In contrasting urine and filtrate by the time it reaches the collecting ducts, it could be said that

The urinary tract is one of the systems that our bodies use to get rid of waste products. The kidneys are the part of the urinary tract that makes urine (pee). Urine has salts, toxins, and water that need to be filtered out of the blood. After the kidneys make urine, it leaves the body using the rest of the urinary tract as a pathway.

What Are the Parts of the Urinary Tract?

People usually have two kidneys, but can live a normal, healthy life with just one. The kidneys are under the ribcage in the back, one on each side. Each adult kidney is about the size of a fist.

Each kidney has an outer layer called the cortex, which contains filtering units. The center part of the kidney, the medulla (pronounced: meh-DUH-luh), has fan-shaped structures called pyramids. These drain urine into cup-shaped tubes called calyxes (pronounced: KAY-luh-seez).

From the calyxes, pee travels out of the kidneys through the ureters (pronounced: YUR-uh-ters) to be stored in the bladder (a muscular sac in the lower belly). When a person urinates, the pee exits the bladder and goes out of the body through the urethra (pronounced: yoo-REE-thruh), another tube-like structure. The male urethra ends at the tip of the penis; the female urethra ends just above the vaginal opening.

What Do the Kidneys Do?

Kidneys have many jobs, from filtering blood and making pee to keeping bones healthy and making a hormone that controls the production of red blood cells.

The kidneys also help regulate blood pressure, the level of salts in the blood, and the acid-base balance (the pH) of the blood. All these jobs make the kidneys essential to keeping the body working as it should.

How Do the Kidneys and Urinary Tract Work?

Blood travels to each kidney through the renal artery. The artery enters the kidney at the hilus (pronounced: HY-luss), the indentation in middle of the kidney that gives it its bean shape. The artery then branches so blood can get to the nephrons (pronounced: NEH-fronz) — 1 million tiny filtering units in each kidney that remove the harmful substances from the blood.

Each of the nephrons contain a filter called the glomerulus (pronounced: gluh-MER-yuh-lus). The fluid that is filtered out from the blood then travels down a tiny tube-like structure called a tubule (pronounced: TOO-byool). The tubule adjusts the level of salts, water, and wastes that will leave the body in the urine. Filtered blood leaves the kidney through the renal vein and flows back to the heart.

Pee leaves the kidneys and travels through the ureters to the bladder. The bladder expands as it fills. When the bladder is full, nerve endings in its wall send messages to the brain. When a person needs to pee, the bladder walls tighten and a ring-like muscle that guards the exit from the bladder to the urethra, called the sphincter (pronounced: SFINK-tur), relaxes. This lets pee go into the urethra and out of the body.

What Can Help Keep the Kidneys and Urinary Tract Healthy?

To help keep your kidneys and urinary tract healthy:

• Get plenty of exercise.
• Eat a nutritious diet.
• Stay hydrated.
• For girls: Wipe from front to back after pooping so germs don't get into the urethra.
• Avoid bubble baths, sitting in the tub after shampoo has been used, and scented soaps. These can irritate the urethra.
• Wear cotton underwear.
• Promptly change out of wet bathing suits.
• Go for regular medical checkups.
• Talk to your doctor before taking any supplements or herbal treatments.
• Let the doctor know about any family history of kidney problems, diabetes, or high blood pressure.
• Let the doctor know if you have any swelling or puffiness, have pain with peeing, need to pee often, have foamy urine or blood in the urine, or are constipated.

Histology Study Guide
Kidney and Urinary Tract

Kidneys excrete urine.  They do this by producing a modified filtrate of blood plasma. 

Note that renal physiology and pathology cannot be properly understood without appreciating some underlying histological detail. 

The histological composition of kidney is essentially that of a gland with highly modified secretory units and highly specialized ducts.  [For an overview of glandular organization, see glands in the introductory unit.]

