None of the functions of blood outlined above could be carried out unless it is in circulation. It is the muscular organ, the heart, which causes blood circulation. The heart is situated above the diaphragm, between the lungs and almost in the middle of the chest, with a slight tilt at its apex, towards the left. The heart is enclosed in a double-walled membranous sac called the pericardium. The heart continuously works without stopping, throughout the life of an individual. The heart of an average person, at rest, beats 72 times in a minute.
Structure of the Mammalian Heart
Longitudinal Section of the Mammalian Heart
The mammalian heart is a four-chambered organ, the right and left halves of which are completely separated by muscular partitions called septa. Each half of the heart has an upper chamber or the auricle and a lower chamber called the ventricle. Each auricle opens into the ventricle of its own side by an auriculo-ventricular aperture. Both the apertures are guarded by valves which allow blood flow only form the auricles to the ventricles but not the other way. The valve guarding the left auriculo-ventricular aperture has two flaps and is called the mitral valve or the bicuspid valve. The corresponding valve on the right side has three flaps and is called tricuspid valve.
The walls of the left ventricle are comparatively thicker. The cavity of the left ventricle is irregular and larger than that of the right ventricle. The ventricle has tendonous cords and papillary muscles which prevent the valves from being pushed into the auricles, when the ventricles contract. From the anterior right border of the left ventricle, arises the large aorta which carries blood to different parts of the body. It is guarded at its origin by three pocket-shaped semilunar valves. The cavities of these valves face away from the ventricle.
The walls of the right ventricle are comparatively thinner. From the anterior left border of the right ventricle, arise the pulmonary aorta, the blood vessels which takes de-oxygenated (venous) blood to the lungs. It is also guarded at its origin by semilunar valves.
Four pulmonary veins bringing in the oxygenated (arterial) blood from the lungs, open into the left auricle. The right and left pre-caval veins and the post-caval veins bringing venous blood from all over the body to the heart by an inferior vena cava and superior vena cava which open into the right auricle.
Course of Blood Flow in the Heart
The venous blood from the body as a whole enters the heart through the superior vena cava (from the head region). It enters the right auricle, goes through the tricuspid valve and into the right ventricle. This blood then goes to the pulmonary artery and to the lungs. In the lungs the blood is oxygenated and returns to the heart via pulmonary veins. Entering the left auricle, the blood passes through the mitral valve and into the left ventricle. From the left ventricle it is pumped to all parts of the body through the aorta.
When the ventricles relax and regain the original size, they draw more blood from the auricles. Blood leaving the ventricles flows only in one direction, as the valves located at the base of the pulmonary trunk and aorta prevent its return into the ventricles.