The heart has four chambers: Two artria (auricles) and two ventricles. The left and right sides of the heart are separated by a muscle wall (septum) so that oxygenated and deoxygenated blood does not mix.
Deoxygenated blood from the rest of the body enters the heart through the vena cava . Blood enters the right atrium, then through tricuspid valve into right ventricle. Then via semi-lunar valve to the pulmonary artery to the lungs. Oxygenated blood from the lungs enters the heart through pulmonary vein.
It enters the left atrium of the heart, then through bicuspid valve into left ventricle. Then via semi-lunar valves to aorta which takes oxygenated blood round the body. A branch of the aorta called coronary artery supplies blood to the heart muscle. The coronary vein carries blood from the heart muscle to the pulmonary artery which then takes it to the lungs for oxygenation.
Pumping Mechanism of the heart
The heart undergoes contraction (systole) and relaxation ( diastole). Systole. When the ventricular muscles contract, the cuspid valves (tricuspid and bicuspid) close preventing backflow of blood into auricles. The volume of the ventricles decreases while pressure increases.
This forces blood out of the heart to the lungs through semi-lunar valves and pulmonary artery,and to the body tissues via semi-lunar valve and aorta respectively. At the same time the atria are filled with blood. The left ventricle has thicker muscles than the right ventricle, and pumps blood for a longer distance to the tissues.
When ventricular muscles relax, the volume of each ventricle increases while pressure decreases. Contractions of atria force the bicuspid and tricuspid valves to open allowing deoxygenated blood from right atrium into right ventricle which oxygenated blood flows from left atrium into the left ventricle.
Semi-lunar valves close preventing the backflow of blood into ventricles. The slight contractions of atria force the , blood flow into ventricles. The Heartbeat The heart is capable of contracting and relaxing rhythmically without fatigue due to its special muscles called cardiac muscles.
The rhythmic contraction of the heart arises from within the heart muscles without nervous stimulation. The contraction is said to be myogenic. The heartbeat is initiated by the pacemaker or sino-artrio-node (SAN) which is located in the right atrium. The wave of excitation spreads over the walls of atria.
It is picked by the artrio-ventricular node which is located at the junction: Of the atria and ventricles, from where the purkinje tissue spreads the wave to the walls of the ventricles. The heart contracts and relaxes rhythmically at an average rate of 72 times per minute. The rate of the heartbeat is increased by the sympathetic nerve, while it is slowed down by the vagus nerve. Heartbeat is also affected by hormones e.g. adrenaline raises the heartbeat.