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CHAPTER 15: The Urinary System

Overview

The urinary system consists of the kidneys, ureters, urinary bladder, and Urethra. This chapter focuses on the most complex component of the urinary system, the kidney. The kidney is an excretory organ of extreme importance. It cleanses the blood minute by minute, constantly maintaining a delicate balance of elements, compounds, molecules, and macromolecules that, via the general circulation, bathe and feed each of the body's cells. The kidney is so important to life that renal failure is lethal; when the kidneys cease their function, the body does, too.

In humans, the fist-sized kidney contains approximately two million functional units called nephrons. To understand the microanatomy of the kidney, one must first understand the microanatomy of the Nephron. Unfortunately for the student of histology, it is quite difficult to learn and understand the structure of the Nephron from sectioned material alone, because even the most favorably oriented sections through the kidney display complex and confusing images. Why is this the case? The answer lies in the compactness of the kidney. To fit millions of nephrons into a small space it is necessary to coil them and pack them tightly together. Consequently, any given histologic section through a kidney will contain hundreds of circular and elliptical profiles of tubules that represent bits and pieces of thousands of nephrons cut at various points along their length. The objective of this overview, then, is to describe the structure of the Nephron in the simplest manner possible to facilitate interpretation of light and electron micrographs of sections taken through the kidney.

In order to understand biologic structures - or any complicated system, for that matter-it is always best to start with the simple and work up to the complex. In its simplest form, the Nephron is basically a hollow tube with an opening at each end. Fluid, called the Glomerular Filtrate, enters at one end and flows out the other. The material that enters the tube is vastly different from the material that leaves it because the fluid's composition is greatly modified as it passes along the length of the tube. This modification is accomplished by cells that line the tubular Nephron. Different kinds of cells, posted at different stations along the Nephron, perform special biochemical operations on the Glomerular Filtrate.

Figures 15-1, 15-2, and 15-3 clarify the structure and function of the Nephron. Figure 15-1 illustrates the overall layout of a human kidney and the relative position of a typical Nephron within it. The kidney is shaped like a common backyard swimming pool. A longitudinal slice through its middle reveals that it has an outer region, called the cortex, and an inner region, called the Medulla. As shown in Figure 15-1, part of the Nephron is in the cortex and part is in the Medulla. The port of entry to the Nephron, called Bowman's Capsule, is out in the cortex. It receives blood Plasma in the form of the glomerlular filtrate. The exit from the Nephron, the dital end of the Collecting Duct, is located in the Medulla. Fully formed urine drips from the tip of the Collecting Duct into tht renal pelvis, where it collects and enters the Ureter. the Ureter then delivers urine to the uniary bladder for storage and eventual elimination. Hence, the Nephron receives blood Plasma in the form of the Glomerular Filtrate and transforms it into urine.

The blood Plasma that enters the Nephron and the urine that leaves it are dramatically different in chemical composition. The difference is crucial; should the kidney's nephrons falter in their conversion of blood Plasma to urine, toxic elements build up in the blood rapidly, and death is sure to follow unless medical intervention is swirf. To understand the transformation of blood to urine by the Nephron we must isolate a Nephron from the kidney, uncoil it, lay it out in a straight line, and examine its parts.

Figure 15-2 shows how an uncoild Nephron would look. (The drwaing is not made to scale, although the parts are shown in the correct order.) At the top sits the entrance into the Nephron, called Bowman's Capsule. Bowman's Capsule is a complex, microscopic funnel that receives filtered blood Plasma -- the Glomerular Filtrate -- from a ball of capillaries called the Glomerulus. (A blood vessel called the Afferent Arteriole brings blood to the glmerulus; an Efferent Arteriole carries it away.) After the filtrate has squeezed out of the capillaries and into the Urinary Space within Bowman's Capsule, it passes on into the prosimal convoluted tubule. The Proximal Convoluted Tubule is lined by a simple Cuboidal Epithelium that, by complex biochemical processes of active and passive transport, moves most of the filtrate out of the Nephron and into the tissue space (called the Interstitial Space) around it. Nearby capillaries then take up the materials from the Interstitial Space and put them back in the bloodstream for further use by the body.

