19.8 The Cardiac Cycle

A cardiac cycle

The inclusive changes within the heart from the initiation of one heartbeat to the start of the next.

One heartbeat involves

the contraction and relaxation of the heart chambers.

Systole

refers to contraction of a heart chamber

Diastole

refers to relaxation of a heart chamber

The alternating contraction and relaxation of both the atria and ventricles causes pressure changes within their chambers. Pressure increases during contraction and decreases during relaxation. These alternating pressure changes are responsible for 2 significant physiologic processes:

• Unidirectional movement of blood through the heart chambers, as blood moves along a pressure gradient (i.e., from an area of greater pressure to an area of lesser pressure)

• Opening and closing of heart valves to ensure that blood continues to move in a "forward" direction without backflow

The contraction and. relaxation of the ___ are the most important driving force to continually move blood through the heart and to open and close heart valves during the cardiac cycle.

ventricles

Ventricular contraction

• Ventricles contract and ventricular pressure rises. The one-way flow of blood occurs as the AV valves are closed, preventing backflow of blood into each atrium, and then the semilunar valves are open, allowing ejection of blood from a ventricle into an arterial trunk (from the right ventricle into the pulmonary trunk and from the left ventricle into the aorta).

Ventricular relaxation

• Ventricles relax and ventricular pressure decreases.
  Now, the semilunar valves close (as blood flows slightly backward and into the cusps to close the valve), preventing backflow of blood into each ventricle, and then the AV valves open, allowing blood to again flow from each atrium into the right or left ventricle.