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Figure

(a) Fluid in the ascending limb of the nephron loop becomes hypotonic as solute is reabsorbed. (b) Fluid in the descending limb becomes hypertonic as it loses water by osmosis.

Figure 20.25

(a) As NaCl completes the countercurrent circuit again and again, more water exits the descending limb, leaving more concentrated solute in the tubular fluid in the ascending limb. This, in turn, transports even more solute into the interstitial fluid. (b) As a result, an NaCl concentration gradient is established in the medullary interstitial fluid.

1. Concentration of water in the blood decreases.

2. Increase in the osmotic pressure of body fluids stimulates osmoreceptors in the hypothalamus.

3. Hypothalamus signals the posterior pituitary gland to release ADH.

4. Blood carries ADH to the kidneys.

5. ADH causes the distal convoluted tubules and collecting ducts to increase water reabsorption by osmosis.

6. Urine becomes more concentrated, and urine volume decreases.

1. Concentration of water in the blood decreases.

2. Increase in the osmotic pressure of body fluids stimulates osmoreceptors in the hypothalamus.

3. Hypothalamus signals the posterior pituitary gland to release ADH.

4. Blood carries ADH to the kidneys.

5. ADH causes the distal convoluted tubules and collecting ducts to increase water reabsorption by osmosis.

6. Urine becomes more concentrated, and urine volume decreases.

Urea moves within the kidney in other ways, including the countercurrent multiplier mechanism that concentrates the medullary interstitial fluid. As a result, urea contributes to the reabsorption of water from the collecting duct.

Uric acid is a product of the metabolism of certain nucleic acid bases (the purines, adenine and guanine). Active transport completely reabsorbs the uric acid that

Blood flow

Increasing NaCI

concentration

Blood flow

Increasing NaCI

concentration

Blood flow

Vasa recta

Medullary interstitial fluid

NaCl NaCl

Blood flow

Vasa recta

Medullary interstitial fluid

NaCl NaCl

Figure

A countercurrent mechanism in the vasa recta helps maintain the NaCl concentration gradient in the medullary interstitial fluid (see fig. 20.13).

is filtered. The fact that some uric acid (equal to approximately 10% of the amount filtered) is excreted in the urine reflects uric acid secretion into the renal tubule.

In the inborn error of metabolism gout, uric acid crystals are deposited in certain joints, particularly of the great toe, causing severe pain. Treatments include taking drugs that increase the kidneys' excretion of uric acid and block an enzyme in the biosynthetic pathway for uric acid; limiting intake of foods that are sources of uric acid, including organ meats, anchovies, and sardines; maintaining a healthy weight; and drinking more fluids to dilute the urine, which enhances uric acid excretion.

Gout is an illness with a long history in medicine. Hippocrates mentioned it, and in 1793, English physician Alfred Baring Garrod isolated and implicated uric acid from the blood of a patient with gout and noted that affected individuals often had relatives suffering from it too. At that time, gout was thought to be the result of being a lazy glutton!

U Describe a countercurrent mechanism.

H What role does the hypothalamus play in regulating urine concentration and volume?

B Explain how urea and uric acid are excreted. Urine Composition

Urine composition reflects the amounts of water and solutes that the kidneys must eliminate from the body or retain in the internal environment to maintain homeostasis. It varies considerably from time to time because of differences in dietary intake and physical activity. Urine is about 95% water and usually also consists of urea from the catabolism of amino acids, uric acid from the catabolism of nucleic acids, and creatinine from metabolism of creatine. Urine may also contain a trace of amino acids, as well as electrolytes whose concentrations reflect the amounts included in the diet (see table 20.1). Appendix C (p. 1030) lists the normal concentrations of urine components.

Part

Function

Renal Corpuscle

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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