Mammalian Kidneys

Filtrate in Blood plasma Bowman's (300 mosm/l) capsule

Proximal convoluted tubule

Distal convoluted tubule

Blood in

Blood out

Plasma Blood

Urine

| The thick segment of the ascending limb pumps NaCl out of the urine and into the tissue fluid, but H2O cannot follow, because this region of the tubule is impermeable to water. Continued pumping of NaCl from the thick ascending limb sets up a concentration gradient in the renal medulla.

I Increased concentration of NaCl in the tissue fluid causes osmotic absorption of water from the descending limb, thus concentrating the tubule fluid that enters the ascending limb.

| The urine entering the collecting duct is less concentrated than the tissue fluid, so as urine passes down the collecting duct it loses water to the tissue fluid and becomes more and more concentrated.

Water resorbed from the descending limb and the collecting duct leaves the medulla in the vasa recta.

The lower collecting duct is permeable to urea as well as to water. Urea is very concentrated in the urine at this point, so it diffuses into the tissue fluid. Some urea enters the ascending limb and is recycled.

51.10 Concentrating the Urine A countercurrent multiplier mechanism enables the kidney to produce urine that is far more concentrated than mammalian blood plasma.

sively) from the tubule fluid and moves it into the surrounding tissue fluid. The thick ascending limb is not permeable to water, so the resorption of Na+ and Cl- raises the concentration of those solutes in the surrounding tissue fluid.

The thin descending limb, in contrast, is rather permeable to water, but not very permeable to Na+ and Cl-. Since the surrounding tissue fluid has been made more concentrated by the Na+ and Cl- resorbed from the neighboring thick ascending limb, water is withdrawn osmotically from the tubule fluid in the descending limb. Therefore, the fluid in the descending limb becomes more and more concentrated as it flows toward the hairpin turn at the bottom of the renal medulla.

Urine

The thin ascending limb, like the thick ascending limb, is not permeable to water. It is, however, permeable to Na+ and Cl-. As the concentrated tubule fluid flows up the thin ascending limb, it is more concentrated than the surrounding tissue fluid, so Na+ and Cl- diffuse out of it. When the tubule fluid reaches the thick ascending limb, active transport continues to move Na+ and Cl- from the tubule fluid to the tissue fluid.

As a result of the processes described above, the tubule fluid reaching the distal convoluted tubule is less concentrated than the blood plasma, and the solutes that have been left behind in the renal medulla have created a concentration gradient in the surrounding tissue fluid. The tissue fluid of the renal medulla becomes more and more concentrated as we move from the border with the cortex down to the tips of the renal pyramids.

You may wonder why the blood flow through the medulla does not wash out the concentration gradient established by the loops of Henle. This is the significance of the parallel arrangement of the descending and ascending per-itubular capillaries—the vasa recta—in the medulla. Coun-tercurrent exchange between the descending and ascending vessels facilitates preservation of the concentration gradient.

Water resorption begins in the distal convoluted tubule

The first portion of the distal convoluted tubule has properties similar to the thick ascending limb of the loop of Henle. Na+ and Cl- are transported out of the tubule fluid, and water cannot follow. As a result, the tubule fluid becomes even more dilute. The later sections of the distal convoluted tubule, however, can be permeable to water, and water can be osmotically drawn into the surrounding tissue fluid. As the tubule fluid flows from the distal tubule to the collecting duct, it can be below or equal to the osmolarity of the blood plasma.

Urine is concentrated in the collecting duct

The tubule fluid entering the collecting duct is at the same solute concentration as the blood plasma, but its composition is considerably different from that of the plasma. The major solute in the tubule is now urea, since salts were resorbed earlier in the nephron. As the tubule fluid flows down the collecting duct, it loses water osmotically to the surrounding tissue fluid.

The concentration gradient established in the renal medulla by the loops of Henle creates the osmotic potential that withdraws water from the collecting ducts. The collecting ducts begin in the renal cortex and run through the renal medulla before emptying into the ureter at the tips of the renal pyramids. As the solute concentration of the surrounding tissue fluid increases, more and more water can be absorbed from the urine in the collecting duct. By the time it reaches the ureter, the urine can become greatly concentrated, with urea as a major solute.

As water is withdrawn from the collecting duct, some urea also leaks out into the medullary tissue, adding to its osmotic potential. This urea diffuses back into the loop of Henle and is returned to the collecting duct. The recycling of urea in the renal medulla contributes significantly to the ability of the kidney to concentrate the urine in the collecting duct.

Overall, the ability of a mammal to concentrate its urine is determined by the maximum concentration gradient it can establish in its renal medulla. An important adaptation for increasing the concentration gradient is to increase the lengths of the loops of Henle. The desert gerbil, for example,

Mammalian Kidney

51.11 The Ability to Concentrate The ability of the mammalian kidney to concentrate urine depends on the lengths of its loops of Henle relative to the overall size of the kidney.The kidney shown here, from a desert gerbil, has a single renal pyramid with loops of Henle so long that they protrude out of the medulla and into the ureter.

has such extremely long loops of Henle that its renal pyramid (each of its kidneys has only one, in contrast to ours) extends far out of the concave surface of the kidney (Figure 51.11). These animals are so effective in conserving water that they can survive on the water released by the metabolism of their food; they do not need to drink!

