The Liver and Homeostasis The liver is technically part of the digestive system because it produces bile, which aids in the digestion of fats. However, it is also fair to say that the liver is one of the body's most important organs for homeostasis.

For example, when proteins are broken down for energy, ammonia, a toxic byproduct, is produced. The liver quickly converts ammonia to urea, which is much less toxic. The kidneys, as you will read a bit later, then remove urea from the blood. The liver also converts many dangerous substances, including some drugs, into compounds that can be removed from the body safely.

One of the liver's most important roles involves regulating the level of a substance we take almost for granted as something completely harmless—the simple sugar, glucose. Glucose is obtained from the foods we eat, and cells take glucose from the blood to serve as a source of energy for their everyday activities. Naturally, right after a meal, as the body absorbs food molecules, the level of glucose in the blood begins to rise. That's where the liver comes in. By taking glucose out of the blood, it keeps the level of glucose from rising too much. As the body uses glucose for energy, the liver releases stored glucose to keep the level of the sugar from dropping too low.

The liver's role in keeping blood glucose levels within a certain range is critical. Too little glucose, and the cells of the nervous system will slow down to the point that you may lose consciousness and pass out. On the other hand, too much glucose gradually damages cells in the eyes, kidneys, heart, and even the immune system. Abnormally high levels of glucose are associated with a disease called diabetes. In diabetes, changes occur in either the pancreas or body cells that affect the cells' ability to absorb glucose. Diabetes, one of the fastest-growing health problems in the developed world, is the unfortunate result of failure of homeostasis with respect to blood glucose levels.

30.1 Assessment

Review Key Concepts

1. 1. Review What are the four types of tissues?

   2. Explain Describe the function of three organ systems depicted in Figure 30–2.

   3. Classify Compare the characteristics of two types of tissues. Identify parts of the body that contain these types of tissues.

2. 1. Review What is homeostasis?

   2. Explain What are two roles of the liver in maintaining homeostasis?

   3. Apply Concepts Do you think that feelings of hunger and fullness are an example of feedback inhibition? Explain.

VISUAL THINKING

3. Draw a Venn diagram to relate the four basic levels of organization in the human body. Provide at least three examples for each level of organization. Hint: Your Venn diagram should have a nesting structure. One set of examples could be skin cells, epithelial tissue, skin, and the integumentary system.

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