11.2 Renal Organs

Image 11.2a shows a diagram of the major organs of the urinary system. Identify all of the labels, tracing the path of the urine from the kidneys to the urethra. Note the direct link of the renal arteries and renal veins into the aorta and vena cava respectively. This direct plumbing of the kidney vessels into the largest blood vessels of the body allows both kidneys to process an enormous (about 180 liters) of blood per day. Normally, both kidneys draw from 20-25% of total cardiac output in order to produce the 1500 ml. of daily urine output. Anything that can significantly impede this high volume renal blood flow can seriously retard kidney function, induce ischemia, and degenerate the kidneys themselves. Image 11.2b shows an example of ischemic atrophy of the kidney. Although both kidneys are affected, note the serious degeneration of the kidney seen on the right, and also observe the general atherosclerotic invasion of the aorta in the center of the specimen.

11.3 Kidneys

The kidneys are paired brown-colored organs located on the posterior wall of the abdominal cavity, one on each side of the vertebral column. Adult kidneys are approximately 5" long, 3" wide, and 1" deep and they extend from the level of the eleventh or twelfth thoracic vertebra to the third lumbar vertebra. Because of the presence of the liver, the right kidney is generally slightly lower than the left. The kidneys are located in a retroperitoneal position which allows surgical exposure through the posterior body wall, without forcing entry into the peritoneal cavity. Each bean-shaped kidney is also capped by a pyramidal-shaped endocrine gland called the adrenal, or more graphically, the suprarenal gland.

The kidneys are surrounded by three layers of tissues. The innermost layer, which covers the surface of the kidney, is the fibrous renal capsule, which can be seen as a thin light-colored line on the right side of the cut kidney specimen in image 11.3a. The outer two layers have been removed in this specimen. In its normal position, each kidney is also surrounded by a protective mass of peri-renal fat, called the adipose capsule, part of which can be seen as fat tissue enveloping the renal pelvis and ureter at the bottom of the kidney specimen. The third tissue layer that covers the kidney is a double layer of fascia called the renal fascia. The renal fascia surrounds the kidney and the adipose capsule, and completely encloses them, anchoring each kidney to the posterior abdominal wall.

Three general regions can be distinguished in each kidney: the cortex, the medulla, and the pelvis. The cortex is the outer layer of the kidney, just deep to the renal capsule (the cortex is the lighter red, outer area in image 11.3a). Extensions of the cortex, called renal columns, pass into the medulla of the kidneys. The medulla is located deep to the cortex and consists of several (up to 18) triangular renal pyramids, which are seen as the darker brownish-red triangles in image 11.3a. The pyramids are oriented so that their broad bases are covered by the cortex and their tips (papillae) project toward the renal pelvis. The pyramids are separated from one another by the cortical renal columns. Blood vessels that supply the cortex and medulla pass through the renal columns. The papilla of each pyramid projects into a funnel-shaped chamber called a minor calyx; however, one minor calyx may receive two or more papilla. Several minor calyces join together to form a major calyx. There are generally 2 or 3 major calyces and up to 13 minor calyces in each kidney. The major calyces join with one another to form the renal pelvis, which is the expanded upper end of the ureter. In image 11.3a, the white areas include the minor and major calyces and the renal pelvis. Urine passes as droplets from tiny pores in the papillae into the minor calyces. From there it travels into the major calyces, the renal pelvis, and finally into the ureter, which carries it to the urinary bladder. The same structures can be viewed in image 11.3b.

Together the cortex and renal pyramids constitute the parenchyma of the kidney. Structurally, the parenchyma of each kidney consists of approximately 1 million microscopic units called nephrons, collecting ducts, and their associated vascular supply. Nephrons are the functional units of the kidney. They help regulate blood composition and form urine. Essentially, a nephron is a renal tubule and its vascular component. It begins as a double-walled cup, called the glomerulus, or Bowman's capsule, Iying in the cortex of the kidney. Image 11.3c shows the glomerulus (arrow in image 11.3c). The area labeled "a" in image 11.3c represents the border between the renal cortex (red to the left) and the renal medulla (pink to the right). Image 11.3d is another view of the cortex of the kidney. Note the numerous round glomeruli (seen as darker purple spots) which are distributed at various levels within the cortex.

