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Hypertonic Blood Vessels

Medullary interstitial fluid

Collecting duct

Concentrated urine

Medullary interstitial fluid

Collecting duct

Concentrated urine

Figure

(a) The distal convoluted tubule and collecting duct are impermeable to water, so water may be excreted as dilute urine. (b) If ADH is present, however, these segments become permeable, and water is reabsorbed by osmosis into the hypertonic medullary interstitial fluid.

the tubular fluid inside becomes hypotonic because it is losing its solute.

In contrast, the epithelium of the descending limb (thin segment) is quite permeable to water but relatively impermeable to solutes. Because this segment is surrounded by hypertonic fluid created by the ascending limb, water tends to leave the descending limb by osmosis. Thus, the contents of the descending limb become more and more concentrated, or hypertonic (fig. 20.24).

This very concentrated tubular fluid now moves into the ascending limb, and sodium chloride (NaCl) is again actively reabsorbed into the medullary interstitial fluid, raising the interstitial NaCl concentration even more. With the increased interstitial fluid solute concentration, even more water diffuses out of the descending limb, further increasing the salt concentration of the tubular fluid. Each time this circuit is completed, the concentration of NaCl increases, or multiplies. For this reason, the mechanism is called a countercurrent multiplier. In humans, this mechanism creates a tubular fluid solute concentration near the tip of the loop that is more than four times the solute concentration of plasma (fig. 20.25a).

The solute concentration of the tubular fluid progressively decreases toward the renal cortex. Since the descending limb of the loop is permeable to water, the interstitial fluid at any level of the loop is essentially in equilibrium with the fluid in the tubule. Thus, the concentration gradient in the loop is also found in the interstitial fluid (fig. 20.25b).

The vasa recta is another countercurrent mechanism that maintains the NaCl concentration gradient in the renal medulla. Blood flows relatively slowly down the descending portion of the vasa recta, and NaCl enters it by diffusion. Then, as the blood moves back up toward the renal cortex, most of the NaCl diffuses from the blood and reenters the medullary interstitial fluid. Consequently, the bloodstream carries little NaCl away from the renal medulla, preserving the gradient (fig. 20.26).

To summarize, the countercurrent multiplier creates a large concentration gradient for water reabsorption in the interstitial fluid surrounding the distal convoluted tubules and the collecting ducts of the nephron. The epithelial lining of these structures is impermeable to water, unless ADH is present. The higher the blood levels of ADH, the more permeable the epithelial lining becomes, leading to increased water reabsorption and the production of concentrated urine. In this way, soluble wastes and other substances can be excreted in a minimum of water, thus minimizing the loss of body water when dehydration is a threat. If the body fluids contain excess water, ADH secretion is decreased, and the epithelial linings of the distal convoluted tubule and the collecting duct become less permeable to water. Thus, less water is reabsorbed, and the urine will be more dilute. Table 20.3 summarizes the role of ADH in urine production. Table 20.4 summarizes the functions of different parts of the nephron.

Urea and Uric Acid Excretion

Urea is a by-product of amino acid catabolism in the liver, and its plasma concentration reflects the amount of protein in the diet. Urea enters the renal tubule by filtration. About 50% of it is reabsorbed (passively) by diffusion, but the remainder is excreted in the urine.

Isotonic fluid

Descending limb

Isotonic fluid

Descending limb

Hypotonic fluid

Thick ascending limb

Impermeable to water

Hypotonic fluid

Thick ascending limb

Impermeable to water

Permeable to water

Permeable to water

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