The kidneys help regulate acid-base balance

Besides regulating salt and water balance and excreting nitrogenous wastes, the kidneys have another important role: They regulate the hydrogen ion concentration (the pH) of the blood. Blood pH is a critical variable because it influences the structure, and therefore the function, of proteins.

One way to minimize pH changes in a chemical solution is to add a buffer—a substance that can either absorb or release hydrogen ions (see Chapter 2). The major buffers in the blood are bicarbonate ions (HCO3-; see Figure 48.14) that are formed from the disassociation of carbonic acid, which in turn is formed by the hydration of CO2 according to the following equilibrium reaction:

H2CO:

You can see that if excess H+ ions are added to this reaction mixture, the reaction will move to the left and absorb the excess H+. On the other hand, if H+ ions are removed from the reaction mixture, the reaction will move to the right and supply more H+ ions.

The HCO3- buffer system is important for controlling the pH of the blood because the reaction can be pushed to the right and pulled to the left physiologically. The lungs control the levels of CO2 in the blood, thus altering the acid portion of the reaction. The kidneys control the base portion of the

1Ï Sodium ions (Na+) and bicarbonate ions (HCO-) are filtered in the glomerulus.

21 Renal tubule cells T secrete H+ in ^ exchange for Na+.

1Ï Sodium ions (Na+) and bicarbonate ions (HCO-) are filtered in the glomerulus.

21 Renal tubule cells T secrete H+ in ^ exchange for Na+.

Renal Cell Ion Exchange

Renal tubule

3] CO2 is formed by the reaction T of HCO- and H+ and diffuses [ into the tubule cell._

CO2 is converted back to HCO3 in renal tubule cell and transported back into the tissue fluid.

Renal tubule

3] CO2 is formed by the reaction T of HCO- and H+ and diffuses [ into the tubule cell._

CO2 is converted back to HCO3 in renal tubule cell and transported back into the tissue fluid.

51.12 The Kidney Excretes Acids and Conserves Bases Bicarbonate ions are filtered out of the glomerulus, and renal tubule cells secrete hydrogen ions into the tubule fluid. In the renal tubule, the filtered bicarbonate buffers the secreted hydrogen ions and keeps the urine from becoming too acidic.The CO2 formed by the reaction of bicarbonate and hydrogen ions is converted back to bicarbonate by the renal tubule cells and transported back into the tissue fluid.

reaction by altering the levels of H+ and HCO3- ions in the blood. The renal tubules secrete H+ into the tubule fluid and resorb HCO3- (Figure 51.12). The kidney has other buffering systems as well, and together they greatly enhance the ability of the kidney to eliminate acid from the blood.

ulatory adjustments compensate for decreases in cardiac output or decreases in blood pressure so that the GFR remains constant (Figure 51.13).

One autoregulatory mechanism is the dilation (expansion) of the afferent renal arterioles when blood pressure falls. This dilation decreases the resistance in the arterioles and helps maintain blood pressure in the glomerular capillaries. If arteriole dilation does not keep the GFR from falling, the kidney releases an enzyme, renin, into the blood. Renin acts on a circulating protein to convert it into an active hormone called angiotensin.

Angiotensin has several effects that help restore the GFR to normal. First, angiotensin constricts the efferent renal arterioles, which elevates blood pressure in the glomerular capillaries. Second, it constricts peripheral blood vessels all over the body—an action that elevates central blood pressure. Third, it stimulates the adrenal cortex to release the hormone aldosterone. Aldosterone stimulates sodium resorption by the kidney, thereby making its resorption of water more effective. Enhanced water resorption helps maintain blood volume and therefore central blood pressure. Finally, angiotensin acts on the brain to stimulate thirst. Increased water intake in response to thirst increases blood volume and blood pressure.

Regulation of Kidney Functions

Several regulatory mechanisms act on the kidneys to maintain blood osmolarity and blood pressure. We will discuss these mechanisms separately, but keep in mind that they are always working together.

The kidneys act to maintain the glomerular filtration rate

If the kidneys stop filtering blood, they cannot accomplish any of their functions. The maintenance of a constant glomerular filtration rate (GFR) depends on an adequate blood supply to the kidneys at an adequate blood pressure. Therefore, the kidneys have mechanisms to maintain their blood supply and blood pressure regardless of what is happening elsewhere in the body. Because these adaptations of the kidney support the maintenance of kidney function, they are called autoregulatory mechanisms. The kidney's autoreg-

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Essentials of Human Physiology

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Responses

  • Jessie
    How kidneys produce urine more concentrated plasma?
    8 years ago
  • sarah
    How can the kidney produce urine more concentrated than plasma?
    8 years ago

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