Functional Anatomy Of The Kidney

A person's kidney is about the size of a clenched fist. When examined in cross section as shown in Fig. 1, the kidney is easily divided into two regions: the cortex and the medulla. The blood, lymphatic, and neural supply of the kidney enter through the hilus together with the ureter, which carries the urine from the kidney to the bladder, where it is stored until emptied by micturition (urination).

In the human kidney, the medulla terminates in several conical structures called papillae. The renal pelvis is essentially an enlarged extension of the ureter. It is divided into individual cup-shaped structures called calyxes that surround each papilla and convey the effluent urine into the ureter.

A layer of tough connective tissue called the capsule, which protects the more delicate parenchyma, covers the kidney. About 1 million nephrons constitute the

Capsule Cortex Calyx

Hilus Region Renal Vein Renal Artery Renal Pelvis Papilla Ureter Medulla

FIGURE 1 Schematic cross section of a human kidney. The ureter, renal artery, and renal vein exit the kidney in the hilus region. The calyces collect the urine as it flows out of the ducts of Bellini (see Fig. 4) at the tips of the papillae and convey it into the ureter, which carries it to the urinary bladder.

majority of the parenchyma in each human kidney. As discussed in detail in subsequent chapters, the nephron is the functional unit that produces an initial ultrafiltrate of the plasma at its point of origin in the glomerulus and modifies that ultrafiltrate by the processes of reabsorption and secretion to control the rate of excretion of solutes and water.

Vasculature

Although the two kidneys together constitute less than 0.5% of the body mass, they receive almost 25% of the total cardiac output when the body is at rest. This blood flows into each of the two kidneys via the right and left renal arteries, which branch directly from the abdominal aorta. On entering the hilus, the renal artery divides into several interlobar arteries that radiate from the hilus toward the cortex (Fig. 2). Near the boundary of the cortex and the medulla, the interlobar arteries divide into arcuate arteries that run parallel to the arc of the corticomedullary junction and give off the radial arteries. These major vessels of the arterial side are paralleled in their course by the interlobar, arcuate, and radial veins. The interlobar veins come together to form the renal vein, which exits from each kidney at the hilus and carries the venous blood to the vena cava.

The network of arterioles, capillaries, and venules connecting the arterial and venous sides of the renal circulation is quite different from that found in other

Paramyxoviren Masern

FIGURE 2 Schematic diagram of the major blood vessels of the kidney. All of the glomeruli are located in the cortex and each is supplied by an afferent arteriole. The efferent arteriole conveys the blood from the glomerular capillary tuft to the peritubular capillary circulation, which is not shown in the figure. The peritubular capillary network arising from glomeruli in the outer regions of the cortex is confined to the cortex in close association with the nephron of origin. The peritubular capillary network arising from glomeruli near the corticomedullary junction penetrates deeply into the medulla in a series of long hairpin loops referred to as vasa recta. OM, outer medulla; IM, inner medulla. (Modified from Kriz W. A standard nomenclature for structures of the kidney. Am J Physiol Renal Fluid Electrolyte Physiol 1988;254:F1-F8.)

FIGURE 2 Schematic diagram of the major blood vessels of the kidney. All of the glomeruli are located in the cortex and each is supplied by an afferent arteriole. The efferent arteriole conveys the blood from the glomerular capillary tuft to the peritubular capillary circulation, which is not shown in the figure. The peritubular capillary network arising from glomeruli in the outer regions of the cortex is confined to the cortex in close association with the nephron of origin. The peritubular capillary network arising from glomeruli near the corticomedullary junction penetrates deeply into the medulla in a series of long hairpin loops referred to as vasa recta. OM, outer medulla; IM, inner medulla. (Modified from Kriz W. A standard nomenclature for structures of the kidney. Am J Physiol Renal Fluid Electrolyte Physiol 1988;254:F1-F8.)

FIGURE 3 Schematic illustration of the glomerulus. The afferent arteriole distributes into the glomerular capillary network. The pressure in these capillary loops is higher than in other systemic capillaries, which results in filtration of fluid from the capillaries into Bowman's space, from which it passes to the proximal convoluted tubule. The blood that remains after filtration flows from the glomerular capillaries into the efferent arteriole, from which it proceeds to the peritubular capillary network. The thick ascending limb from the same nephron is closely associated with the afferent arteriole to form the juxtaglomerular apparatus. This structure is comprised of the macula densa cells of the thick ascending limb and the specialized juxtaglomerular smooth muscle cells of the afferent arteriole.