11.4 Renal Blood Vessels

Image 11.4a shows a cast of the renal vasculature of the kidney, from which all of the cellular material has been digested away by a strong alkaline solution. Note the renal artery (labeled "RA" in image 11.4a) entering at the hilus of the kidney. It quickly branches into a number of interlobar (labeled "Ilo" in image 11.4a) arteries which then curve to form arcuate arteries (labeled "AA" in image 11.4a) that branch off interlobular arteries (labeled "Ilu" in image 11.4a) into the cortex. These interlobular arteries then branch off afferent arterioles to service the numerous glomeruli capillaries (labeled "Gl" in image 11.4a). From these glomeruli the efferent arterioles carry blood into an extensive capillary plexus (labeled "CP" in image 11.4a) that surrounds the nephron tubules. Once the blood has traversed the capillary network it enters the venous circulation (labeled "VC" in image 11.4a), a series of blood vessels that have identical names to the arteries named above.

Image 11.4b shows a close-up view of a kidney model which illustrates the main vessels. Locate the renal pyramid (labeled "H" in image 11.4b) which is an aggregation of collecting ducts (labeled "C" in image 11.4b) that converge to a nipple-like tip (labeled "D" in image 11.4b), called the renal papilla, that releases urine into the minor calyx (labeled "E" in image 11.4b). Next to the minor calyx, an interlobar artery and vein (labeled "I" in image 11.4b) pass blood upward into the renal cortex. At the cortico-medullary junction the interlobar vessels arc to form the arcuate arteries and veins (labeled "B" in image 11.4b). From the arcuate artery a perpendicular vessel, the interlobular artery (labeled "A" in image 11.4b), penetrates into the cortex to service the nephrons with blood. Interlobular veins (such as the blue vessel next to "J") drain blood back into arcuate veins (labeled "B" in image 11.4b), interlobar veins (labeled "I" in image 11.4b), and finally to the renal vein to leave the kidney. Note the presence of glomeruli or renal corpuscles (labeled "G" in image 11.4b) in the cortex, many of which drop loops of Henle (labeled "K" in image 11.4b) down into the renal pyramids of the medulla. These loops ultimately pass the filtrate into terminal collecting ducts (labeled "F" in image 11.4b) which then release the fully processed urine out of the papilla and into the calyx.

11.5 Renal Corpuscle

The renal corpuscle is the part of the nephron that initially filters the blood, forming a filtrate that eventually becomes urine. The renal corpuscle consists of two parts, the glomerulus and the glomerular capsule. The glomerulus is a ball-like clump of capillaries in the center of the renal corpuscle, as seen in the electron microscope photo of image 11.5a. The numerous capillaries are their exposed lumens (labeled "Lu" in image 11.5a). The glomerular (Bowman's) capsule is the portion of the nephron that encloses the glomerulus like a hand wrapped around a ball. This capsule is always located in the cortex of the kidney and is the first part of a nephron. The inner and outer walls form a cavity called the capsular space (labeled "CS" in image 11.5a). The outer layer of the glomerular capsule is the parietal layer (labeled "PL" in image 11.5a), and is composed of simple squamous epithelial cells which have a thin basement membrane. The inner visceral layer is composed of specialized epithelial cells, called podocytes, which surround the glomerular capillaries. Outside of the renal corpuscle you can identify the afferent arteriole (labeled "Ar" in image 11.5a) which brings blood to the glomerulus, the polkissen cells (labeled "Po" in image 11.5a), and the distal convoluted tubule (labeled "DCT" in image 11.5a).

Image 11.5b shows a light microscope slide of a glomerulus surrounded by Bowman's capsule (labeled "a" in image 11.5b). Note that the outer membrane of Bowman's capsule (tip of arrow in image 11.5b) is composed of a thin simple squamous cell layer. Also note the presence of a proximal convoluted tubule (labeled "c" in image 11.5b) and a distal convoluted tubule (labeled "b" in image 11.5b) in this section.

