The urinary system, composed of the kidneys, ureters, urinary bladder, and urethra, functions in the formation of urine, regulation of blood pressure

and ßuid volume of the body, acid-base balance, and formation and release of certain hormones.

The functional unit of the kidney is the uriniferous tubule (see Graphic 16-1), consisting of the nephron and the collecting tubule, each of which is derived from a different embryologic primordium.

KIDNEY

The kidneys possess a convex and a concave border, the latter of which is known as the hilum. It is here that arteries enter and the ureter, veins, and lypmh vessels leave the kidney. Each kidney has a capsule that has two layers, the outer Þbrous layer and an inner, more cellular layer.

¥Outer Þbrous layer is composed of type I and type III collagen and occasional Þbroblasts

¥The inner layer consists of types I and III collagen and myoÞbroblasts

Each kidney is divided into a cortex and a medulla.

¥The cortex is subdivided into the cortical labyrinth and the medullary rays (see Table 16-1),

The cortical labyrinth is composed of the renal corpuscles and the convoluted tubular portions of the nephron

Each medullary ray is an extension of the renal medulla into the cortex, where it forms the core of a kidney lobule.

Each of the 500 or so medullary rays are composed of pars recta of proximal and distal convoluted tubules as well as of collecting ducts

¥the medulla is composed of 10 to 18 renal pyramids, each of which is said to constitute a lobe of the kidney.

The apex of each pyramid is perforated by 15 to 20 papillary ducts (of Bellini) at the area cribrosa.

The region of the medulla between neighboring renal pyramids is occupied by cortical-like material known as renal columns (of Bertin).

The vascular supply of the kidney must be appreciated to understand the histophysiology of the kidney. Each kidney is supplied by a renal artery, a direct branch of the abdominal aorta. This vessel subdivides into several major branches as it enters the hilum of the kidney, each of which subsequently divides to give rise to two or more interlobar arteries.

•Interlobar arteries pass between neighboring pyramids toward the cortex and, at the corticomedullary junction, give rise to

•arcuate arteries that follow the base of the pyramid.

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¥Small, interlobular arteries derived from arcuate arteries enter the cortical labyrinth, equidistant from neighboring medullary rays, to reach the renal capsule. Along the extent of the interlobular arteries, smaller vessels, known as

•afferent glomerular arterioles, arise, become enveloped by Bowman’s capsule, and form a capillary plexus known as the glomerulus.

Collectively, BowmanÕs capsule and the glomerulus are referred to as the renal corpuscle (see Graphic 16-2).

•Efferent glomerular arterioles drain the glomerulus, passing into the cortex.

In the cortex, they form the peritubular capillary network

In the medulla, they form the arteriae spuriae, a part of the vasa recta.

¥The interstitium of the cortical labyrinth and the capsule of the kidney are drained by interlobular veins, most of which enter the arcuate veins, tributaries of the interlobar veins.

¥Blood from the interlobar veins enters the renal vein, which delivers its blood to the inferior vena cava.

Uriniferous Tubule

The functional unit of the kidney is the uriniferous tubule (see Table 16-1), consisting of the nephron and the collecting tubule, each of which is derived from a different embryologic primordium.

Nephron

There are three types of nephrons, classiÞed by the location of their renal corpuscles in the kidney cortex:

•juxtamedullary nephrons, possessing long, thin limbs of HenleÕs loop,

•cortical (subcapsular) nephrons located just beneath the capsule, and

•midcortical (intermediate) nephrons, whose renal corpuscles are located in the midcortical region.

It is the long, thin limbs of HenleÕs loop that assist in the establishment of a concentration gradient in the renal medulla, permitting the formation of hypertonic urine.

Bowman’s Capsule

¥The nephron begins at Bowman’s capsule, a distended, blindly ending, invaginated region of the tubule.

The modiÞed cells of the inner, visceral layer are known as podocytes. Some of their

primary (major) processes but mainly their secondary processes and terminal pedicels wrap around the glomerular capillaries.

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TABLE 16-1 • Location of the Various Regions of the Uriniferous Tubule

The spaces between adjoining pedicels, known as

filtration slits, are bridged by thin slit diaphragms that extend from one pedicel to the next.

Pedicels are richly endowed with actin Þlaments permitting slight movement of the pedicels to adjust the size of the Þltration slits.

Glomerular capillaries are fenestrated with large pores (60 to 90 nm in diameter) lacking diaphragms (see Graphic 16-2). The endothelial cell membranes possess aquaporin-1 channels designed for the rapid passage of water through them.

A thick glomerular basal lamina (see Table 16-2), manufactured by the podocytes and the endothelial cells of the capillary, is interposed between them.

Interstitial tissue composed of intraglomerular mesangial cells (seeTable 16-3) and extraglomerular

mesangial cells and the extracellular matrix they manufacture is also associated with the glomerulus.

Intraglomerular mesangial cells share the basal lamina of the glomerular capillaries.

¥The ultraÞltrate from the capillaries enters Bowman’s (urinary) space by passing through the filtration barrier and is drained from there by the neck of the proximal tubule (see below).

