78 Steps Health Journal

Nephron Tubules

• Human Anatomy & Physiology Study Course

The tubular portion of a nephron consists of a glomerular capsule, a proximal convoluted tubule, a descending limb of the loop of Henle, an ascending limb of the loop of Henle, and a distal convoluted tubule (fig. 17.5).

The glomerular (Bowman's) capsule surrounds the glomerulus. The glomerular capsule and its associated glomerulus are located in the cortex of the kidney and together constitute the renal corpuscle. The glomerular capsule contains an inner visceral layer of epithelium around the glomeru-lar capillaries and an outer parietal layer. The space between these two layers is continuous with the lumen of the tubule and receives the glomerular filtrate, as will be described in the next section.

Filtrate that enters the glomerular capsule passes into the lumen of the proximal convoluted tubule. The wall of the proximal convoluted tubule consists of a single layer of cuboidal cells containing millions of microvilli; these microvilli increase the surface area for reabsorption. In the process of reabsorption, salt, water, and other molecules needed by the body are transported from the lumen, through the tubular cells and into the surrounding peritubular capillaries.

The glomerulus, glomerular capsule, and convoluted tubule are located in the renal cortex. Fluid passes from the proximal convoluted tubule to the loop of Henle. This fluid is carried into the medulla in the descending limb of the loop and returns to the cortex in the ascending limb of the loop. Back in the cortex, the tubule again becomes coiled and is called the distal convoluted tubule. The distal convoluted tubule is shorter than the proximal tubule and has relatively few mi-crovilli. The distal convoluted tubule terminates as it empties into a collecting duct.

The two principal types of nephrons are classified according to their position in the kidney and the lengths of their loops of Henle. Nephrons that originate in the inner one-third of the cortex—called juxtamedullary nephrons because they are

Physiology of the Kidneys

Physiology of the Kidneys

Ureter

Interlobular artery and vein

Renal cortex

Arcuate artery and vein

Renal medulla

Interlobar artery and vein

Renal artery

Renal pelvis

Renal vein

Ureter

Interlobular artery and vein

■ Figure 17.4 The vascular structure of the kidneys. (a) An illustration of the major arterial supply and (b) a scanning electron micrograph of a glomerulus (300x),

Glomerulus Glomerular capsule

Efferent arteriole Afferent arteriole

Peritubular capillaries

Distal convoluted tubule

Peritubular capillaries

Distal convoluted tubule

Interlobular vein

Collecting duct

Interlobar artery and vein

Interlobar artery and vein

Interlobular vein

Collecting duct

■ Figure 17.5 The nephron tubules and associated blood vessels. In this simplified illustration, the blood flow from a glomerulus to an efferent arteriole, to the peritubular capillaries, and to the venous drainage of the kidneys is indicated with arrows. The names for the different regions of the nephron tubules are indicated with boldface type.

528 Chapter Seventeen

■ Figure 17.6 The contents of a renal pyramid. (a) The position of cortical and juxtamedullary nephrons is shown within the renal pyramid of the kidney. (b) The direction of blood flow in the vessels of the nephron is indicated with arrows.

next to the medulla—have longer loops of Henle than the more numerous cortical nephrons, which originate in the outer two-thirds of the cortex (fig. 17.6). The juxtamedullary nephrons play an important role in the ability of the kidney to produce a concentrated urine.

A collecting duct receives fluid from the distal convoluted tubules of several nephrons. Fluid is then drained by the collecting duct from the cortex to the medulla as the collecting duct passes through a renal pyramid. This fluid, now called urine, passes into a minor calyx. Urine is then funneled through the renal pelvis and out of the kidney in the ureter.

Polycystic kidney disease is a condition inherited as an autosomal dominant trait (see chapter 20) that affects 1 in every 600 to 1,000 people. This disease is thus more common than sickle-cell anemia, cystic fibrosis, or muscular dystrophy, which are also genetic diseases. In 50% of the people who inherit the defective gene (located on the short arm of chromosome 16), progressive renal failure develops during middle age to the point that dialysis or kidney transplants are required. The cysts that develop are expanded portions of the renal tubule. Cysts that originate in the proximal tubule contain fluid that resembles glomerular filtrate and plasma. Cysts that originate in the distal tubule contain fluid with a lower NaCl concentration and a higher potassium and urea concentration than plasma as a result of the transport processes that occur during the passage of fluid through the tubules.

Test Yourself Before You Continue

1. Describe the "theme" of kidney function in a single sentence. List the components of this functional theme.

2. Trace the course of blood flow through the kidney from the renal artery to the renal vein.

3. Trace the course of tubular fluid from the glomerular capsules to the ureter.

4. Draw a diagram of the tubular component of a nephron. Label the segments and indicate which parts are in the cortex and which are in the medulla.

Physiology of the Kidneys

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