Excretion and the Kidneys

How do the kidneys clean the blood?

What does a kidney do? As waste-laden blood enters the kidney through the renal artery, the kidney removes urea, excess water and minerals, and other waste products. The clean, filtered blood leaves the kidney through the renal vein and returns to circulation.

Each kidney contains nearly a million individual processing units called nephrons. These nephrons are where most of the work of the kidney takes place—impurities are filtered out, wastes are collected, and purified blood is returned to circulation. Blood purification in the kidneys is complex and involves two distinct processes: filtration and reabsorption.

Filtration Passing a liquid or gas through a filter to remove wastes is called filtration. The filtration of blood mainly takes place in the glomerulus (gloh MUR yoo lus). A glomerulus is a small but dense network of capillaries (very small blood vessels) encased in the upper end of each nephron by a hollow, cup-shaped structure called Bowman's capsule. A glomerulus is shown in Figure 30–19.

Because the blood is under pressure and the walls of the capillaries and Bowman's capsule are permeable, much of the fluid from the capillaries flows into Bowman's capsule. The material that is filtered from the blood is called the filtrate. The filtrate contains water, urea, glucose, salts, amino acids, and some vitamins. Large substances in the blood, such as proteins and blood cells, are too large to pass through the capillary walls.

Reabsorption Nearly 180 liters of filtrate pass from the blood into nephron tubules every day. That's the equivalent of 90 2-liter bottles of soft drink. Thank goodness, not all of those 180 liters are excreted. In fact, nearly all of the material that moves into Bowman's capsule makes its way back into the blood. The process by which water and dissolved substances are taken back into the blood is called reabsorption.

A number of materials, including salts, vitamins, amino acids, fats, and glucose, are removed from the filtrate by active transport and reabsorbed by the capillaries. Because water follows these materials by osmosis, almost 99 percent of the water that enters Bowman's capsule is actually reabsorbed into the blood. In effect, the kidney first throws away nearly everything and then takes back only what the body needs. This is how the kidney is able to remove drugs and toxic compounds from the blood—even chemicals the body has never seen before.

A section of the nephron tubule called the loop of Henle is responsible for conserving water and minimizing the volume of the filtrate. The waste material—now called urine—that remains in the tubule is emptied into a collecting duct.

Urine Excretion From the collecting ducts, urine flows to the ureter of each kidney. The ureters carry urine to the urinary bladder for storage until the urine leaves the body through the urethra.

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