Anatomy and Physiology: The Excretory System

The Excretory System

When many people look at the cardiovascular system they often tend to accentuate the positive contents in the blood—food, water, and oxygen—and rarely focus on the negative. True, I did talk about CO₂ and the lungs, but what about other wastes, particularly nitrogenous wastes from the breakdown of proteins. This section rectifies that error and concentrates on eliminating the negative.

Have you ever looked at the alcohol content of wine? It is always around 12 percent because at that concentration the alcohol (a toxic waste) kills the yeast that makes it. If we did not have our own sewage system, we would poison ourselves! This condition, a toxic level of urea (a toxic, nitrogenous waste) in the blood, is called uremia.

Not all of the waste is removed by the stars of this section, the kidneys, but the kidneys are members of the only body system concerned mainly with waste removal. All the other organs that also act as waste removers are covered elsewhere, and they are members of other systems: the lungs (remove CO₂, heat, and H₂O), skin (removes heat, H₂O, CO₂, salts, and a little urea), and the gastrointestinal tract (removes solid wastes, H₂O, CO₂, salts, and heat). This section focuses on the kidneys.

Function Junction

Compared to some other body systems, the excretory (or urinary) system has a fairly short list of functions. All the functions, however, are related to the composition of the blood, which undergoes some dramatic changes before it makes its way out through the renal vein. The kidneys, in fact, receive anywhere from 20 to 25 percent of the blood pumped out of the heart every minute (about 1200 ml of the cardiac output). The excretory system …

• Regulates blood volume and composition by removing wastes (and thus making urine) and excess water, and by excreting specific wastes (such as H+ ions—lowering pH).
• Regulates blood pressure through release of renin, and the action of the nephron.
• Aids the metabolism by making new glucose (gluconeogenesis), stimulating production of RBC, and converting D₃ made by the skin into the hormone calcitrol.