Brown and Goldstein also found that some people carry genes that produce defective LDL receptors. This causes two problems. First, without working LDL receptors, liver cells can't remove cholesterol from blood. Second, these liver cells don't get the signal to stop producing cholesterol. People with defective LDL receptors have very high cholesterol levels, even if they don't eat much cholesterol or fat.

From Genetic Disease to the Public Does understanding this genetic defect help us understand high cholesterol in the general public? Brown and Goldstein learned that people who eat high-fat diets store excess cholesterol in their liver cells. Those cells then stop making LDL receptors and removing cholesterol from blood. The excess cholesterol is then deposited in arteries. So a diet that is high in cholesterol can cause symptoms similar to those of a genetic disease!

Brown and Goldstein's work led to the development of drugs that can help people with high cholesterol. For example, statins block the synthesis of cholesterol in liver cells. This stimulates the liver to produce more LDL receptors, which then remove excess cholesterol from the blood.

Keeping Your Circulatory System Healthy It is much easier to prevent heart disease than to cure it. Prevention starts when you're young, with healthy habits that include a balanced diet, regular exercise, and not smoking. A healthy diet may protect your arteries from atherosclerosis. Exercise strengthens your heart and helps your circulatory system work efficiently. Never starting to smoke will protect your circulatory system from the many dangerous chemicals in tobacco smoke.

33.2 Assessment

Review Key Concepts

1. 1. Review List the main function of plasma, red blood cells, white blood cells, and platelets.

   2. Infer Hemophilia is a genetic disorder that results from a defective protein in the clotting pathway. What do you think happens to a person with hemophilia who has a minor cut?

2. 1. Review Describe the role of the lymphatic system.

   2. Compare and Contrast How are the functions of veins and lymphatic vessels similar? How are they different?

3. 1. Review What are the risk factors for the three common diseases of the circulatory system?

   2. Form a Hypothesis Why do you think atherosclerosis may lead to hypertension?

4. 1. Review What are two types of cholesterol carriers found in the blood?

   2. Compare and Contrast Explain how high blood cholesterol develops in someone with a genetic disorder versus someone who eats a high-fat diet.

WRITE ABOUT SCIENCE

Creative Writing

5. Use library or Internet resources to research the connection between diet and circulatory disease. Write a short commentary that could be used on a television news program that explains the connection. (Hint: Prepare a cause-and-effect diagram to organize your ideas.)

Table of Contents