FIGURE 3 Schematic illustration of the glomerulus. The afferent arteriole distributes into the glomerular capillary network. The pressure in these capillary loops is higher than in other systemic capillaries, which results in filtration of fluid from the capillaries into Bowman's space, from which it passes to the proximal convoluted tubule. The blood that remains after filtration flows from the glomerular capillaries into the efferent arteriole, from which it proceeds to the peritubular capillary network. The thick ascending limb from the same nephron is closely associated with the afferent arteriole to form the juxtaglomerular apparatus. This structure is comprised of the macula densa cells of the thick ascending limb and the specialized juxtaglomerular smooth muscle cells of the afferent arteriole.

organs and is responsible for many of the functional characteristics of this organ. The unique feature of the microcirculation is the presence of two capillary beds in series. Afferent arterioles branch off the radial arteries and feed the first of the two capillary networks, the glomerular capillaries. As discussed later, the blood pressure in these capillaries is relatively high, so it serves to filter fluid. The glomerular capillary bed is effectively embedded in the epithelial layer referred to as Bowman's capsule (which forms the initial portion of the nephron, as described later), and fluid filtered from the glomer-ular capillaries enters Bowman's space (Fig. 3). The complex of the glomerular capillaries and Bowman's capsule is termed the renal corpuscle, or, more frequently, the glomerulus.

In contrast to other capillary beds, the glomerular capillary network empties not into a venule but into another resistance vessel, the efferent arteriole. The efferent arteriole gives rise to a second capillary bed, the peritubular capillaries. As the name implies, this capillary network lies adjacent to the tubular components of the nephron. In the cortex, the peritubular capillaries form a dense plexus surrounding all the tubular components. Efferent arterioles from glomeruli that lie near the corticomedullary junction give rise to a different type of capillary network that supplies the renal medulla as long, hairpin-shaped capillary loops.

Juxtamedullary Nephrons

FIGURE 4 Primary structural elements of the nephron. A juxtamedullary nephron is shown on the left, and a superficial nephron on the right. Note the longer loop of Henle, which occurs in many but not all juxtamedullary nephrons. Also, in the juxtamedullary nephron the connecting tubule (CNT) is elongated in structures called arcades to reach from the deeper cortex to the superficial cortex where it merges with other CNTs, including shorter CNTs of superficial nephrons, to form the cortical collecting duct. (Modified from Kriz WA. Standard nomenclature for structures of the kidney. Am J Physiol Renal Fluid Electrolyte Physiol. 1988;254:F1-F8.)

FIGURE 4 Primary structural elements of the nephron. A juxtamedullary nephron is shown on the left, and a superficial nephron on the right. Note the longer loop of Henle, which occurs in many but not all juxtamedullary nephrons. Also, in the juxtamedullary nephron the connecting tubule (CNT) is elongated in structures called arcades to reach from the deeper cortex to the superficial cortex where it merges with other CNTs, including shorter CNTs of superficial nephrons, to form the cortical collecting duct. (Modified from Kriz WA. Standard nomenclature for structures of the kidney. Am J Physiol Renal Fluid Electrolyte Physiol. 1988;254:F1-F8.)

Blood in these capillaries descends first downward into the medulla via the descending vasa recta and then returns to the cortex via the ascending vasa recta. Blood pressure in both the cortical and medullary regions of the peritubular capillaries is relatively low and, thus, these capillaries can take up the fluid and solutes that have been reabsorbed by the renal tubules from the ultrafiltrate formed by the glomerulus.

Glomeruli and Nephrons

The nephrons begin as an extension of Bowman's space, which surrounds the glomerular capillaries. All glomeruli are located in the cortex, but those that lie near the cortical surface give rise to what are referred to as superficial nephrons, whereas those near the cortico-medullary junction are referred to as juxtamedullary nephrons. Although the pattern is not invariant, superficial nephrons give rise to short loops of Henle that end in the outer medulla and have at most a short, thin ascending limb. A larger proportion of the juxtamedul-lary nephrons have long loops of Henle that extend to varying depths in the inner medulla and even to the tip of the papilla. The various regions of the nephron have quite different structural characteristics that reflect differences in their metabolism and function.