Please note that this is an ancillary resource, NOT a substitute for textbooks or for time spent studying real specimens with a microscope.  If you use this on-line study aid, please refer to your textbooks and atlases and resource sessions for richer, more detailed information. 

Click here or scroll down to continue this introduction to kidney.  To go directly to specific topics, see the table below.

Essential microanatomy of kidney, from Kölliker's 1852 Handbuch der Gewebelehre.  Vasculature is shown on left side, epithelial tubules on right.

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Renal pathology

CLINICAL NOTE:  From a clinical perspective, the most significant histological feature of the kidney is probably the renal corpuscle.  Several pathological processes interfere with glomerular filtration and thus have a critical impact on kidney function.  Diagnosis of renal pathology often involves biopsy of renal cortex and careful examination of renal corpuscles.  Corpuscle details such glomerular basement membranes, podocytes, and mesangial cells can be revealed by several special stains as well as by electron microscopy. 

The thumbnail to right links to some images of membranoproliferative glomerulonephritis.   

External links to WebPath provide additional information and images:

• WebPath electron micrographs of renal pathology
  • "Minimal change disease (MCD) with effacement of podocyte foot processes."  Note that podocytes form a continuous surface over the capillary; filtration slits are missing.
  • "Membranous glomerulonephritis with thickened glomerular basement membrane containing electron dense deposits."
     
• WebPath tutorial on renal cystic disease.
• WebPath tutorial on urinalysis (including brief descriptions of how urinary casts are related to glomerular and tubular function)
• Additional renal pathology at WebPath.  (This page is worth an extended visit, after you have become comfortable with basic kidney histology.)
  

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Overview of kidney histology

The essential tissue composition of kidney is that of a gland with highly modified secretory units and highly specialized ducts.  If you are not yet familiar with basic patterns of tissue organization, you might find this page to be helpful.  You needn't master all the details of glands before you study kidney, but it might be helpful to get a sense of how epithelial cells arrange themselves to form the basic structure of secretory units and ducts.

Image contrast, gland vs. kidney
(click on any thumbnail for enlarged comparison)

gland
kidney

 

	Gland (alveoli and ducts)      The bulk of a typical exocrine gland, such as a salivary gland, consists of secretory epithelial cells.  These cells form secretory units, called acini, which drain their secretory product into a branching tree of ducts.      In this cartoon, orange color represents acini, blue color indicates the short striated ducts, where secretory product is concentrated by active reabsorption, and lavender color indicates interlobular and interlobar ducts, whose function is more passive.	salivary gland
	Kidney (corpuscles and tubules)      In contrast, the bulk of the kidney consists of tubules.  The secretory units of the kidney, called renal corpuscles, comprise a relatively small proportion of the organ.  Most of the kidney consists of highly specialized renal tubules, which correspond to the duct tree of a typical gland.  Together, one renal corpuscle and its associated tubule is called a nephron.      In this cartoon, orange color represents renal corpuscles, blue color indicates tubules associated with each corpuscle, whose function is analogous to that of striated ducts, and lavender color indicates collecting ducts, whose function is more passive.	renal corpuscles renal tubules
	Glandular alveolus In a typical exocrine gland, each acinus uses raw materials supplied by blood to manufacture a secretory product.  As this product drains away, it may be concentrated by a striated duct.	pancreas
	Kidney corpuscle In the kidney, each renal corpuscle is essentially just a highly modified secretory acinus.  However, each corpuscle "secretes" a filtrate of blood plasma which drains into its associated renal tubule.	renal corpuscle
	Kidney tubules Renal tubules have wiggly cortical portions, called convoluted tubules, and straight medullary portions, called loops of Henle and collecting ducts.  These different segments carry out different aspects of filtrate modification.  The cortical tubules, analogous to striated ducts, modify the filtrate by reabsorbing everything that is not waste.	renal cortex
	Everything in the paragraphs above is described in greater detail further down this page.

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Cortex and Medulla

This diagram is not drawn to scale.  Each kidney comprises hundreds of thousands of nephrons.

