutrients from the lumen of the intestine. Simple columnar epithelia are also located in the stomach where it secretes acid, digestive enzymes and mucous.

### Stratified Squamous Epithelium

Stratified squamous epithelia consist of multiple layers of cells with the outer most layer being squamous. The other layers may contain cells that are cuboidal and/or columnar, but the classification of the epithelium is based only on the shape of the outermost layer of cells. This type of epithelium provides protection to mechanical stress and dessication and is found in the skin.

### Pseudostratified

Pseudostratified epithelia consist of a single layer of cells, but due to the different heights of the cells, it gives the appearance of having mutliple layers of cells, hence the name pseudostratified. Importantly, all cells are attached to the basement membrane. This type of epithelium is found in the respiratory tract and functions to secrete mucous and move material up the respiratory tract through the beating of cilia. Cilia are long extensions of the cell membrane that contain a microtubule-based structure called the axoneme. Dynein motors within the axoneme generate force that cause a wave-like motion in the cilia.

### Basement membrane

All epithelia rest on a basement membrane. The basement membrane provides structural support and integrity to epithelia by providing a common framework of proteins to which epithelial cells adhere. The basement membrane separates the epithelia from the underlying or surrounding tissue. Importantly, because epithelia lack blood vessels and depend upon capillaries in surrounding tissues, all nutrients, hormones and other proteins must diffuse across the basement membrane before reaching an epithelia. Epithelial cells interact with the basement membrane via receptors in their cell membrane called integrins.

## Epidermis

Don Bliss / National Cancer Institute

The outermost layer of the skin, composed of epithelial tissue, is known as the epidermis. It contains squamous cells, or keratinocytes, which synthesize a tough protein called keratin. Keratin is a major component of skin, hair, and nails. Keratinocytes on the surface of the epidermis are dead and are continually shed and replaced by cells from beneath. This layer also contains specialized cells called Langerhans cells that signal to the immune system when there is an infection. This aids in the development of antigen immunity.

The innermost layer of the epidermis contains keratinocytes called basal cells. These cells constantly divide to produce new cells that are pushed upward to the layers above. Basal cells become new keratinocytes, which replace the older ones that die and are shed. Within the basal layer are melanin-producing cells known as melanocytes. Melanin is a pigment that helps protect the skin from harmful ultraviolet solar radiation by giving it a brown hue. Also found in the basal layer of the skin are touch receptor cells called Merkel cells.

The epidermis is composed of five sublayers:

• Stratum corneum: The top layer of dead, extremely flat cells. Cell nuclei are not visible.
• Stratum lucidum: A thin, flattened layer of dead cells. Not visible in thin skin.
• Stratum granulosum: A layer of rectangular cells that become increasingly flattened as they move to the surface of the epidermis.
• Stratum spinosum: A layer of polyhedral-shaped cells that flatten as they get closer to the stratum granulosum.
• Stratum basale: The innermost layer of elongated column-shaped cells. It consists of basal cells that produce new skin cells.

The epidermis includes two distinct types of skin: thick skin and thin skin. Thick skin is about 1.5 mm thick and is found only on the palms of the hands and the soles of the feet. The rest of the body is covered by thin skin, the thinnest of which covers the eyelids.

## BIO 140 - Human Biology I - Textbook

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### Gross Anatomy of the Kidney

• Describe the external structure of the kidney, including its location, support structures, and covering
• Identify the major internal divisions and structures of the kidney
• Identify the major blood vessels associated with the kidney and trace the path of blood through the kidney
• Compare and contrast the cortical and juxtamedullary nephrons
• Name structures found in the cortex and medulla
• Describe the physiological characteristics of the cortex and medulla

The kidneys lie on either side of the spine in the retroperitoneal space between the parietal peritoneum and the posterior abdominal wall, well protected by muscle, fat, and ribs. They are roughly the size of your fist, and the male kidney is typically a bit larger than the female kidney. The kidneys are well vascularized, receiving about 25 percent of the cardiac output at rest.