From this discussion, it is evident that the kidney is not a device that simply filters harmful substances out of the blood. Quite the reverse happens; the kidney recycles useful materials. Bowman's Capsule receives a Plasma filtrate from the Glomerulus; the remainder of the Nephron pumps most of the "good" substances back into the bloodstream, leaving urea and harmful nitrogenous wastes behind to be eliminated from the body in the form of urine.

Although much of the work of the Nephron is performed by the Proximal Convoluted Tubule, the fluid that leaves it is further modified by the rest of the Nephron before it is eliminated from the collecting ducts as urine. Having passed through the Proximal Convoluted Tubule, the fluid passes into a long segment of the Nephron called the loop of Henle. From there, urine enters the distal convoluted tubule, where more salts and water are eliminated, thereby further concentrating the urine. After passing through the Distal Convoluted Tubule, the urine enters the Collecting Duct, which carries it through the Medulla of the kidney and releases it into a container at the base of the kidney - the renal pelvis - which, in turn, empties into the Ureter, which takes the urine to the bladder.

The actual Nephron is coiled back upon itself along its length, as shown in Figure 15-3. lt is largely by virtue of this coiling that many nephrons, some two million of them, can be packed into the small volume of a single kidney.

Fig. 15-1. Drawing that demonstrates the position of the nephron within the kidney.

 

Fig. 15-2. A cutaway drawing of a nephron, straightened out and shortened, that shows the relative positions of the parts of the nephron along its length.

 

Fig. 15-3. Drawing of a "typical" nephron.

 

Kidney, Part 1: The Renal Cortex

Figure A illustrates that the cortex of the kidney is an extremely complex histologic structure. To interpret this image, it is necessary to understand the basic structure and function of the Nephron, as described in the overview.

The entrance to the Nephron is the Renal Corpuscle (RC). When viewed by light microscopy, the Renal Corpuscle stands out in the cortex as a round, tightly packed mass of cells of types difficult to distinguish from one another. Under the electron microscope, however, the structure of the Renal Corpuscle appears almost comprehensible (RC, Figure B). As explained and illustrated in the overview, the Renal Corpuscle consists of two separate parts: the Glomerulus and Bowman's Capsule. The Glomerulus is a ball of capillaries that delivers blood Plasma to Bowman's Capsule; Bowman's Capsule is an elaborate two-layered cup that receives the Glomerular Filtrate. Figure B shows the parietal (outer) layer (arrow) of Bowman's Capsule-a thin layer of epithelial cells that lines the capsule itself. Also present is a capillary (C) of the Glomerulus, which is surrounded by fuzzy-looking cellular extensions (arrowhead). These tiny cellular extensions that surround the glomerular capillaries are called pedicles, parts of cells called podocytes that constitute the visceral (inner) layer (VL) of Bowman's capsule. These will be examined in more detail in Plate 15-2.

The base of Bowman's Capsule is coextensive with the lumen of the first tubular portion of the Nephron, the Proximal Convoluted Tubule. When viewed with the light microscope (Figure A), the Proximal Convoluted Tubule (PCT) is a thick-walled, darkly stained tubule. It is lined by a high cuboidal or low Columnar Epithelium that has a number of dark vertical lines called Basal Striations that extend upward from the Basement Membrane. The electron micrograph in Figure B reveals that these basal striations are actually many long, thin Mitochondria.