A gross section of the kidney reveals that the outer cortex has a somewhat different texture from the deeper medulla.  This difference reflects the disposition of various portions of the many nephrons which comprise the kidney.  

• The cortex consists of convoluted tubules together with the renal corpuscles. 
   
• The medulla consists of loops of Henle and collecting ducts.
   
  • The medulla may be divided into "zones" or "stripes," visible grossly, which reflect the structural differentiation of the tubules that form the loops of Henle.
     
  • The medulla has a remarkable interstitial environment, hypertonic and poorly oxygenated.  (Consult your physiology resources for more info.)
     
• The cortex and medulla together comprise millions of individual nephrons, all packed together.
     
• The cortex and medulla surround and drain into the hollow pelvis, the funnel-shaped beginning of the ureter.  Like the ureter, the pelvis is lined by transitional epithelium. 

renal cortex	renal medulla	renal pelvis

The cortex / medulla organization has considerable functional significance, reflected in the arrangement of renal tubules and vasculature.

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Lobes and lobules

(Renal lobes and lobules are details, of little clinical significance.  Awareness of their existence does help explain some of the terminology for describing renal vasculature.  Their existence also reflects embryonic development.)

Each kidney lobe consists of a medullary pyramid (a roughly pyramidal region that projects into the pelvis) and its associated cortex.  Lobes are visible to gross inspection of a sectioned kidney.  Medullary pyramids are separated from one another by cortical tissue that extends deeper into the kidney, the so-called "columns of Bertin" (named after E.-J. Bertin, b. 1712).

A renal lobule is defined as a portion of the kidney containing those nephrons that are served by a common collecting duct.  

Lobules are generally inconspicuous, even to microscopic inspection. 

Lobules are centered on "medullary rays," bundles of straight tubules (collecting ducts and loops of Henle) which resemble the substance of the medulla but extend into the cortex.

A complementary grouping of nephrons is provided by the distribution of afferent arterioles from a common interlobular artery.  The interlobular artery ascends through the cortex between lobules and sends out afferent arterioles to nearby glomeruli.

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The NEPHRON

The nephron is the functional unit of the kidney.  Each nephron consists of one renal corpuscle and its associated tubule.  The kidney as a whole consists of many nephrons (millions) with their associated blood vessels.

The renal corpuscles are the sites where the process of urine formation begins with a filtrate of blood plasma.

Renal tubules are differentiated into several segments.  In the diagram to right, click on any tubule segment for more detailed information.

Each of these nephron components is described in greater detail below.

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The RENAL CORPUSCLE The most distinctive microscopic feature of the kidney is the renal corpuscle.  Renal corpuscles produce a filtrate of blood plasma.  (For more on this glomerular function, see below.  Also see CLINICAL NOTE, above.) Historical note:  Renal corpuscles may also be called "Malpighian corpuscles," after Marcello Malpighi (b. 1628), the researcher who introduced microscopy to medicine.

Click on image for enlargement.

Examples of renal corpuscles H&E stain PAS stain

Each renal corpuscle has several parts.  In the diagram to right, click on any feature for more detailed information, or continue down the page.

Bowman's capsule is the outer, epithelial wall of the corpuscle.   Bowman's space, also called "urinary space," is the space (white in the diagram) lying within Bowman's capsule.   The glomerulus, comprising several distinct elements, is the conspicuous "little knot" which occupies most of the corpuscle.   Glomerular capillaries have an endothelium that is fenestrated (full of holes).   Podocytes are epithelial cells covering the glomerular capillaries.   Immediately adjacent to each glomerular capillary, between the podocytes and the capillary endothelium, is the filtration membrane (unlabelled red line on this diagram).   Mesangium is a supporting tissue consisting of mesangial cells and matrix (green in the diagram).   The beginning of the proximal tubule (blue in the diagram) is the "drain" carrying fluid away from Bowman's space.

