The human heart is the circulatory system's pump, which forces blood through the blood vessels of the body. To pump blood, the muscle tissue of the heart contracts. The contraction begins in the upper two chambers of the heart, called the atria, and then proceeds to the lower two chambers, the ventricles. A complete contraction and relaxation of the heart is a single heartbeat, also called a cardiac cycle.
In the accompanying animation, we examine the events of the cardiac cycle, which can be divided into a phase in which the ventricles are contracted (systole) and a phase in which the ventricles are relaxed (diastole). We also correlate these phases of the cycle with the changing pressures in the ventricles and aorta and the changing blood volume in the ventricles.
The heart operates through the expansion and contraction of its chambers and by the opening and closing of valves at the right time in the cycle.
The heart pumps blood by expanding and then contracting, first taking in blood and then pushing it through the circulatory system. During each beat of the heart, the ventricles relax, expanding their volume to collect blood from the atria. The ventricles then contract, pumping half their blood volume through the lungs and through the rest of the body. As the ventricles relax again, blood flows in from the atria, and then the atria contract, topping off the blood in the ventricles.
We can also examine the cardiac cycle from the perspective of blood pressure and the function of heart valves. Increasing blood pressure in the heart drives the pumping of blood throughout the circulatory system. During diastole, when the ventricles are relaxed, the blood is under relatively low pressure, but when the ventricles contract, the pressure increases approximately ten fold. The higher pressure in the ventricles relative to the atria causes the atrioventricular valves to snap shut ("lub"), preventing backflow into the atria. The blood from the left ventricle is then forced through the aortic valve and into the aorta. As the blood passes through the aorta, the pressure in this large artery increases, and the aortic walls elastically expand to accommodate the flowing blood. During diastole, the aortic walls recoil. The recoil maintains the relatively high blood pressure in the aorta, even as the pressure drops significantly in the ventricles. The higher pressure in the aorta relative to the left ventricle forces the aortic valve to slam shut ("dub"), and prevents blood from flowing back into the heart. Together, the changes in blood pressure and the anatomy of the valves force blood to flow through the heart in one direction.
Textbook Reference: Concept 32.3 A Beating Heart Propels the Blood