There have never been sufficient kidney donations to provide a kidney to each person needing one. Watch the video linked to below to learn about a cutting-edge technique in which a new kidney is &ldquoprinted.&rdquo The successful utilization of this technology is still several years in the future, but imagine a time when you can print a replacement organ or tissue on demand.

#### External Anatomy

The left kidney is located at about the T12 to L3 vertebrae, whereas the right is lower due to slight displacement by the liver. Upper portions of the kidneys are somewhat protected by the eleventh and twelfth ribs (Figure 1). Each kidney weighs about 125&ndash175 g in males and 115&ndash155 g in females. They are about 11&ndash14 cm in length, 6 cm wide, and 4 cm thick, and are directly covered by a fibrous capsule composed of dense, irregular connective tissue that helps to hold their shape and protect them. This capsule is covered by a shock-absorbing layer of adipose tissue called the renal fat pad , which in turn is encompassed by a tough renal fascia. The fascia and, to a lesser extent, the overlying peritoneum serve to firmly anchor the kidneys to the posterior abdominal wall in a retroperitoneal position.

Figure 1: The kidneys are slightly protected by the ribs and are surrounded by fat for protection (not shown).

On the superior aspect of each kidney is the adrenal gland. The adrenal cortex directly influences renal function through the production of the hormone aldosterone to stimulate sodium reabsorption.

#### Internal Anatomy

A frontal section through the kidney reveals an outer region called the renal cortex and an inner region called the medulla (Figure 2). The renal columns are connective tissue extensions that radiate downward from the cortex through the medulla to separate the most characteristic features of the medulla, the renal pyramids and renal papillae . The papillae are bundles of collecting ducts that transport urine made by nephrons to the calyces of the kidney for excretion. The renal columns also serve to divide the kidney into 6&ndash8 lobes and provide a supportive framework for vessels that enter and exit the cortex. The pyramids and renal columns taken together constitute the kidney lobes.

#### Renal Hilum

The renal hilum is the entry and exit site for structures servicing the kidneys: vessels, nerves, lymphatics, and ureters. The medial-facing hila are tucked into the sweeping convex outline of the cortex. Emerging from the hilum is the renal pelvis, which is formed from the major and minor calyxes in the kidney. The smooth muscle in the renal pelvis funnels urine via peristalsis into the ureter. The renal arteries form directly from the descending aorta, whereas the renal veins return cleansed blood directly to the inferior vena cava. The artery, vein, and renal pelvis are arranged in an anterior-to-posterior order.

##### Nephrons and Vessels

The renal artery first divides into segmental arteries, followed by further branching to form interlobar arteries that pass through the renal columns to reach the cortex (Figure 3). The interlobar arteries, in turn, branch into arcuate arteries, cortical radiate arteries, and then into afferent arterioles. The afferent arterioles service about 1.3 million nephrons in each kidney.

Nephrons are the &ldquofunctional units&rdquo of the kidney they cleanse the blood and balance the constituents of the circulation. The afferent arterioles form a tuft of high-pressure capillaries about 200 µm in diameter, the glomerulus . The rest of the nephron consists of a continuous sophisticated tubule whose proximal end surrounds the glomerulus in an intimate embrace&mdashthis is Bowman&rsquos capsule . The glomerulus and Bowman&rsquos capsule together form the renal corpuscle . As mentioned earlier, these glomerular capillaries filter the blood based on particle size. After passing through the renal corpuscle, the capillaries form a second arteriole, the efferent arteriole (Figure 4). These will next form a capillary network around the more distal portions of the nephron tubule, the peritubular capillaries and vasa recta , before returning to the venous system. As the glomerular filtrate progresses through the nephron, these capillary networks recover most of the solutes and water, and return them to the circulation. Since a capillary bed (the glomerulus) drains into a vessel that in turn forms a second capillary bed, the definition of a portal system is met. This is the only portal system in which an arteriole is found between the first and second capillary beds. (Portal systems also link the hypothalamus to the anterior pituitary, and the blood vessels of the digestive viscera to the liver.)