In addition to the proximal convoluted tubules, profiles of the distal convoluted tubules (DCT) are evident in the renal cortex (Fig. A). At first, it is easy to confuse the cross-sectional images of proximal and distal tubules. Several key features, listed below, facilitate their individual identification. First, proximal tubules have a prominent brush border (*) consisting of many Microvilli; distal tubules do not. Instead, the apical surfaces of the cells lining the lumen of the distal tubules are smooth and have only a few Microvilli. Second, the Cytoplasm of the proximal tubules is dense and stains darkly; that of distal tubules is more clear and takes less stain. Third, the epithelial lining of the distal tubules is thinner than that of the proximal tubules. Unfortunately, both types of tubules have Basal Striations (i.e., Mitochondria), which can make them hard to distinguish.

A third type of tubule, the Collecting Duct (CD), is evident in Figure A. Collecting ducts carry out of the Nephron urine received from the distal convoluted tubules. Collecting ducts have a low Cuboidal Epithelium and a large lumen and are lined by cells with rounded cell surfaces.

Plate 15-1

Plate 15-1, Figure A. Light micrograph of the kidney cortex. CD, Collecting Duct; DCT, Distal Convoluted Tubule; PCT, proximal convoluted tubule; RC, Renal Corpuscle; *, brush border of PCT. 600 X Figure B. Electron micrograph of the kidney cortex. C, capillary; DCT, Distal Convoluted Tubule; PCT, Proximal Convoluted Tubule; RC, Renal Corpuscle; US, Urinary Space; VL, Visceral Layer of Bowman's Capsule; *, brush border of lumen of PCT; arrow, parietal layer of Bowman's Capsule; arrowhead, pedicles of podocytes. 1,100 X

 

Kidney, Part II: The Renal Corpuscle

The Renal Corpuscle is the site where blood enters the Nephron and undergoes the process of glomerular filtration. In order to understand the ultrastructure of the Renal Corpuscle, first recall that it is made up of two basic parts - the Glomerulus and Bowman's Capsule. Both structures are illustrated in the electron micrographs at right.

The Glomerulus à ± Is a tightly coiled ball of capillaries (C) that sits within Bowman's Capsule. Bowman's capsule resembles a double-walled chalice that contains the capillaries of the Glomerulus. The walls of the chalice are extremely thin. The outer wall is made up of a simple Squamous Epithelium called the Parietal Layer (PL); the inner wall, called the Visceral Layer, is made of a monolayer of intricately sculpted, thin cells called podocytes (P) (Figure A). The podocytes send out fine foot-processes (arrow, Figure A) that, in turn, send out even thinner extensions of Cytoplasm, called pedicles (arrowhead, Figure B).

The pedicles wrap tightly around the fenestrated capillaries of the Glomerulus in the same way you might wrap your fingers around a leaky garden hose. In this case, the leaky hose would be a Fenestrated Capillary; your wrist, a foot-process of a Podocyte; your fingers, the pedicles. Just as water would leak out through holes in the hose and squeeze through the spaces between your fingers, so does blood Plasma leak out of holes in the glomerular capillaries and squeeze in between the pedicles of the podocytes. In this process of microfiltration, large molecules and blood cells are left behind in the blood space (BS) within the capillary; smaller molecules and ions pass along with the Plasma through the holes in the capillary wall, through the slits between the pedicles, and move on out into the urinary space (US) of Bowman's Capsule to form the glomerular filtrate. Because the Urinary Space of Bowman's Capsule is continuous with the lumen of the proximal convoluted tubule (PCT), it is easy to see that the Glomerular Filtrate, once produced within the Renal Corpuscle, continues on through the lumen of the tubular Nephron for further biochemical processing-the end product, of course, being urine.

Portions of proximal and distal convoluted tubules are shown in cross section in the micrographs at left. As often happens in histologic preparation of kidneys, the lumens of the tubules have collapsed. The proximal tubules have many radially oriented, basally located Mitochondria that provide ATP for the highly bioenergetic processes of Active Transport of materials out of the tubule. The inner surface of the proximal convoluted tubule is modified to form an elaborate brush border (BB) consisting of thousands of tightly packed Microvilli. This structure contrasts greatly with the lumenal surface of the Distal Convoluted Tubule (DCT) shown in Figure B. Here, no Microvilli are evident; instead, the cell surface appears rounded.