Each of these glomerular components is described in greater detail below.

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BOWMAN'S SPACE and BOWMAN'S CAPSULE

Historical note:  Bowman's capsule and Bowman's space are both named after English surgeon William Bowman.  At right is an image of Bowman's 1842 diagrammatic drawing of basic renal cortical structure.  Click on this image for additional historical information.

Bowman's space, also called the urinary space, is the space within Bowman's capsule surrounding the loops and lobules of the glomerulus.  This is the space into which the glomerular plasma filtrate collects as it leaves the capillaries through the filtration membrane. 

Bowman's capsule is the outer epithelium which encloses Bowman's space.  This epithelium is simple squamous, becoming cuboidal at the proximal tubule. 

Although Bowman's capsule is rather obviously a simple squamous epithelium, it is less apparent that the glomerulus is also closely enveloped by epithelium.  The peculiar structure of podocytes obscures the fact that they are indeed epithelial cells.  Thus Bowman's space is entirely lined by epithelium.  

The outer, parietal epithelium of the renal corpuscle is Bowman's capsule. 
The inner, visceral epithelium is comprised of podocytes. 

One way to appreciate the essential epithelial construction of renal corpuscles is by examining fetal development.  Click on diagram or image to view a series of micrographs of developing renal corpuscles.

Each renal corpuscle is roughly spherical and has two "poles," not necessarily at opposite ends of the corpuscle. 

A random section across a renal corpuscle will only rarely display either pole.  Yet, as you might have noticed, images of renal corpuscles in this website (and elsewhere) commonly include both poles.  These micrographs were taken during hours of searching across various slides, with examination of many, many corpuscles, to find those which had been sliced through both poles by a fortuitous plane of section.

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The GLOMERULUS The glomerulus ("little ball of yarn") is essentially a small knot of capillaries and supporting structures suspended within Bowman's capsule.  The glomerulus is the source of the initial filtrate of plasma that is eventually processed into urine.  With this function, the glomerulus is arguably the most significant component of the nephron. Examples of glomeruli H&E stain PAS stain

Several elements comprise the glomerulus.  Click on any feature for more detailed information, or continue down the page.

In typical histological specimens, it is often difficult to discern the relationships among these elements (i.e., the glomerulus looks like a jumbled mass of cells).  Very thin sections (2µ or less) are much better than routine thick (5-10µm) sections for resolving cell types and their relative positions. 

Electron microscopy is essential for resolving functional details like capillary fenestrations and podocyte filtration slits (consult your histology text or atlas, e.g., Rhodin figures 32-9, 32-10, 32-11).

Several pathological processes modify glomerular structure and thereby interfere with glomerular filtration.  Diagnosis of renal pathology routinely calls for histological examination of glomerular details, including electron microscopy.  See examples from WebPath:

• "Minimal change disease (MCD) with effacement of podocyte foot processes."  Note that podocytes form a continuous surface over the capillary; filtration slits are missing.
• "Membranous glomerulonephritis with thickened glomerular basement membrane containing electron dense deposits."

Examples of glomerular pathology

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Glomerular capillary endothelium 

Introductory note about capillaries.

The endothelium of glomerular capillaries is perforated with many small holes, or fenestrations (from Latin fenestra, window).  Each fenestrated endothelial cell has a shape like a slice of very holey Swiss cheese, rolled into a cylinder to make a segment of capillary.  The fenestrations are too small to allow blood cells through, but plasma can pass freely out of the holes and into the filtration membrane. The capillaries of renal glomeruli are exceptionally leaky.  Although the filtration membrane holds back cells and plasma proteins, the remaining fluid (water, mineral ions, and small molecules) passes freely into Bowman's space and hence along the renal tubule.  Consult your histology textbook and/or atlas (e.g., Rhodin, figure 32-9 , 32-10, and 32-11) for additional detail and electron micrographs of these cells.

