The walls of the atria are thinner than those of the ventricles because the atria
- A. pump blood only into the adjacent ventricles
- B. never contract at all
- C. pump blood to the whole body
- D. contain no muscle tissue
Explanation
Atria need only enough force to top up the ventricles below, so their muscle is thin. The left ventricle, which drives blood around the entire body, has the thickest wall of all four chambers.