30.4 The Excretory System

THINK ABOUT IT about it It's a hot day, and you've been getting thirsty for hours. Finally, you get the chance to go inside, and you gulp down more than a liter of water. The water tastes great, but as you drink, you begin to wonder. Where's all that water going? Will it just dilute your blood, or is something in your body making sure that everything stays in balance? Structures of the Excretory System

What is the principal role of the structures of the excretory system?

The chemistry of the human body is a marvelous thing. An intricate system of checks and balances controls everything from your blood pressure to your body temperature. Nutrients are absorbed, stored, and carefully released when they are needed. However, every living system, including the human body, produces chemical waste products, some of which are so toxic that they will cause death if they are not eliminated.

For example, as a normal consequence of being alive, every cell in the body produces waste compounds, including excess salts and carbon dioxide. Ammonia, one of the most toxic of these waste compounds, is produced when the amino acids from proteins are used for energy. Ammonia is converted to a less toxic compound called urea, but it, too, must be eliminated from the body. The process by which these metabolic wastes are eliminated to maintain homeostasis is called excretion. Excretion is one part of the many processes that maintain homeostasis.

The excretory system, which includes the skin, lungs, liver, and kidneys, excretes metabolic wastes from the body. The ureters, urinary bladder, and urethra are also involved in excretion. Figure 30–18 shows the major organs of excretion.

In Your Notebook Make a two-column table that lists the organs of excretion in the first column and their function in the second column.

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