Figure 4: The two capillary beds are clearly shown in this figure. The efferent arteriole is the connecting vessel between the glomerulus and the peritubular capillaries and vasa recta.

##### Cortex

In a dissected kidney, it is easy to identify the cortex it appears lighter in color compared to the rest of the kidney. All of the renal corpuscles as well as both the proximal convoluted tubules (PCTs) and distal convoluted tubules are found here. Some nephrons have a short loop of Henle that does not dip beyond the cortex. These nephrons are called cortical nephrons . About 15 percent of nephrons have long loops of Henle that extend deep into the medulla and are called juxtamedullary nephrons .

#### Chapter Review

As noted previously, the structure of the kidney is divided into two principle regions&mdashthe peripheral rim of cortex and the central medulla. The two kidneys receive about 25 percent of cardiac output. They are protected in the retroperitoneal space by the renal fat pad and overlying ribs and muscle. Ureters, blood vessels, lymph vessels, and nerves enter and leave at the renal hilum. The renal arteries arise directly from the aorta, and the renal veins drain directly into the inferior vena cava. Kidney function is derived from the actions of about 1.3 million nephrons per kidney these are the &ldquofunctional units.&rdquo A capillary bed, the glomerulus, filters blood and the filtrate is captured by Bowman&rsquos capsule. A portal system is formed when the blood flows through a second capillary bed surrounding the proximal and distal convoluted tubules and the loop of Henle. Most water and solutes are recovered by this second capillary bed. This filtrate is processed and finally gathered by collecting ducts that drain into the minor calyces, which merge to form major calyces the filtrate then proceeds to the renal pelvis and finally the ureters.

## Lymph: Formation and Functions | Body Fluids | Humans | Biology

In this article we will discuss about:- 1. Introduction to Lymph 2. Properties of Lymph 3. Composition 4. Functions 5. Rate of Flow 6. Formation 7. Circulation.

1. Introduction to Lymph
2. Properties of Lymph
3. Composition of Lymph
4. Functions of Lymph
5. Rate of Flow in Lymph
6. Formation of Lymph
7. Circulation of Lymph

#### 1. Introduction to Lymph:

The lymphatic vessels at the periphery are micro­scopic blind (closed) end vessels, known as lymphat­ic capillaries. These tiny vessels are situated in the intercellular spaces and their walls formed by en­dothelial cells supported by the fibrous connective tissue (Fig. 5.3).

These capillaries repeatedly join together to form bigger lymphatic vessels, which pass through the lymph nodes, receive more tribu­taries and gradually increase in size. All the lymph from the body is finally collected into two big chan­nels—the right lymphatic duct and the thoracic duct (or left lymphatic), which open respectively at the right and left subclavian veins.

The right lymphat­ic duct, about 1.25 cm long, drains from the right forelimb and the right side of the neck and chest (Fig. 5.4). The thoracic duct, being about 38—45 cm long and about 4-6 mm in diameter, emerges from the cisterna (receptaculum) chyli and also receives the left cervical duct, which collects lymph from the left forelimb, left side of the neck and chest. The cister­na chyli, being situated on the front of the body of the second lumbar vertebra, receives all the lymph coming from two hind-limbs and alimentary canal (Fig. 5.4A).

The lymphatic vessels are provided with valves which help the lymph stream to flow in the direc­tion of the chest. The primary lymphatic vessels that remain in the centre of small intestinal villi are known as lacteals and during the course of digestion lacteals are filled with milk-white fluid, chyle. The chemical composition of chyle, except for its high fat content, is similar to that of the lymph in other parts of the body. In the central nervous system there are no lymphatics.

