Regulatory Functions of the Kidneys
The kidneys are involved in maintaining the fluid and electrolyte balance, and also the acid-base balance, of the blood. If the kidneys fail to carry out these vital functions, either hemodialysis or a kidney transplant is needed.
Fluid and Electrolyte Balance
The average adult male body is about 60% water by weight. The average adult female body is only about 50% water by weight because females generally have more subcutaneous adipose tissue, which contains less water. About two-thirds of this water is inside the cells (called intracellular fluid), and the rest is largely distributed in the plasma, tissue fluid, and lymph (called extracellular fluid). Water is also present in such fluids as cerebrospinal fluid and synovial fluid; in Figure 16.6, these fluids are referred to as “other” fluids. For body fluids to be normal, it is necessary for the body to be in fluid balance. The total water intake should equal the total water loss. Table 16.2 shows how water enters the body-namely, in liquids we drink, in foods we eat, and as a by-product of metabolism. We drink water when the osmolarity of the blood rises as determined by the hypothalamus. Table 16.2 also shows how water exits the body-namely, in urine, sweat, exhaled air, and feces. Similar to the gain and loss of water, the body also gains and loses electrolytes. Despite these changes, the kidneys keep the fluid and electrolyte balance of the blood within normal limits. In this way, they also maintain the blood volume and blood pressure.
Figure 16.6 Location of fluids in the body. Most of the body’s water is inside cells (intracellular fluid), and only about one-third is located outside cells (extracellular fluid).
Reabsorption of Water
Because of the process of osmosis, the reabsorption of salt (NaCl) automatically leads to the reabsorption of water until the osmolarity is the same on both sides of a plasma membrane. Most of the salt, and therefore water, present in the filtrate are reabsorbed across the plasma membranes of the cells lining the proximal convoluted tubule. But the amount of salt and water reabsorbed is not sufficient to result in a hypertonic urine-one in which the osmolarity is higher than that of blood. How is it, then, that humans produce a hypertonic urine? We now know that the excretion of a hypertonic urine is dependent upon the reabsorption of water from the loop of the nephron and the collecting duct.
Loop of the Nephron and Collecting Duct
A loop of the nephron has a descending limb and an ascending limb. A long loop of the nephron penetrates deep into the renal medulla. In the ascending limb, salt (NaCl) passively diffuses out of the lower portion and is actively transported out of the upper portion into the tissue of the outer renal medulla (Fig. 16.7). Less and less salt is available for active transport as fluid moves up the thick portion of the ascending limb. Therefore, the concentration of salt is greater in the inner medulla than in the outer medulla. (It is important to realize that water cannot leave the ascending limb because the ascending limb is impermeable to water).
The large arrow to the side in Figure 16.7 indicates that the lowest portion of the inner medulla has the highest concentration of solutes. You can see that this is due not to the presence of salt, but to the presence of urea. Urea is believed to leak from the lower portion of the collecting duct, and it is this molecule that contributes to the high solute concentration of the lowest portion of the inner medulla. Because of the osmotic gradient within the renal medulla, water leaves the descending limb along its entire length. There is a higher concentration of water at the top of the descending limb, and so it takes a lesser amount of solute in the medulla to pull it out. The remaining fluid within the descending limb encounters an even greater osmotic concentration of solute as it moves along; therefore, water continues to leave the descending limb from the top to the bottom. Such a mechanism is called a countercurrent mechanism. At the top of the ascending limb, any remaining water enters the collecting duct. Surprisingly, the fluid inside the nephron is still not hypertonic-the net effect of reabsorption of salt and water so far is the production of a fluid that has the same tonicity as blood plasma. However, the collecting duct also encounters the same osmotic gradient as did the descending limb of the loop of the nephron (Fig. 16.7). Therefore, water diffuses out of the collecting duct into the renal medulla, and the urine within the collecting duct becomes hypertonic to blood plasma.
Antidiuretic Hormone (ADH)
ADH released by the posterior lobe of the pituitary plays a role in water reabsorption at the collecting duct. In order to understand the action of this hormone, consider its name. Diuresis means flow of urine, and antidiuresis means against a flow of urine. When ADH is present, more water is reabsorbed (blood volume and pressure rise), and a decreased amount of urine results.
Figure 16.7 Reabsorption of water at the loop of the nephron and the collecting duct. A hypertonic environment in the tissues of the medulla of a kidney draws water out of the descending limb and the collecting duct. This water is returned to the cardiovascular system. (The thick black line means the ascending limb is impermeable to water).
In practical terms, if an individual does not drink much water on a certain day, the posterior lobe of the pituitary releases ADH, causing more water to be reabsorbed and less urine to form. On the other hand, if an individual drinks a large amount of water and does not perspire much, ADH is not released. In that case, more water is excreted, and more urine forms.