11.6 Bowman's Capsule

Study the model of Bowman's capsule shown in image 11.6a. Bowman's capsule is a filtrate-collecting bag (labeled "E" in image 11.6a) that is tightly applied to the glomerular capillaries (labeled "G" in image 11.6a) of the nephron. These capillaries are fed blood by the afferent arteriole (labeled "A" in image 11.6a) and drained by the efferent arteriole (labeled "C" in image 11.6a). The capsule consists of an outer simple squamous membrane, the parietal layer (labeled "D" in image 11.6a); the lumen (labeled "E" in image 11.6a) which receives the filtrate; and the internal visceral layer (labeled "F" in image 11.6a) which, together with the glomerular capillary walls, forms the filtering membrane of the renal corpuscle. Note that this visceral layer is formed of interlacing, white, spider-like cells called podocytes. These podocytes have a highly branching cell structure.

In image 11.6b note the large bulbous cell body (labeled "CB" in image 11.6b) which radiates many primary branches (labeled "PB" in image 11.6b) over the surface of the capillaries. Also note how these primary branches further subdivide into secondary branches (labeled "SB" in image 11.6b) and then into tertiary branches (visible to the right of "SB") and finally into terminal branches, called pedicels (branching out from the tertiary process in the bottom center of image 11.6b). This interlacing of primary, secondary, tertiary, and pedicel branches by adjacent podocytes produces a covering over the capillary wall that allows the formation of slit-like pores (seen in between the pedicels). These slit-like pores, called "filtration slits", together with the capillary wall restrict the passage of all large molecules (like the large blood proteins with molecular weights greater than 40,000), but allow the passage of smaller solutes (like salts, glucose, urea, etc.) and water into the filtrate of Bowman's capsule.

11.7 Glomerulus

image 11.7a shows a model of the renal corpuscle in which the glomerular capillaries have been exposed by dissecting off the visceral layer of Bowman's capsule. The podocytes of the visceral layer form one essential part of the capsular filter in the nephron by providing filtration slit-like pores; the second major contribution to the filter is provided by the glomerular capillary membranes. The glomerular membrane has two major components: (1) a thin fenestrated epithelium, called endothelium, and (2) a non- cellular basal lamina underlying the endothelium.

An inner surface view of a glomerular capillary can be seen in image 11.7b as a scanning electron micrograph. The thickened regions (labeled "Th" in image 11.7b) of the endothelial cells show extensive branching to produce a surface peppered by very small (500-1000 Angstrom units in diameter) polygonal-shaped pores, called "fenestrations" (labeled "Fe" in image 11.7b) which makes the entire endothelial layer a miniature sieve. The basal lamina (which underlies the endothelium) also provides a filtering function. This basal layer, which is approximately 2500 Angstrom units in thickness, consists of fine fibrils of unpolymerized collagen embedded in an amorphous glycoprotein matrix.

11.8 Renal Tubules

Image 11.8a shows a model of the nephron that will allow you to study the major tubules that comprise it. The renal corpuscle is seen on the upper left of the image. The red-colored glomerulus is shown surrounded by a gray-colored Bowman's capsule. Filtrate is passed from the blood into Bowman's capsule and then into the proximal convoluted tubule (seen here as a twisted, dusty white tube attached to the right of the capsule). After passing through the proximal tubule, the filtrate moves downward along the descending limb of the loop of Henle, around the loop itself, and up the ascending limb (note in this model that the descending limb appears dusty white in its thicker portion and then darkens as it thins towards the bottom; the actual loop is out of view; and the ascending limb thickens as it rises to ultimately fuse with the distal convoluted tubule-seen here as a highly coiled green tubule). After the filtrate has traversed the distal tubule, it enters the collecting duct (seen here as a bright white vertical tube) which drains numerous nephrons. Also note on this model the presence of a "cortical" nephron located to the right of the "medullary" nephron just described. Observe that the cortical nephron (which is shown covered with its extensive peritubular capillary network) has a short loop of Henle and, consequently, cannot concentrate urine as efficiently as the medullary type. Once the glomerular filtrate exits the Bowman's capsule, it drains into the proximal convoluted tubule.

