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The Renal System Study Guide (page 2)

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Updated on Sep 22, 2011

How Does a Kidney Work?

In each kidney are at least a million individual units called nephrons. Functionally, nephrons are similar to the alveoli in the lungs. The alveoli are structured to function as gas exchange interfaces, whereas the kidney nephrons are structured to function as fluid interchange points.

Each nephron consists of a bed of capillaries with thin walls surrounded by a tube structure called Bowman's capsule. Filtration of the blood occurs for water, nutrients, and wastes through the capillaries into the Bowman's capsule. Most of the water and nutrients are reabsorbed right away. This concentrates the wastes in the fluid inside the Bowman's capsule tubules. We now call this fluid urine.

The tubules leading away from Bowman's capsule eventually arrive at the collecting duct where even more water may be absorbed. The collecting duct leads to the interior of the kidney where urine collects and flows into the ureters, which take it to the urinary bladder. Urine will collect in the urinary bladder until the urge to urinate is strong enough that the urine is expelled from the body through the urethra.

Monitoring Water Levels

Part of what the kidneys do is regulate the amount of water that circulates in the bloodstream. If the brain detects low levels of water in the blood, then more antidiuretic hormone (ADH) is released. As its name implies, it causes the kidneys to reabsorb water into the bloodstream, thus concentrating the urine and preserving water for the body.

The brain is very good at keeping a balance on many interrelated factors. Our nervous system respond to sensors in the body that keep track of blood sugar, blood pressure, blood carbon dioxide, blood oxygen, blood dissolved salts, and so on. Lack of water affects all these values, which is how the body detects it.

When you drink too much alcohol, you usually urinate more often. This is because alcohol inhibits the action of the ADH signal from the brain. Thus, nothing tells the kidneys to conserve water.

In Short

The series of physical and chemical processes that work to maintain an internal equilibrium is called homeostasis. The circulatory system and the filtration/excretion system work very closely together to help maintain homeostasis, and wastes other than carbon dioxide are produced during cellular metabolism. For the most part, these wastes cannot be eliminated in the lungs, and instead, they must be filtered out of the blood by the kidneys and then be excreted.

The kidneys are sophisticated filters. They are able to take urea—which is less toxic than ammonia but still not well tolerated in high concentrations—and convert it to urine, which is very soluble in water and can be excreted. The quantity of water in the body is tightly controlled, and the kidneys play an important role in that necessary function. The amount of water in the blood is controlled in what is called a negative feedback loop.

Practice problems of this concept can be found at: The Renal System Practice Questions

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