Throughout its length, the nephron is comprised of a single epithelial cell layer with an underlying basal lamina (basement membrane). However, the cells that constitute the different tubular segments of the nephron shown in Fig. 4 differ both anatomically and functionally. To simplify these details, the nephron is discussed in subsequent chapters as if it were divided into six functional regions: (1) the proximal tubule, comprised of the convoluted and straight tubules; (2) the thin descending limb of the loop of Henle; (3) the thin and thick segments of the ascending limb of the loop of Henle; (4) the distal convoluted tubule; (5) the connecting tubule; and (6) the collecting duct, including the initial cortical collecting tubule, and the cortical and medullary collecting ducts.

As shown in Fig. 4, the proximal convoluted tubule arises from Bowman's space and folds in a complex series of turns in the region of its own glomerulus. The proximal straight tubule is a straight continuation of the convoluted segment, and the convoluted and straight segments are called collectively the proximal tubule. The straight segment runs radially through the cortex into the outer medulla, and in this region, its morphology changes to become the thin descending limb of the loop of Henle.

The thin descending limb of the loop of Henle runs radially into the medulla, where it makes a hairpin turn at a level determined, in part, by the location in the cortex of its glomerulus of origin (see later discussion). The thin ascending limb of the loop of Henle then returns toward the outer medulla. Both these regions of the nephron are comprised of a very thin epithelial cell layer, thus giving rise to their names. The thin ascending limb changes in the outer medulla to an epithelium with taller cells and numerous mitochondria, which is called the thick ascending limb, but is sometimes referred to as the distal straight tubule. The thick ascending limb proceeds through the outer medulla into the cortex, where it comes into close contact with the afferent and efferent arterioles associated with the glomerulus from which the nephron originated. This region of contact is referred to as the juxtaglomerular apparatus (see Figure 3).

The juxtaglomerular apparatus consists of specialized regions of both the cortical thick ascending limb and the afferent arteriole. The thick ascending limb cells in this region of contact form a plaque of taller and larger cells referred to as the macula densa (see Fig. 3), which monitors the flow and composition of the tubular fluid. Specialized cells of the afferent arteriole in the juxtaglo-merular apparatus, referred to as granular cells or juxtaglomerular cells, store a hormone, renin, that can be released into the circulation. The functions of this hormone and the juxtaglomerular apparatus are discussed in Chapters 24 and 29.

The distal convolution lies between the macula densa and the collecting ducts, and it consists of two segments, the distal convoluted tubule and the connecting tubule (see Fig. 4), which are structurally and functionally distinct. Beyond the macula densa, the thick ascending limb cells continue for a short distance until they are abruptly replaced by the distal convoluted tubule cells (DCT cells). DCT cells have the greatest density of mitochondria and Na+-K+-ATPase of any nephron segment and the basolateral membrane is highly folded and interdigitated with adjacent cells. The connecting tubule, which is populated primarily by connecting tubule cells (CNT cells), follows the distal convoluted tubule but the transition is gradual and CNT cells begin to appear together with DCT cells near the end of the distal convoluted tubule. Finally, the connecting tubule at its distal end has the same structural and functional characteristics as the cortical collecting duct, and therefore, it is referred to as the initial collecting tubule.

In the human kidney, individual nephrons do not begin to merge until the cortical collecting duct, which is characterized by yet another cell type, the principal cell. Principal cells begin to appear among CNT cells at the transition from the connecting tubule to the initial collecting tubule. on the average, the confluence of 10-12 initial collecting tubules near the cortical surface forms a cortical collecting duct, which then runs unbranched through the cortex and the outer medulla until it reaches the inner medulla. The inner medullary collecting ducts fuse successively to form the ducts of

Bellini, each of which carries the urine originating from approximately 2800 glomeruli.

Yet one additional cell type has not been discussed. The intercalated cells, which are responsible for urinary acidification, begin to appear in the early part of the connecting tubule where they are intermingled in varying proportions with CNT cells. In the cortical collecting duct, the proportion of intercalated to principal cells is about 1:2, but this proportion decreases in the medullary collecting duct until intercalated cells disappear in the inner medullary collecting duct.

The localization of different cells along the distal nephron segments is of more than histological interest because the cell type determines both the transport processes that occur and the hormones that regulate them. For example, as will be discussed in detail in Chapter 27 in the section titled Transport in the Distal Tubule and Collecting Duct, CNT and principal cells respond to the hormones aldosterone and vasopressin that regulate, respectively, Na+ reabsorption via an electrogenic Na+ channel and water reabsorption. In contrast, DCT cells reabsorb Na+ and Cl- by an electroneutral cotransporter that is inhibited by thiazide diuretics, and they do not reabsorb water in the presence or absence of vasopressin.

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