It is the sequence of events which make up one heart beat. Contraction is called systole and relaxation is called diastole.
Heart muscle in atrial walls contract simultaneously. Blood is conveyed into the ventricles. Backflow of blood into vena cava and pulmonary veins is prevented by closure of semi lunar valves in the veins. Pressure of blood in atria is higher than that in the ventricles and so, forces the atrioventricular valves (bicuspid and tricuspid) open.
Pressure developed by the contraction is not very great because muscular walls of the atria are thin but pressure is not enough to force blood into ventricles.
0.1seconds later, both ventricles contract. Ventricular pressure rises and exceeds blood pressure in aorta and pulmonary artery forcing aortic and pulmonary valves (semi lunar) open. Blood is expelled into the elastic walled vessels. Blood flows upwards from ventricles into arteries.
Thick muscular walls of ventricles squeeze on blood increasing pressure. As pressure in ventricles becomes greater than pressure in atria; the pressure difference pushes atrioventricular valves shut preventing blood from going back into atria. Closing of these valves produces the first heart beat sound ‘lub’ as blood is forced against the closed valves. Ventricular systole lasts 0.3s.
Contraction of papillary muscles, attached to the valve by tendons, prevents the valve from being forced inside out.
It is the resting period of heart chambers.
Muscle relaxes and pressure in ventricles drops. High pressure in aorta and pulmonary arteries tend to flow back into ventricles and this closes the semilunar valves in aorta and pulmonary artery to prevent backflow into heart. Closing of valves and impact of bloodflow against valves causes the second heart sound ‘dub’.
When atria relax, blood flows from veins through the two atria. Oxygenated blood...