During this step, both the atria and both ventricles relax.
The Right atrium then relaxes. Later, the Deoxygenated blood enters the right side of the heart through two large veins called the vena cavae. The superior vena cava returns blood from the head and arms; the inferior vena cava from the rest of the body (except, of course, the lungs!) Both vena cavae empty into the right atrium. This is where the blood pressure is lowest (even negative). When the heart relaxes (between beats), the pressure in the circulatory system causes the right atrium to fill with blood.
At the same time left atrium relaxes. Oxygenated blood leaves the lungs and returns to the heart through the pulmonary veins. These are the only veins to carry oxygenated blood. This blood enters the left atrium.
Joint diastole lasts for 0.4 s.
During this step both the atria contract.
When the right atrium contracts, pressure inside it rises, the right atrioventricular (AV) valve opens, and blood is squeezed from the right atrium into the right ventricle. This valve is also known as the tricuspid valve. The closing of this valve makes a sound – ‘lub’.
When the atrium is empty, the pressure inside it falls, and the pressure inside the ventricle begins to rise. This causes the atrioventricular valve to shut quickly, preventing the back-flow of blood.
When left auricle contracts, the left atrioventricular (AV) valve, (or bicuspid or mitral valve) opens. When the atrium is empty and pressure begins to rise in the ventricle, the bicuspid valve closes.
Atrial systole lasts for 0.1 s.
During this step both the ventricles contract.
When the right ventricle contracts, blood is forced out through the semi-lunar valve (also known as the pulmonary valve), into the pulmonary arteries, where it goes to lungs. These are only arteries to carry deoxygenated blood. When the right ventricle is empty, the pressure inside falls below that in the pulmonary artery, and this causes the semilunar valve to snap shut The closing of these valves also causes a sound – dup’. A normal heartbeat is thus ‘lub…dup’.
When left ventricle contracts, the blood in it is forced at very high pressure through another semi-lunar valve (the aortic valve), into the aorta. Which carries blood throughout the body (apart from the lungs!). This surge of blood from the ventricles causes the walls of the aorta to expand and the muscles within to stretch we can detect this as a pulse. When the ventricle is almost empty, the pressure begins to fall below that in the aorta, and this causes the semilunar valve to snap shut, as the elastic walls of the aorta recoil, thus preventing back-flow of blood into the heart.
Ventricular systole lasts for 0.1 s.
Sound of Heart Beat
The cardiac cycle also creates the heart sounds: each heartbeat produces two sounds, often called lub-dup, that can be heard with a stethoscope.
The first sound is caused by the contraction of the ventricles (ventricular systole) closing the A-V valves. The second sound is caused by the snapping shut of the Aortic and Pulmonary Valves (Semi-lunar valves).
If any of the valves do not close properly, an extra sound called a heart murmur may be heard.