Image 11.8b shows a light microscope slide of the major tubules seen in the medulla. Locate the proximal convoluted tubule (labeled "a" in image 11.8b) as most of the twisted tubule surrounding the glomerulus in image 11.8a; the distal convoluted tubule (labeled "b" in image 11.8b), and the collecting duct (labeled "c" in image 11.8b). The epithelial cells making up the proximal tubule are cuboidal, and the surfaces that face the lumen of the tubule are lined with microvilli, forming a brush border. The microvilli enormously increase the epithelial surface area over which transport can take place. The proximal tubule is the site for the reabsorption of many substances filtered from the blood, such as water, sodium, potassium, chloride, glucose, some amino acids and polypeptides, and bicarbonate ions.

After passing through the proximal convoluted tubule, glomerular filtrate enters the loop of Henle. The loop is composed of descending and ascending limbs of cuboidal epithelium connected by a thin limb of simple squamous epithelium. In image 11.8c, the thin limb of the loop of Henle (labeled "TLH" in image 11.8c, lies adjacent to several capillaries and the vasa rectae (labeled "VR" in image 11.8c) of the peritubular capillary network. After the descending (thin) limb extends into the medulla of the kidney, it makes an abrupt upward U-turn and widens again. This ascending (thick) limb passes through the medulla back into the cortex. The purpose of this looping is to generate a concentration gradient which allows the formation of concentrated urine. After the filtrate passes through the entire loop, it moves into the distal convoluted tubule in the cortex. The cuboidal epithelial cells of the distal tubule are similar in size to those of the proximal tubule, but they have very few microvilli. The cells are abundantly supplied with mitochondria near their basal surfaces, where potassium and hydrogen ions are actively transported into the glomerular filtrate. Finally, the filtrate passes from the distal convoluted tubule into the collecting duct.

Image 11.8d is a cross section view of a collecting duct and note the columnar shape to the cells and the basally located nuclei. Actually, a single collecting duct receives the distal tubules of many nephrons as it travels down through the cortex and medulla. The collecting ducts join to form larger and larger tubes until they reach the minor calyx. From there, the final filtrate (now called urine) drains into the renal pelvis that acts as a terminal funnel directing the urine into the ureter.

11.9 Juxtaglomerular Apparatus

Image 11.9a shows a diagram of the juxtaglomerular apparatus, a special anatomical arrangement of cells found in numerous nephrons in each kidney. In these nephrons the distal convoluted tubule presses tightly against the afferent arteriole and the cells of these two structures can communicate. This image shows a cross sectional view of these two structures. On the left of image 11.9a is the afferent arteriole which, along its contact surface to the distal convoluted tubule (on the right of image 11.9a), shows specially modified smooth muscle cells, called "juxtaglomerular cells". These cells are thicker than the normal smooth muscle fibers, are modified for secretion, and are in close proximity to the special "macula densa" cells of the distal convoluted tubule. It is believed that the juxtaglomerular apparatus is a special autoregulation device for the nephron that allows it to control the rate and content of its filtrate processing (and thereby overall urine formation). If too much filtrate fluid is passing through the distal convoluted tubule or too much sodium is being filtered, the macula densa cells sense this aberration and inform the neighboring juxtaglomerular cells which, in turn, secrete chemicals that will vasoconstrict the afferent arterioles of nephrons and also enhance their sodium retention. In this way nephrons can reduce their filtrate processing and reduce their sodium excretion rates.

In image 11.9b, you can see a close-up dissected view of the juxtaglomerular apparatus on a model. The afferent arteriole (labeled "A" in image 11.9b) has been dissected open, revealing numerous juxtaglomerular cells (labeled "B" in image 11.9b), which are the brown cells with white nuclei and dark black nucleoli. They lie in close proximity to the macula densa cells of the distal convoluted tubule (labeled "H" in image 11.9b) which are clearly taller than most of the cells of the distal convoluted tubule.

