The anatomical organization of nephrons makes it possible for the mammalian kidney to produce a urine more concentrated than the blood, thereby conserving water to maintain extracellular fluid volume. Bulk reabsorption of salts, other valuable solutes, and water takes place in the proximal convoluted tubule. The loops of Henle act as a countercurrent multiplier, creating a concentration gradient of the interstitial fluids in the renal medulla. Collecting ducts run through the renal medulla and lose water osmotically to the surrounding interstitial fluids, concentrating the urine. The renal tubules contribute to acid–
learning outcomes
You should be able to:
Describe the internal organization of the mammalian kidney.
Describe the mechanism whereby most of the glomerular filtrate is reabsorbed in the proximal convoluted tubule.
Describe how the loop of Henle maintains a concentration gradient in the extracellular fluid of the medulla of the kidney.
Describe how the mammalian excretory system is involved in regulating pH of the extracellular fluid.
If you could measure the concentration of mitochondria in mammalian kidney tissues, where would you find the highest concentration, the lowest concentration, and an intermediate concentration? Explain your answer.
The highest concentration of mitochondria would be in the renal cortex because of all of the active transport mechanisms in the convoluted tubules. The lowest concentration would be at the bottom of the renal medulla where there are only thin regions of the loops of Henle. The intermediate concentration would be in the upper level of the renal medulla because of the active transporters of Na+ in the thick ascending limb of the loop of Henle.
How do the loops of Henle act as a countercurrent multiplier?
The thick ascending limb of the loop of Henle actively transports Na+ from the tubular fluid to the extracellular fluid, and Cl– follows. The increase in NaCl in the extracellular fluid of the medulla pulls water out of the descending limb osmotically, increasing the concentration of the tubular fluid. Since the ascending limb is not permeable to water, the more concentrated tubular fluid cannot pull water into the ascending limb. When it reaches the thick ascending limb, more NaCl is pumped out into the extracellular space. Through this mechanism, the concentration of the extracellular fluid in the renal medulla is maintained higher than the blood osmolarity.
If you compared a water rat and a desert rat of the same size, how would you expect their kidneys to differ in anatomical structure? Explain your answer.
The desert rat would have longer loops of Henle and therefore a longer renal medulla. The desert rat has evolved under selective pressure to conserve water. One way of doing that is to produce a more concentrated urine. The longer the loops of Henle, the greater the concentration gradient that can be created in the renal medulla, so the desert rat should have longer loops of Henle.
How does the active secretion of H+ ions by the renal tubule cells result in an increase in the bicarbonate concentration in the blood?
Bicarbonate in the blood is filtered into the tubular fluid, but the H+ ions transported into the tubular fluid by the tubule cells combine with that bicarbonate to produce H2CO3 (carbonic acid), which dissociates into H2O and CO2. The CO2 diffuses into the tubule cells, where its hydration to H2CO3 is catalyzed by carbonic anhydrase. The resulting HCO3– is transported across the basal end of the tubule cells and into the interstitial fluid.
The kidneys contribute to homeostasis in several ways, including regulating extracellular fluid volume, maintaining the osmotic concentration and ionic composition of the extracellular fluid, and regulating pH. As you will see next, the kidneys also play a major role in regulating blood pressure.