Here, cerebrospinal fluid takes the place of lymph. Lymphatic capillar­ies are not also found in the cartilage, spleen, epidermis, internal ear and eyeball. The function of lymphatics is to carry tissue fluid from tissues to veins and the return of water and protein from the interstitial fluid to blood from which they come. And the function of lacteals is to help in the absorption of digested food materials generally fats from the intestine.

#### 2. Properties of Lymph:

Lymph should be regarded as modified tissue fluid. Lymph is the clear watery-appearing fluid found in lymphatic vessels and is formed by the passage of substances from blood capillaries into tissue spaces. This process is known as transudation which involves the processes of diffusion and filtration. A pure sample of lymph can be obtained by inserting a canula in the thoracic duct of an animal.

Lymph, as collected from thoracic duct during fasting, is transparent, yellowish in colour, faintly alkaline in reaction and clots slowly. Its colloidal osmotic pressure is lower than that of plasma but is believed to be higher than that of the tissue fluid. Its hydrostatic pressure is very low. After a fatty food, the lymph of the thoracic duct appears milky due to the presence of minute droplets of emulsified fat absorbed from the alimentary canal.

#### 3. Composition of Lymph:

Microscopic examination of lymph depicts that it contains a large number of leucocytes (mostly lymphocytes) ranging from 500 to 75,000 per cu. mm. No blood platelets present.

The composition of the non-cellular part of lymph (fasting) is as follows:

Total protein content is roughly half that of plasma and varies from 2.0—4.5%. It varies according to the part of the body from which it is collected and also according to the degree of activity of the region. Lymph from the liver contains three times (6%) as much proteins as that coming from the limb (2%).

Lymph from the intestine contains protein which is intermediate between these two (4%). Three varieties of proteins are found— albumin, globulin and fibrinogen. In addition to this, traces of prothrombin are also found. Fibrinogen content is very low. Probably it is due to this, that lymph coagulates very slowly. Albumin is proportionally much more than globulin, as compared with plasma.

The albumin/globulin ratio, which is about 1.5:1.0 in plasma, is much higher in the lymph. The protein content of lymph is higher than that of tissue fluid. But since lymph is derived from tissue fluid, this difference is not easily understood. It has been suggested that water is possibly removed from the lymph as it flows along the lymphatics in this way, the proteins become concentrated. The higher amount of albumin is supposed to be due to its lesser molecular weight and size, and consequently higher rate of diffusion.

In fasting condition fat content is low but after a fatty diet it may be 5.0—15%.

Sugar, 132.2 mgm per 100 ml (Dog’s plasma contains 123.0 mgm per 100 ml on the average).

iv. Other Constituents:

(Expressed in mgm per 100 ml) urea, 23.5 mgm (plasma, 21.7 mgm) non-protein nitrog­enous substance, 34.8 mgm (plasma 32.6 mgm) creatinine, 1.4 mgm (plasma 1.37 mgm) chlorides, 711 mgm (plasma 678 mgm) Total phosphorus, 11.8 mgm (plasma 22 mgm) inorganic phosphorus 5.9 mgm (plasma5.6 mgm) calcium 9.84 mgm (plasma 11. 7 mgm). Enzymes and antibodies are also present.

From the above, the difference between plasma and lymph may be noted. Protein, calcium and total phos­phorus are lower than in plasma. Chlorides and glucose are distinctly higher. Other constituents are also higher than in plasma to some extent.

#### 4. Functions of Lymph:

It supplies nutrition and oxygen to those parts where blood cannot reach.

It drains away excess tissue fluid and the metabolites and in this way tries to maintain the volume and composition of tissue fluid constant.

iii. Transmission of Proteins:

Lymph returns proteins to the blood from the tissue spaces.

iv. Absorption of Fats:

Fats from the intestine are also absorbed through the lymphatics.

The lymphocytes and monocytes of lymph act as defensive cells of the body. The lymphatics also remove bacteria from tissues.