Proximal Tubule

The proximal tubule has two regions, the convoluted portion (proximal convoluted tubule) and the straight portion (pars recta). The simple cuboidal epithelium of the proximal tubule adjoins the simple squamous epithelium of the parietal layer of BowmanÕs capsule.

¥The simple cuboidal cells of the proximal convoluted tubule possess an extensive brush border (microvilli) on their luminal surface.

Their lateral and basal plasma membranes are considerably convoluted, and the lateral membranes form numerous interdigitations with membranes of adjoining cells.

The exaggerated folding of the basal plasmalemma presents a region rich in mitochondria and provides a striated appearance when viewed with the light microscope.

¥The straight portion, or pars recta, of the proximal tubules is also referred to as the descending thick limb of Henle’s loop. It is histologically similar to the convoluted portion; however, its brush border becomes shorter at its distal terminus, where it joins the descending thin limb of HenleÕs loop.

Henle’s Loop

Henle’s loop is composed of a simple squamous epithelium and has three regions: descending thin limb, HenleÕs loop, ascending thin limb.

TABLE 16-3 • Functions of Intraglomerular

Mesangial Cells

Phagocytosis of glomerular basement membrane and molecules trapped in it (69,000 Da or greater)

Physically support podocytes and their primary and secondary processes

Secretion of cytokines (e.g., PDGF, IL-1)* to facilitate repair of damaged glomerular components

Contractile elements assist in reducing the luminal diameter of glomerular capillaries to increase filtration rate

*PDGF, platelet-derived growth factor; IL-1, Interleukin 1.

¥The descending thin limb of Henle’s loop of juxtaglomerular nephrons extends to the apex of the medullary pyramid (those of midcortical and cortical

nephrons are very short and will not be discussed).

•Henle’s loop is near the apex of the medullary pyramid, and it connects the descending and ascending thin limbs in a hairpin-like loop.

¥The ascending thin limb of Henle’s loop parallels the descending thin limb as the corticalward continuation of HenleÕs loop.

¥The descending and ascending thin limbs of HenleÕs loop are composed of simple squamous epithelial cells (types I through IV) whose structure varies according to their permeability to water, organelle content, and complexity of tight junctions. Type I cells are present only in cortical nephrons, whereas types II, III, and IV cells are present in juxtaglomerular nephrons.

Distal Tubule

The distal tubule is composed of two regions, distal convoluted tubule and pars recta of the distal tubule. Since the present discussion follows the path of the nephron and the ascending thin limb of HenleÕs loop ends in the pars recta of the distal tubule, the pars recta is discussed Þrst.

¥The ascending thick limb of Henle’s loop, also known as the pars recta of the distal tubule, is composed of simple cuboidal cells that resemble the cells of the distal tubule.

The pars recta of the distal tubule begins much deeper in the medulla than the end of the pars recta of the proximal tubule.

The pars recta of the distal tubule ascends into the cortex to contact the afferent and efferent glomerular arterioles of its own renal corpuscle.

¥Cells of the distal tubule that contact the afferent (and efferent) glomerular arteriole are modiÞed, in that

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they are thin, tall cuboidal cells whose nuclei are close to one another. This region is referred to as the macula densa of the distal tubule.

Cells of the macula densa communicate with modiÞed smooth muscle cells, juxtaglomerular (JG) cells, of the afferent (and efferent) glomerular arterioles.

The macula densa and the JG cells together form the juxtaglomerular apparatus.

The extraglomerular mesangial cells, modiÞed interstitial tissue cells, also known as lacis cells, are likewise considered to belong to the juxtaglomerular apparatus.

¥The distal convoluted tubule is shorter than the proximal convoluted tubule; therefore, in any histological section of the renal cortex, there are fewer proÞles of it surrounding the renal corpuscle. The cells of the distal convoluted tubule resemble those of the pars recta of the distal tubule, and instead of cilia, they possess short, blunt microvilli.

Collecting Tubules

Collecting tubules begin at the terminal ends of distal convoluted tubules as either connecting tubules or arched collecting ducts. Several distal convoluted tubules join each collecting tubule, a structure composed of a simple cuboidal epithelium whose lateral cell membranes are evident with the light microscope.

¥The cortical collecting tubules descend from the medullary rays of the cortex to enter the renal pyramids of the medulla.

¥As they enter the medulla, they are known as medullary collecting tubules.

¥Several medullary collecting tubules merge to form the papillary ducts (ducts of Bellini), which terminate at the area cribrosa.

The cuboidal cells of the collecting tubule are of two types, the lightly staining principal cells and the intercalated cells that stain darker.

•Principal cells (light cells) possess a single, nonmotile, apically situated cilium that probably functions as a mechanosensor that monitors ßuid ßow along the lumen of the tubule.

Principal cells possess antidiuretic hormone (ADH)Ðsensitive aquaporin-2 channels that permit the cell to be permeable to water.

They also have polycystin-1 and polycystin-2 in their plasmalemma. The latter of the two proteins is a calcium channel.

•Intercalated cells (dark cells) are fewer in number and are of two types, A and B:

Type A cells secrete H+ into the tubular lumen and