20.13 In humans, the kidneys regulate water balance.

The kidney is an organ in vertebrates that helps maintain homeostasis by regulating water balance and solute concentration in body fluids. It accomplishes this by filtering blood—as much as 2,000 liters (or almost 400 times the total volume of blood in your body!) passes through the human kidneys each day—and reabsorbing water and other substances needed by the body. The kidneys accomplish three primary functions.

• 1. Filtration. As blood passes through the kidneys, it is filtered. During this process, water, ions, and other dissolved substances are removed from the blood as a filtrate, leaving behind just cells and large proteins.
• 2. Reabsorption. The filtrate is then processed and modified, as water, nutrients (including glucose and amino acids), and some ions are reabsorbed into the blood, concentrating the filtrate.
• 3. Secretion. Following concentration of the filtrate, the substances not needed by the body are excreted in the urine.

The amount of urine excreted each day depends on an individual’s fluid intake and water loss (such as due to perspiration) and typically ranges from 0.7 to 2.0 liters. In controlling the amount of water excreted in urine or reabsorbed, the kidneys play an important role in preventing dehydration, even as fluid consumption and fluid loss may vary tremendously from day to day.

The filtering of blood by the kidneys begins as blood flows into blood vessels within each kidney via a renal artery. The filtered blood leaves the kidney via a renal vein, while waste products isolated during filtration and any excess water removed from the bloodstream pass to the bladder, and from there are excreted as urine (FIGURE 20-24).

A human has two kidneys, each about the size of a fist, located on either side of the spine, just above the waist. Each kidney is made up of approximately a million nephrons, the structural units that accomplish the work of the kidneys. A nephron consists of two basic components: a nephron tubule and a mass of blood vessels that work together to accomplish the tasks of filtration, reabsorption, and secretion (FIGURE 20-25). The blood-filtering unit of the nephron is a mass of capillaries called a glomerulus, and the ball-like structure that surrounds it is called Bowman’s capsule. Each Bowman’s capsule is connected to a single, long, urine-producing tube that excretes its filtered fluid into a collecting duct.

The capillaries in the glomerulus are porous, and blood pressure forces out water and small molecules and ions (but not blood cells or most proteins) through the capillary walls. Fluid that accumulates in Bowman’s capsule, called filtrate, contains salts, sugars, amino acids, vitamins, and many other molecules, all at the same concentration as in the blood.

As the filtrate moves from Bowman’s capsule through the urine-collecting tubule of the nephron, the vast majority of its water and dissolved solutes must be reabsorbed. In fact, of the 2,000 liters of blood that pass through the kidneys each day, only about 1.5 liters of urine are produced and excreted.

Reabsorption in the long tubule of the nephron is a complex process. It begins with the active transport of sodium ions out of the tubule, which causes water and chloride ions to follow, moving out by passive transport.

The tubule (and the filtrate it contains) then loops from the outer part of the kidney, called the cortex (see Figure 20-24), down into the innermost part of the kidney and back again, in a path called the loop of Henle. As the tubule passes to the innermost part of the kidney, it encounters a higher solute concentration in the interstitial fluid, outside the tube. This creates an osmotic gradient, which causes more and more water to be drawn out of the tubule, moving from the filtrate into the interstitial fluid.

Then, as the filtrate moves up toward the kidney cortex again, more salt is lost, through diffusion and active transport. In the cortex, the tubule empties into the collecting duct, which passes through the innermost part of the kidney again, returning additional water to the interstitial fluid and further concentrating the urine for excretion. The collecting duct eventually merges with collecting ducts from other nephrons, ultimately forming the ureter, which empties into the urinary bladder. From the bladder, urine passes through the urethra and is excreted from the body.

Filtering the blood and producing urine not only follows a complicated path but is very energetically expensive. Considerable amounts of energy are expended in the active transport of solutes that leads to the recovery of water. Nonetheless, some animals, such as kangaroo rats, are so efficient at reabsorbing water that they never have to drink water at all. They can recover nearly all of the water filtered by their kidneys so that the water contained in their food and generated as a by-product during cellular respiration is sufficient.

Nitrogen-containing compounds are among the most important of the metabolic waste products that are removed from the blood by the kidneys. Produced from the breakdown of proteins and nucleic acids, this nitrogen tends to be in the form of ammonia, which is generally very toxic to organisms. Some organisms—mostly aquatic organisms—are able to rid their bodies of excess nitrogen by simply excreting the ammonia. Terrestrial animals (and many marine animals), however, cannot consume sufficient water to safely dilute toxic ammonia. Instead, these organisms use an energetically expensive process that combines ammonia with carbon dioxide to produce urea, which can be stored for longer periods of time and at higher concentrations (thereby requiring much less water). Insects, terrestrial snails, birds, and many reptiles use the least water but the most energy in removing nitrogenous wastes, which are excreted as a paste called uric acid.

The human kidneys are so effective at filtering blood and concentrating the many waste products of metabolism in urine that it is possible to detect the use of many drugs through urinalysis. Most drug-screening urinalysis does not actually involve testing for the presence of the drugs themselves, which may remain in the body for only a short time. Instead, the tests look for the presence of chemicals, called metabolites, that result from breakdown of the drugs (FIGURE 20-26). Metabolites have been identified for many drugs, including Ecstasy, cocaine, methamphetamines, and marijuana. In the case of marijuana, the metabolites are fat-soluble. This means that they are stored in fat cells indefinitely and released only when fat from those cells is metabolized for energy. As a consequence, marijuana can be detected a month or longer after the last use.