Pumping Action of the Heart

Blood Flow Through the Heart
Blood is pumped through the chambers of the heart and out through the great vessels by a simple squeezing action of the heart chambers. You have probably seen a bulb syringe with a glass nozzle like the one pictured in Figure 3-1. Suppose it is full of water. If you squeeze forcefully, expelling the water, you would be imitating the contraction of a heart chamber. This is called systole (sis-toe-lee). After the syringe had been emptied, imagine that you placed the nozzle in a container of water and let the bulb expand so that it filled. This is what a heart chamber does when it relaxes and fills with blood. The movement is called diastole (die-as-toe-lee). You can picture the process by holding your left hand over your right, fists clenched. If your left hand represents the atria, your right hand will represent the ventricles. Now clench your left fist (the atria) while opening your right fist (the ventricles). This is what happens during atrial systole when the atria are pumping blood down into the ventricles. Next, open your left fist and clench your right. This is what happens during ventricular systole when the ventricles are pumping blood out into the two great arteries and the atria are refilling. By alternately opening and clenching your two fists you can similate the coordinated beat of the heart.
Note: The cycle of a heartbeat, in other words, goes through these stages:
Atrial systole: The atria contract, forcing the blood down into the ventricles.
Ventricular systole: The ventricles contract, forcing the blood out the pulmonary artery and aorta.
Atrial diastole: This starts during ventricular systole as the atria begin refilling with blood from the great veins.
Ventricular diastole: This takes place during atrial systole as blood from the atria fills the ventricles.

The rhythmic contraction and relaxation of the ventricles does the work of pumping the blood: atrial contraction is much less important and, in fact, many patients live for years without any pumping action from the atria. If the ventricles stop beating, death follows within minutes.