The semilunar valves allow blood to leave the ventricles and enter the pulmonary and systemic circulations, but these valves prevent blood from returning from the arteries to the ventricles. The Purkinje fibers transmit the impulse into the ventricular muscle and cause it to contract. The heart is a two-step pump. Blood is ejected from the ventricles until the pressure within the falls below the pressure in the arteries. When the pressure in the ventricles falls below the pressure in the atria, a phase of rapid filling of the ventricles occurs, followed by the final filling caused by contraction of the atria.
Abnormal valves can cause abnormal sounds called murmurs.
The long action potential and long refractory period of myocardial cells allows the entire mass of cells to be in a refractory period while it contracts. This prevents the myocardium from being stimulated again until after it relaxes. Medium and small arteries and arterioles are less distensible, and thus provide greater resistance to blood flow. The capillary wall may be continuous, fenestrated, or discontinuous. The flow of blood back to the heart is aided by contraction of the skeletal muscles that surround veins.