The cardiac cycle consists of one heart beat or one cycle of contraction and relaxation of the cardiac muscle. During a heart beat there is contraction and relaxation of atria and ventricles. The contraction phase is called the systole while the relaxation phase is called the diastole. When both the atria and ventricles are in diastole or relaxed phase, this is referred to as a joint diastole. During this phase, the blood flows from the superior and inferior venae cavae into the atria and from the atria to the respective ventricles through auriculo-ventricular valves. But there is no flow of blood from the ventricles to the aorta and pulmonary trunk as the semilunar valves remain closed. The successive stages of the cardiac cycle are briefly described below:
- Atrial systole: The atria contract due to a wave of contraction, stimulated by the SA node. The blood is forced into the ventricles as the bicuspid and tricuspid valves are open.
- Beginning of ventricular systole: The ventricles begin to contract due to a wave of contraction, stimulated by the AV node. The bicuspid and tricuspid valves close immediately producing part of the first heart sound.
- Complete ventricular systole: When the ventricles complete their contraction, the blood flows into the pulmonary trunk and aorta as the semilunar valves open.
- Beginning of ventricular diastole: The ventricles relax and the semilunar valves are closed. This causes the second heart sound.
- Complete ventricular diastole: The tricuspid and bicuspid valves open when the pressure in the ventricles falls and blood flows from the atria into the ventricles. Contraction of the heart does not cause this blood flow; it is due to the fact that the pressure within the relaxed ventricles is less than that in the atria and veins.
The duration of a cardiac cycle is 0.8 sec. Double circulation: The heart of birds and mammals is four chambered, it comprises of 2 atria and 2 ventricles. Right and left atria are separated by inter-atrial septum and right and left ventricles are separated by inter-ventricular septum, thus, preventing the mixing of oxygenated and deoxygenated blood. It results in double circulation which includes systemic and pulmonary circulation. Systemic circulation is the flow of oxygenated blood from the left ventricle to all parts of the body and deoxygenated blood from various body parts to the right atrium. All systemic arteries arise from the aorta. All the veins of the systemic circulation bring deoxygenated blood into the superior vena cava, the inferior vena cava or the coronary sinus, which open into the right atrium. The purpose of systemic circulation is to carry oxygen and nutrients to body tissues and to remove carbon dioxide and other wastes from the tissues. Pulmonary circulation is flow of deoxygenated blood from the right ventricle to the lungs and the return of oxygenated blood from the lungs to the left atrium. The pulmonary trunk arises from the right ventricle and then divides into the right pulmonary artery and left pulmonary artery which supply deoxygenated blood to the right and left lungs respectively. Exchange of gases takes place in the lungs.