In the first phase of a cardiac cycle the atria are at rest, allowing blood to pour in from the body and lungs. This phase is called ____ ____. As the pressure rises, the tricuspid & bicuspid (mitral) valves open to allow blood to pass into the relaxed ventricles

• A. Atrial Diastole
• B. Atrial Systole
• C. Ventricular Diastole
• D. Ventricular Systole

Answer: (A)

The phase described is atrial diastole. During this time the atria are relaxed and filling from the venous return (blood coming from the body and lungs). As atrial pressure rises, it becomes greater than the ventricular pressure, causing the tricuspid and mitral valves to open so blood can flow passively into the relaxed ventricles. This is the period of ventricular filling that occurs before atrial contraction. The alternative would be atrial systole, when the atria actively contract to push any remaining blood into the ventricles, but the scenario given fits atrial diastole.