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Filtration membrane

Immediately outside the capillary endothelium is the filtration membrane (blue-greenish in the diagram to right).  This membrane represents a fusion of the endothelial basement membrane with the basement membrane of the glomerular epithelium (the podocytes).  Pathology:  Anything which clogs or thickens the filtration membrane can interfere with the passage of fluid and hence reduce the rate of filtration.  See WebPath. As plasma passes through the capillary fenestrations, water, ions, and small molecules pass freely through the filtration membrane into Bowman's space, while serum proteins are retained in the capillaries.  The outside of the filtration membrane is supported by podocytes.

The filtrate which accumulates in Bowman's space drains into the proximal tubule, and hence to the loop of Henle, the distal tubule, and the collecting duct.  In these various segments of the renal tubule, the filtrate is modified into urine, chiefly by reabsorption of non-waste components.  The filtration membrane is not apparent on H&E stained histological specimens but may be demonstrated with PAS or silver stain.  Electron microscopy is the best way to visualize the filtration membrane.

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Podocytes

Podocytes ("footed cells") are extraordinary (one might say bizarre) epithelial cells which support the filtration membrane without obstructing the flow of filtrate.  Each podocyte stands upon branched pedicels, or "foot processes," which rest on the filtration membrane.  Between adjacent pedicels are gaps called filtration slits which permit free passage of fluid filtrate into Bowman's space. In mature renal corpuscles, the epithelial nature of podocytes is not obvious.  Nevertheless, observation of fetal development reveals that podocytes form from the epithelial lining of Bowman's capsule. In typical histological preparations, podocyte nuclei tend to be oval and fairly euchromatic.  They may be recognized because they look more "epithelial" than those of capillary endothelial cells or mesangial cells, and because they lie immediately adjacent to Bowman's space (no intervening tissue). Consult your histology textbook and/or atlas (e.g., Rhodin, figure 32-9 , 32-10, and 32-11) for additional detail and electron micrographs of these cells.   Pathology:  Anything which occludes filtration slits or Bowman's space can interfere with the passage of fluid and hence reduce the rate of filtration.   Fusion of adjacent pedicels can block filtration (e.g., see WebPath), as can occlusion of Bowman's space caused by proliferation of any of the cell types of the renal corpuscle (podocytes, mesangial cells, Bowman's capsule cells, capillary endothelial cells).
This simplified diagram is not drawn to scale, and does not show the filtration membrane.

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Mesangial cells and matrix

Glomerular mesangial cells are inconspicuous and rather non-descript cells concentrated toward the vascular pole of the glomerulus.  These cells produce the mesangial matrix and may contribute to maintenance of the filtration membrane.

Mesangial cell nuclei may sometimes be recognized as small, irregularly shaped, and rather heterochromatic nuclei within the glomerulus.

Consult your histology textbook and/or atlas (e.g., Rhodin, figure 32-9 and 32-10) for additional detail and electron micrographs of these cells.  

Exra-glomerular mesangial cells, also called lacis cells or cells of Goormaghtigh (commemorating Norbert Goormaghtigh, b. 1890) are found in the space between the glomerulus and the macula densa of the distal tubule; they form part of the juxtaglomerular apparatus.

The mesangial matrix is extracellular material which surrounds the mesangial cells.  Apart from offering some mechanical support to the glomerular capillaries, the function of mesangial matrix is unknown.

The mesangial matrix is not apparent on H&E stained histological specimens, but (like the filtration membrane) it may be visualized with PAS or silver stain. 

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RENAL TUBULES

In each nephron, the renal tubule receives plasma filtrate draining from Bowman's space and processes it into urine. 

The functional differentiation of the different tubular segments is associated with variation in the structure of tubular epithelial cells.  This structural specialization is in turn reflected in the microscopic appearance of the tubules.

Click on any feature for more detailed information, or continue down the page, where each segment is described further.

