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A&P 2
Lab Manual

3) Microscopic Anatomy of the Kidney

Each kidney contains over a million nephrons, which are the anatomical units responsible for forming urine. Figure 4.7 depicts the detailed structure and the relative positioning of the nephrons in the kidney.

Each nephron consists of two major structures: a glomerulus (a capillary knot) and a renal tubule. During embryologic development, each renal tubule begins as a blind-ended tubule that gradually encloses an adjactent capillary cluster, or glomerulus. The enlarged end of the tubule encasing the glomerulus is the glomerular (Bowman’s) capsule, and its inner, or visceral, wall consists of highly specialized cells called podocytes. Podocytes have long, branching processes (foot processes) that interdigitate with those of other podocytes and cling to the endothelial wall of the glomerular capillaries, thus forming a very porous epithelial membrane surrounding the glomerulus. The glomerulus-capsule complex is sometimes called the renal corpuscle.

The rest of the tubule is approximately 3 cm (1.25 inches) long. As it emerges from the glomerular capsule, it becomes highly coiled and convoluted, drops down into a long hairpin loop, and then again coils and twists before entering a collecting duct. In order from the glomerular capsule, the anatomical areas of the renal tubule are: the proximal convoluted tubule, loop of Henle (descending and ascending limbs), and the distal convoluted tubule. The wall of the renal tubule is composed almost entirely of cuboidal epithelial cells, with the exception of part of the descending limb (and sometimes part of the ascending limb) of the loop of Henle, which is simple squamous epithelium. The lumen surfaces of the cuboidal cells in the proximal convoluted tubule but in greatly reduced numbers, revealing its less significant role in reclaiming filtrate contents.

Most nephrons, called cortical nephrons, are located entirely within the cortex. However, parts of the loops of Henle of the juxtamedullary nephrons (located close to the cortex-medulla junction) penetrate well into the medulla. The collecting ducts, each of which receives urine from many nephrons, run downward throught eh medullary pyramids, giving them their striped appearance. As the collecting ducts approach the renal pelvis, they fuse to form larger papillary ducts, which empty the final urinary product into the calyces and pelvis of the kidney.

The function of the nephron depends on several unique features of the renal circulation. The capillary vascular supply consists of two distinct capillary beds, the glomerulus and the peritubular capillary bed. Vessels leading to and from the glomerulus, the first capillary bed, are both arterioles: the afferent arteriole feeds the bed while the efferent arteriole drains it. The glomerular capillary bed has no parallel elsewhere in the body. It is a high-pressure bed along its entire length. Its high pressure is a result of two major factors: (1) the bed is fed and drained by arterioles (arterioles are high-resistance vessels as opposed to venules, which are low-resistance vessels), and (2) the afferent feeder arteriole is larger in diameter than the efferent arteriole draining the bed. The high hydrostatic pressure created by these two anatomical features forces out fluid and blood components smaller than proteins from the glomerulus into the glomerular capsule. That is, it forms the filtrate which is processed by the nephron tubule.

Figure 4.7

The peritubular capillary bed arises from the efferent arteriole draining the glomerulus. This set of capillaries cling intimately to the renal tubule and empty into the interlobular veins that leave the cortex. The peritubular capillaries are low-pressure very porous capillaries adapted for absorption rather than filtration and readily take up the solutes and water reabsorbed from the filtrate by the tubule cells. The juxtamedullary nephrons have additional looping vessels, called the nephrons have additional looping vessels, called the vasa recta (“straight vessels”, that parallel their long loops of Henle in the medulla. Hence, the two capillary beds of the nephron have very different, but complementary, roles: The glomerulus produces the filtrate and the peritubular capillaries reclaim most of the filtrate.

Urine formation is a result of three processes: filtration, reabsorption, and secretion (Figure 4.8 and 4.9).

Filtration, the role of the glomerulus, is largely a passive process in which a portion of the blood passes from the glomerular bed into the glomerular capsule. This filtrate then enters the proximal convoluted tublue where tubular reabsorption and secretion begin.

During tubular reabsorption, many of the filtrate components move through the tubule cells and return to the blood in the peritubular capillaries. Some of this reabsorption is passive, such as that of water, which passes by osmosis, but the reabsorption of most substances depends on active transport processes and is highly selective. Which substances are reabsorbed at a particular time depends on the composition of the blood and needs of the body at that time. Substances that are almost entirely reabsorbed from the filtrate include water, glucose, and amino acids. Various ions are selectively reabsorbed or allowed to go out in the urine according to what is required to maintain appropriate blood pH and electrolyte composition. Waste products (urea, creatinine, uric acid, and drug metabolites) are reabsorbed to a much lesser degree or not at all. Most (75% to 80%) of tubular reabsorption occurs in the proximal convoluted tubule; the balance occurs in other areas, especially the distal convoluted tubules and collecting ducts.

Tubular secretion is essentially the reverse process of tubular reabsorption. Substances such as hydrogen and potassium ions and creatinine move either from the blood of the peritubular capillaries through the tubular cells or from the tubular cells into the filtrate to be disposed of in the urine. This process is particularly important for the disposal of substances not already in the filtrate (such as drug metabolites), and as a device for controlling blood pH.

Observe photos of slides of the nephron

1) Identify a glomerulus, which appears as a ball of tightly packed material containing many small nuclei (Figure 4.10). It is usually delineated by a vacant-appearing region (corresponding to the space between the visceral and parietal layers of the glomerular capsule) that surrounds it.

2) Notice that the renal tubules are cut at various angles. Also try to differentiate between the thin-walled loop of Henle portion of the tubules and the cuboidal epithelium of the proximal convoluted tubule, which has dense microvilli.

Figure 4.8 and 4.9 Figure 4.10

Sketch the nephron and label:
Glomerulus, Renal tubule, Bowman’s capsule, Afferent arteriole,
Efferent arteriole, Artery, Proximal convoluted tubule, Loop of Henle, Distal convoluted tubule, Peritubular capillaries, Vein, Descending loop, Ascending loop, Collecting duct, Renal cortex, Renal medulla

Sketch of the longitudinal section of the kidney and label (Figure 4.8 and 4.9):
Renal tubules, Glomeruli, Bowman’s capsule