#### 5. Rate of Flow in Lymph:

Rate of flow of lymph along the human thoracic duct is from 1.0—1.5 ml per minute. In dogs it is much higher. Lymphatogogue is the substance that increases the rate of lymph flow.

Regulation of the rate of lymph flow depends upon:

(c) Intrathoracic pressure, and

#### 6. Formation of Lymph:

Since lymph is formed from tissue fluid, anything that increases the amount of tissue fluid will increase the rate of lymph formation. Lymph formation depends upon physical factors. There is no vital secretory process involved in it.

The following factors are responsible for lymph formation:

If the capillary pressure is raised, the rate of lymph formation increases. This is seen in venous obstruction. [But after some time, the rate slows down due to increased accumulation of fluid in the tissue spaces and the consequent rise of hydrostatic pressure of the tissue fluid.]

2. Permeability of the Capillary Wall:

Under any condition, where the permeability of the capillary wall is increased, more tissue fluid will be formed and consequently more lymph.

The following factors increase capillary permeability:

i. Rise in Temperature:

Increased temperature of a particular locality increases capillary permeability.

ii. Substances acting directly on the Capillary Wall:

Peptone, foreign proteins, histamine and extracts of straw­berries, crayfish, mussels, leech, etc., exert an injurious effect upon the capillaries and thereby increase their permeability.

iii. Reduced Oxygen Supply:

Under conditions of oxygen lack lymph flow increases due to higher permeabil­ity of the vessels. It acts probably by damaging the capillary endothelium. Anoxia, anaemia, stasis of blood due to vascular congestion, produces such results.

3. Substances that Alter the Osmotic Pressure:

Anything that reduces the colloidal osmotic pressure of blood will increase the formation of tissue fluid and lymph. Normal or hypotonic saline, when given intravenously, will dilute the plasma colloids and reduce the osmotic pressure. Moreover, blood pressure will be raised. Both these factors will favour formation of tissue fluid and lymph. Hypertonic solutions will exert the same effect in a better way.

Hypertonic solutions, introduced in the blood, will draw in more fluid from the tissue spaces at first and will increase the blood volume further. Blood pressure will be raised to a great extent, and plasma colloids will be further diluted. In this way hypertonic solutions will increase the formation of lymph much more than the normal or hypotonic solutions. Solutions of NaCl, glucose, Na2SO4, etc., may be used for this purpose.

4. Increased Metabolic Activity of an Organ:

Increased activity of a particular area increases the flow of lymph in the locality.

i. Formation of more metabolites which increase the osmotic pressure of the tissue fluid.

ii. Local vasodilatation and increased capillary pressure and permeability.

iv. Increased temperature of the locality.

The last two also act by increasing the capillary permeability.

5. Massage and Passive Movements:

These increase lymphatic flow to some extent just like active muscular contraction.

#### 7. Circulation of Lymph:

In the frogs circulation is maintained by rhythmically contracting lymph hearts. But in higher animal no such pump exists and the flow is maintained by a number of physical factors only.

In the tissues, the pressure of lymph (8 to 10 mm of Hg) is higher than that in the thoracic duct (0 to 4 mm Hg).

ii. Presence of Valves:

Presence of valves in the lymphatic channels helps to maintain the flow in one direction.

Muscular (skeletal) contraction, active or passive, compresses the lymphatic vessels and carries the lymph onwards because the valves within lymphatics prevent backflow. Contraction of the villi helps to pump the chyle from the central lacteals into the basal lymphatics. This lymph is carried up to the cisterna chyli by the help of the intestinal movements.

iv. Respiratory Movements:

During inspiration, due to the descent of the diaphragm intrathoracic pressure falls, which thereby sucks in lymph into the thorax? Moreover, intra-abdominal pressure rises. This com­presses the cisterna chyli, so that lymph flow through the thoracic duct is increased. These pressure chang­es during inspiration are very important factors in maintaining lymphatic circulation.