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Proximal convoluted tubule The initial segment of the tubule is the proximal convoluted tubule.  It is called proximal because it is nearest to the starting point (the renal corpuscle) and convoluted because it twists about (in contrast to the straight segments of tubule which form the loop of Henle).  This segment of the renal tubule restores much of the filtrate to the blood in the peritubular capillaries, by actively pumping small molecules out of the tubule lumen into the interstitial space.  Water then follows the concentration gradient. The length of a proximal convoluted tubule tends to be several times greater than that of a distal convoluted tubule, so sections of proximal tubules are several times more numerous than those of distal tubules in a typical histological slide of renal cortex. The epithelium of the initial descending segment of the loop of Henle is similar to that of the proximal convoluted tubule, and is sometimes called the pars recta of the proximal tubule (in contrast to the convoluted pars convoluta).

The proximal convoluted tubule is lined by a simple cuboidal epithelium whose cells have several characteristic features, each of which affects the appearance of the tubule. The apical end of each proximal tubule cell has a brush border of microvilli.  This provides an increased surface area to accommodate the membrane channels that are responsible for absorbing into the cell small molecules from the filtrate in the tubular lumen.   Appearance: The brush border is seldom plainly visible in routine histological preparations, but proximal tubule cells tend to have indistinct apical ends (in contrast to the more definite apical border of cells comprising distal tubules and collecting ducts).   Proximal tubule cells have a high proportion of mitochondria in their cytoplasm, to provide the energy for pumping ions and molecules against their concentration gradient.   Appearance: The abundance of mitochondria makes proximal tubule cells rather intensely acidophilic.    The plasma membranes of adjacent proximal tubule cells are extensively interdigitated.  This increases the basal membrane surface area available for pumping molecules out the basal end of each cell.   Appearance: As a consequence of such interdigitated cell membranes, boundaries between adjacent proximal tubule cells are inconspicuous (i.e., in section the epithelium looks like a continuous band of cytoplasm with nuclei appearing at irregular intervals). Consult your histology textbook and/or atlas (e.g., Rhodin, figure 32-13 and 32-14) for additional detail and electron micrographs of these cells.

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Loops of Henle (medullary tubules)

The loop of Henle is a remarkable feature of the renal tubule, associated with the remarkable function of the renal medulla in water conservation.  Basically, the loop helps to establish and maintain a hypertonic saline environment in the medulla, via a counter-current exchange process, which in turn allows subsequent recovery of water from collecting ducts (and associated concentration of urine within the collecting ducts).  For further explanation of this process, see your physiology resources.

Historical note:  The loop of Henle is named for Friedrich Henle, b. 1809.

The loop of Henle consists of a descending limb, having an initial short thick segment followed by a long thin segment, and an ascending limb, having a thin segment followed by a thick segment.

Medullary tubules (loops of Henle and collecting ducts)

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Distal convoluted tubule The distal convoluted tubule continues into the cortex from the ascending limb of the loop of Henle.  Like the proximal convoluted tubule, this segment of the tubule is called convoluted because it twists about.  It is distal because it is further "downstream" from the starting point, the renal corpuscle.  Like the proximal convoluted tubule, this distal segment of the renal tubule continues the return of useful materials from the filtrate to the blood in the peritubular capillaries, by actively pumping small molecules out of the tubule lumen into the interstitial space.  Each distal tubule returns to the vascular pole of the renal corpuscle from which the tubule arose.  At this site is a special region of the distal tubule called the macula densa (macula densa = "dense spot," named for the close clustering of epithelial nuclei in the wall of the distal tubule) which is part of the juxtaglomerular apparatus. In typical histological specimens of renal cortex, profiles of distal convoluted tubules tend to be relatively less numerous than those of proximal convoluted tubules, because distal tubules tend to be considerably shorter than proximal tubules.

