Cardiac output is defined as which of the following?

• A. The ventricles' fibers stretch.
• B. The resistance to pumping.
• C. The amount of blood flow through the heart in 1 minute.
• D. The amount ejected from the atria.

Answer: (C)

Cardiac output is the volume of blood the heart pumps per minute. It rises when either the heart rate or the amount of blood ejected with each beat (stroke volume) increases, since CO = heart rate × stroke volume. The ventricles' fibers stretch describe preload, which influences stroke volume but isn’t the definition itself. The resistance to pumping is afterload, which also affects CO but again isn’t the definition. The amount ejected from the atria isn’t cardiac output—the atria mainly move blood into the ventricles, not out of the heart. At rest, cardiac output is typically about 4–8 liters per minute and can increase with activity. Therefore, the best definition is the amount of blood the heart pumps per minute.