Image 11.9c is an actual low-power slide of the macula densa cells (labeled "a" in image 11.9c). The afferent arteriole entering the glomerulus can be seen just above and to the left of these cells and the distal convoluted tubule (labeled "b" in image 11.9c) is also visible. Image 11.9d is a highly magnified view of macula densa cells (seen on an oblique section in the middle of the slide as a pink mass of cells showing spherical nuclei). Also note the afferent arteriole above and to the left of the macula densa cells. This blue-colored afferent arteriole delivers blood to the glomerulus seen on the far left of the slide. Image 11.9e is a close-up view of the juxtaglomerular cells (arrow in image 11.9e). They are in close contact with macula densa cells (seen with dark blue-ringed nuclei between the juxtaglomerular cells and the lumen (labeled "a" in image 11.9e) of the distal convoluted tubule.

11.10 Ureters

The body has two ureters-one for each kidney. Each ureter is an extension of the pelvis of the kidney and extends 10 to 12 inches to the urinary bladder. As the ureters descend, their thick walls increase in diameter, but at their widest point they measure less than l inch in diameter. Like both of the kidneys, the ureters are retroperitoneal in placement. Three coats of tissue form the wall of the ureters (see electron microscope scan in image 11.10a). The inner coat, or mucosa, is mucous membrane with transitional epithelium (labeled "TE" in image 11.10a). It is supported by an underlying lamina propria (labeled "LP" in image 11.10a) that contains numerous blood vessels (labeled "BV" in image 11.10a). Throughout most of the length of the ureters, the second or middle coat, called the muscularis (labeled "ML" in image 11.10a), is composed of inner longitudinal and outer circular layers of smooth muscle. Peristalsis is the major function of this muscularis layer. The third, or external, coat of the ureters is a fibrous layer, called the adventitia (labeled "Ad" in image 11.10a), which also contains larger divisions of blood vessels.. Extensions of this fibrous coat anchor the ureters in place.

Image 11.10b shows a light microscope cross section slide of a ureter. The arrow in image 11.10b points to the epithelium which is transitional (allowing for stretch). Surrounding the epithelium is a connective tissue and muscle layer (dark red) and an outer thin tunica. Image 11.10c is a more magnified view of the ureter epithelium. It is seen here as extensively folded, projecting numerous folds into the white-colored lumen. Note that the epithelium is multicellular; studded with small, dark, purple nuclei.

11.11 Urinary Bladder

The urinary bladder is the hollow, muscular sac that collects urine from the ureters and stores it until it is excreted from the body through the urethra. The bladder is located on the floor of the pelvic cavity, and like the kidneys, it is retroperitoneal. image 11.11a is a saggital section through a female model and the bladder is seen as a flattened, horizontal, red-colored structure located above and to the right of the pubic bone (the oval blue structure on the left) and just below and to the left of the curved uterus (similarly colored to the bladder). Note also the short urethra that drains urine from the bladder down to the vestibule area of the vulva (blue). Posterior to the opening of the urethra is the opening of the vagina, seen as a long vertical tube leading up to the uterus.

The inner lining of the bladder, which is called the mucosa, is highly folded, as seen in the electron microscope photo of image 11.11b. The mucosa of the urinary bladder consists of transitional epithelium in association with an underlying lamina propria. Image 11.11c is a low-power view of the bladder wall. The gray epithelium is seen at the top of the slide with a blue-colored submucosa supporting it underneath. The extensive smooth muscle layer of the bladder wall, called the "detrusor" muscle, is seen in a deep purple color with white adipose tissue separating it from the submucosa above.

Image 11.11d is a very high magnification of the transitional epithelium of the urinary bladder. Note the multicellular nature of the epithelium and that the cells enlarge as they approach the surface of the lumen. Here they show the typical "pear" shape of this type of epithelium.