The distal convoluted tubule is lined by a simple cuboidal epithelium whose cells have several characteristic features, each of which affects the appearance of the tubule. Unlike the proximal convoluted tubule, the apical end of each distal tubule cell does not have a brush border, although there may be scattered microvilli.  Because most of the "heavy lifting" has already been done in the proximal tubule, distal tubule cells are not so highly specialized.   Appearance: The apical ends of distal tubule cells tend to be more distinct than those of proximal tubule cells, conferring the usual appearance of a larger, clearer lumen in each distal tubule.   Like proximal tubule cells, distal tubule cells have a high proportion of mitochondria in their cytoplasm, to provide the energy for pumping ions and molecules against their concentration gradient.    Appearance: However, distal tubule cells are less extremely specialized than those of the proximal tubules.  They have relatively fewer mitochondria than proximal tubule cells and therefore display a lesser degree of acidophilia.    The plasma membranes of adjacent distal tubule cells are extensively interdigitated (as are those of proximal tubules).  This increases the basal membrane surface area available for pumping molecules out the basal end of each cell.    Appearance: As a consequence of such interdigitated cell membranes, boundaries between adjacent distal tubule cells are inconspicuous (i.e., in section the epithelium looks like a continuous band of cytoplasm with nuclei appearing at irregular intervals). Consult your histology textbook and/or atlas (e.g., Rhodin, figure 32-19 and 32-20) for additional detail and electron micrographs of these cells.

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Macula densa / juxtaglomerular apparatus (juxtaglomerular complex)

The juxtaglomerular apparatus is a complex of structures associated with the vascular pole of each renal corpuscle.  The juxtaglomerular apparatus has two principal components: The macula densa is a patch of densely-packed epithelial cell nuclei along the distal convoluted tubule, adjacent to the afferent arteriole at the vascular pole of the corpuscle from which the tubule arose.  It may function as a sensor for sodium and/or chloride concentration.   Juxtaglomerular cells ("J-G cells") in the wall of the afferent arteriole are specialized smooth muscle cells containing secretory granules, the source of the hormone renin. Consult your histology textbook and/or atlas (e.g., Rhodin, figure 32-6 and 32-7) for additional detail and electron micrographs of these cells.     The juxtaglomerular region also includes extra-glomerular mesangial cells, also called cells of Goormaghtigh (commemorating Norbert Goormaghtigh, b, 1890), or lacis cells. The juxtaglomerular apparatus participates in the regulation of blood pressure and the rate blood flow through the glomerular capillaries (and hence the rate of urine formation).

Understanding the detailed histophysiology of the juxtaglomerular apparatus has been a principal achievement of modern nephrology.  For a detailed historical account of this understanding, see "The juxtaglomerular apparatus of Norbert Goormaghtigh -- a critical appraisal," by G. Eknoyan, et al. (2009) Nephrology Dialysis Transplantation, V. 24, pp. 3876-3881 (https://doi.org/10.1093/ndt/gfp503).

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Collecting ducts

Collecting ducts are named because they "collect" the urine from distal tubules. 

Larger collecting ducts may also be called papillary ducts or ducts of Bellini. 

Historical note:  The name "ducts of Bellini" commemorates Lorenzo Bellini, the 17th century Italian anatomist who first described them.

Collecting ducts are lined by simple cuboidal epithelium which appears less specialized than that of the proximal or distal tubules.  Cytoplasm of collecting duct cells is relatively clear (i.e., not as intensely eosinophilic as that of proximal or distal tubules), and cell borders are usually distinct.  Collecting ducts merge and become larger as they descend through the medulla, so different sizes of collecting ducts may be observed at different levels in the kidney, with the smallest in the cortex and the largest near the pelvis.

• Collecting duct epithelium has the unusual physiological property of adjustable permeability to water (under control of the pituitary anti-diuretic hormone, ADH).  If permeability to water is high, then water diffuses across the collecting duct epithelium into the hypertonic interstitium of the medulla, resulting in a concentration of urine within the duct.  But if water permeability is low, then water is retained in the urine and excreted from the body.