Plate 15-2

Plate 15-2. Figure A. Electron micrograph of the cortex of the kidney. BB, brush border of Proximal Convoluted Tubule; BS, blood space; C, capillary; E, Erythrocyte; P, Podocyte; PCT, Proximal Convoluted Tubule; PL, Parietal Layer of Bowman's Capsule; RC, Renal Corpuscle; US, Urinary Space; arrow, foot-process extending from Podocyte to capillary; arrowhead, Basement Membrane surrounding renal corpuscle. 2,000 X

 

Figure B. Electron micrograph of same kidney cortex shown at higher magnification. BB, brush border lining collapsed lumen of Proximal Convoluted Tubule; BS, blood space; C, capillary; DCT, Distal Convoluted Tubule; E, Erythrocyte; PCT, proximal convoluted tubule; PL, Parietal Layer of Bowman's Capsule; US, Urinary Space; arrowhead, pedicles of podocytes; *, collapsed lumen of distal convoluted tubule with no brush border. 3,000 X

 

Kidney, Part III: The Renal Medulla

The microanatomy of the renal Medulla differs dramatically from that of the renal cortex. In the Medulla, the Renal Corpuscle, proximal convoluted tubule, and Distal Convoluted Tubule are absent. Instead, the collecting ducts, loops of Henle, and capillaries called the Vasa Recta are present. These components of the Nephron are illustrated by electron microscopy in Figures A, B, and C at right. (To prepare yourself to understand these images, refer to the drawings of the kidney and Nephron in the overview to this chapter).

Figure A is an electron micrograph of a longitudinal thin section taken through the Medulla of the monkey kidney. The field of view includes longitudinal images of collecting ducts (CD), loops of Henle (LH), and capillaries (C) of the Vasa Recta. The Collecting Duct is characterized by a large-caliber, open lumen lined by a simple Cuboidal Epithelium. The epithelium of the Collecting Duct is distinctive and easy to recognize. Each of the large, clear cells has a rounded cell surface that lacks a true brush border. The cells have round, euchromatic, centrally located nuclei. The lateral surfaces of neighboring cells form extensive interdigitations with one another (arrow), forming a dense zone so prominent that it is even detectable by light microscopy.

The loop of Henle is radically different in structure and function from the Collecting Duct. Its open lumen is lined by a very thin simple Squamous Epithelium that is about twice as thick as the Endothelium lining the adjacent capillaries (C). Loops of Henle and capillaries are easily distinguished from one another by the presence of blood cells in capillaries. Under normal conditions, the loops of Henle have only urine in them and are free from erythrocytes.

All of the structures named above are shown in cross section in Figure B. Here the differences in wall structure between the collecting ducts (CD), thin loops of Henle (LH), and capillaries (C) are immediately apparent. Also evident are the connective tissue spaces (CT) that lie between each of the fluid-bearing tubes. These connective tissue spaces are of great functional significance, for not only do they contain Collagen fibers that bind the soft tissues of the kidney together, but they also provide space in which fluids can accumulate during the vital exchange of ions, water, and metabolites between Nephron and bloodstream.

The lower recesses of the Medulla of the kidney are shown in Figure C. Here, the end of a Collecting Duct (CD) has been cut in longitudinal section as it opens into the renal pelvis (P). As described in the overview, urine drips out of the collecting ducts and into the renal pelvis, where it collects before passing through the Ureter en route to the bladder.

 

Plate 15-3

Plate 15-3. Figure A. Electron micrograph of longitudinal section through the Medulla of the kidney of the squirrel monkey. C, capillary; CD, Collecting Duct; CT, connective tissue; LH, loop of Henle; arrow, interdigitated lateral borders of adjoining Collecting Duct cells; *, Basement Membrane of Collecting Duct. 2,300 X

 

Figure B. Electron micrograph of cross section through Medulla of the same monkey kidney. C, capillary; CD, Collecting Duct; CT, connective tissue; LH, Loop of Henle. 1,500 X Figure C.

