This permits blood to flow in one direction.

This permits blood to flow in one direction.

• A. Valve
• B. Chamber
• C. Ventricle
• D. Atrium

Answer: (A)

Valves regulate the direction of blood flow. They open to allow blood to move forward and close to prevent it from backing up as pressures change during the cardiac cycle. This one-way flow happens both between the atria and ventricles (the atrioventricular valves) and at the exits to the great arteries (the semilunar valves). For example, when the ventricle contracts, the semilunar valves open to push blood into the aorta or pulmonary artery and then close to stop it from flowing back into the ventricle. Likewise, the atrioventricular valves close during ventricular contraction to keep blood from moving back into the atria and open during filling to let the ventricles receive blood. Without valves, the pressure shifts would cause backflow, preventing the heart from maintaining a unidirectional flow.

Valves regulate the direction of blood flow. They open to allow blood to move forward and close to prevent it from backing up as pressures change during the cardiac cycle. This one-way flow happens both between the atria and ventricles (the atrioventricular valves) and at the exits to the great arteries (the semilunar valves). For example, when the ventricle contracts, the semilunar valves open to push blood into the aorta or pulmonary artery and then close to stop it from flowing back into the ventricle. Likewise, the atrioventricular valves close during ventricular contraction to keep blood from moving back into the atria and open during filling to let the ventricles receive blood. Without valves, the pressure shifts would cause backflow, preventing the heart from maintaining a unidirectional flow.