Collecting ducts are readily recognized in the renal medulla, as relatively large tubules lined by cuboidal epithelium, in which the epithelial cells are relatively clear (i.e., not as eosinophilic as proximal and distal tubules) and have distinct cell borders.

Medullary tubules (loops of Henle and collecting ducts)

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RENAL VASCULATURE

In most organs the particular disposition of arteries, veins and capillaries serves only to distribute blood uniformly throughout the organ.  In the kidney, however, the arrangement of blood vessels has special functional significance.  Here we will offer a more-detailed-than-usual account of the within-organ arrangement of blood vessels.  (Click here for a general introduction to blood vessels.)

Distributing vessels.  See clinical note for the surgical significance of renal vessel arrangement. Interlobar arteries and veins arise from the renal artery and vein and ascend between lobes (as the adjective interlobar suggests) from the pelvis across the medulla toward the cortex.   Arcuate arteries and veins branch from the interlobar vessels and "arch" (as the adjective arcuate suggests) across the boundary between cortex and medulla. CLINICAL NOTE:  Interlobar and arcuate vessels do not extend into the cortex.  Therefore, to minimize the risk of bleeding during renal biopsy, the biopsy needle should be aimed tangentially through the cortex to avoid damaging one of these larger vessels. Interlobular arteries and veins ascend from the arcuate vessels and pass up into the cortex perpendicular to the surface of the kidney.  These may also be called cortical radial vessels.  "Radial" describes their orientation in the cortex.  "Interlobular" refers to renal lobules, which are an inconspicuous pattern of organization for renal cortex.) In some of our kidney specimens, the distributing vessels display conspicuous atherosclerosis.

Microvasculature.  The kidney has three distinct capillary networks, each with a different function. Afferent arteriole and glomerular capillaries Each glomerulus receives its blood from one afferent arteriole.  (Afferent means "incoming."  The point of reference for "afferent" is the glomerular capillaries.)  Afferent arterioles arise from interlobular arteries. Within each renal corpuscle, glomerular capillaries perform the critical role of expressing a filtrate across the capillary wall and through the filtration membrane.  Blood leaving the glomerulus has been thickened by loss of water and other components; most of these components will be immediately reabsorbed from the proximal tubule into peritubular capillaries.  The wall of the afferent arteriole includes specialized smooth cells which together with the macula densa of the distal tubule comprise the juxtaglomerular apparatus. Efferent arteriole and peritubular capillaries   Blood leaves the glomerulus through a short and inconspicuous efferent arteriole (efferent means "outgoing") and enters either peritubular capillaries or vasa recta. Peritubular capillaries envelope all of the convoluted tubules of the cortex and recover the materials (water, ions, nutrient molecules) which are pumped across the tubular epithelium.  Plasma volume lost from glomerular capillaries is thus almost immediately restored in peritubular capillaries.    Peritubular capillaries eventually return blood to the interlobular veins.   Vasa recta   Vasa recta ("straight vessels") are bundles of thin vessels (but generally larger than capillaries) which carry blood into and out of the medulla.   The parallel clustering of arterial and venous flow creates a counter-current exchange so that blood flow does not erase the osmotic gradient of the medulla.   Vasa recta eventually return blood to arcuate veins.   Stroma of the kidney includes both peritubular capillaries (in cortex) and vasa recta (in medulla).  But this tissue is typically inconspicuous.  Renal stroma can be made more evident with a trichrome stain, which renders more noticeable the collagen associated with these small vessels.

Vasa recta

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Urinary Tract

The tissue composition of renal pelvis, ureters, bladder and urethra is relatively simple, compared to kidney.  Each of these elements has a wall lined by transitional epithelium (urothelium) and containing smooth muscle.

renal pelvis	ureter	bladder	urethra

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Renal Image Index

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SIUC / School of Medicine / Anatomy / David King https://histology.siu.edu/crr/rnguide.htm

Last updated:  30 October 2022 / dgk