 

Longitudinal section through tip of Collecting Duct of the same monkey kidney. C, capillary; CD, Collecting Duct; CT, connective tissue; P, renal pelvis. 900 X

 

The Urinary Bladder

When urine has been formed and collects in the pelvis of the kidneys, it passes through the ureters and enters the urinary bladder, wherein it is stored until one finds an appropriate time and place for its release into the environment. The urinary bladder, then, is a storage tank, and a rather remarkable one at that. It is particularly unusual in two ways. First, it is a highly elastic container faced with a serious physical problem: it must repeatedly undergo formidable volumetric changes. When full, it is a large, turgid, tight-skinned sphere that can hold upwards of 1 L of urine. When empty, it is a flaccid, folded sac lying limp on the pelvic floor. This conformational change can occur with astonishing rapidity. Second, the bladder is faced with a serious physiologic problem: urine is highly hypertonic to Cytoplasm, and the epithelial cells lining the lumen must somehow protect themselves from instant death at the hands of osmotic extraction.

The bladder solves these physical and physiologic problems with a number of structures built into its wall. Figures A and B at right are a matched pair of light and electron micrographs of serial thick and thin cross sections taken through the wall of the bladder of the squirrel monkey. This bladder was partially filled with urine at the time of tissue preparation.

The lumen (L) of the bladder is lined by an unusual type of epithelium called Transitional Epithelium (TE). Unique to the conduits and containers of the urinary tract, Transitional Epithelium is so named because it undergoes a marked morphologic transition in response to the degree to which the epithelium is stretched - which, in turn, depends on the degree to which the container that it lines is filled. Consequently, transitional epithelium has several layers of cells, and the number of layers one can count at any time depends on how slack or taut the epithelium happens to be. In the empty bladder, for example, the epithelium seems to consist of more than five layers of chubby, cuboidal cells. In the full bladder, on the other hand, the epithelium seems to consist of two layers of thin, attenuated cells. In Figures A and B, the Transitional Epithelium averages three cells thick. The superficial cells, which contain many Mitochondria (*), have a characteristically convex free surface (arrow). This surface, which borders on the lumen (L), consists of a highly modified, thickened Plasma membrane, too small to see at this low magnification, that protects the epithelium from extraction by the highly hypertonic urine.

Beneath the epithelium lies the Lamina Propria (LP). The Lamina Propria rests directly on the thick muscularis (M), which comprises the bulk of the wall of the bladder. There is no Muscularis Mucosae or Submucosa. The muscularis consists of a very strong, elastic, distensible, and powerful combination of Smooth Muscle (SM, SM' in Figure A) and connective tissue (CT). The connective tissue consists of collagenous and elastic fibers, intertwined among the Smooth Muscle fibers, that run in many directions. Although the smooth muscles, too, run in all directions, a semblance of the "layering" present in the Ureter remains; that is, the inner fibers tend to be longitudinally oriented (SM), the middle fibers tend to be circularly oriented (SM'), and the outermost fibers-not shown here-tend to be longitudinally oriented. Since this is a cross section through the bladder, the Smooth Muscle fibers of the middle circular layer are cut in longitudinal section (SM', Figure A). The wall of the bladder, then, is a very strong web that is capable not only of withstanding great distension without bursting, but of generating powerful muscular contractions as well.

Urinary Bladder

Plate 15-4

Plate 15-4, Figures A and B. Matched pair of light and electron micrographs of serial cross sections through the wall of the bladder of the squirrel monkey. A, artery; C, capillary; CT, connective tissue; L, lumen of bladder; LP, Lamina Propria; M, muscularis; SM, Smooth Muscle cut in cross section; SM' (Figure A), Smooth Muscle cut in longitudinal section; TE, Transitional Epithelium; V. vein; *, Mitochondria; arrow, surface of epithelium. 600 X

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