Before reading this section make sure you understand how pressure affects blood flow (read Understanding Blood Pressure).
pressure changes in the major blood vessels and the four chambers causes the
to open and close and directly affect the flow of blood through the heart. The
following graph shows the atrial,
pressure over time (one cycle).
To see an animation of the cardiac cycle with explanations of the pressure changes
in each phase click here.
The contraction of the atrium causes a rise in pressure in the atria. This pressure rise is called the a atrial pressure wave (marked 'a' on the graph); it pushes any blood left in the atria into the ventricles.
As the ventricles contract the ventricular pressure begins to rise. As soon as the ventricular pressure surpasses the atrial pressure the A-V valves close (marked '1' on the graph). The rapid closing of the valves causes a rise in pressure in the atria which is known as the c atrial pressure wave (marked 'c' on the graph). What follows is a short period of time called isovolumic (or isometric) contraction, during which all four valves of the heart are closed. During this short phase (approximately .025 seconds) the volume in the ventricle remains unchanged (hence the term isovolumic) but the the tension in the muscle and the pressure in the ventricle is increasing rapidly. The pressure quickly rises to a point above that of aortic and pulmonary pressures (marked '2' on the graph); when this occurs the aortic and pulmonary valves are forced open and blood flows into the aorta and pulmonary artery. Both pressures begin to fall to a point when the ventricular pressures are lower than the aortic and pulmonary pressures. At this point (marked '3' on the graph) the pressure difference causes the aortic and pulmonary valves to close. The rapid closing of the valves causes a pressure rise in the aorta and isovolumic (or isometric) relaxation, during which all four valves of the heart are closed. During this phase, again, the volume of the ventricles remains unchanged even though the muscle relaxes and the pressure in the ventricle rapidly drops.
At the point when the ventricular pressure drops below that of atrial pressure (marked '4' on the graph) the A-V valves open. The blood pressure which has slowly built up in the atrium during ventricular contraction causes blood to quickly flow into the ventricle (this is known as rapid inflow). The pressure build up and release in the atrium during ventricular contraction can be seen as a wave and is called the v atrial pressure wave (marked 'v' on the graph).