The Mammalian Heart
...d all round the body. Therefore the blood entering the aorta from the left ventricle is at a much higher blood pressure than the blood entering the pulmonary artery. The circulation to and from the lungs is called the pulmonary circulation and the circulation round the body is called the systematic circulation. As the atria contract they force blood into the ventricles, and rings of muscle which surround the venae cavae and pulmonary veins. This prevents blood returning into the veins. The left atrium is separated from the left ventricle by a bicuspid valve, whilst a tricuspid valve separates the right atrium from the right ventricle. These are known as the atrioventricular valves. Attached to the ventricle side of the flaps are fibrous cords which in turn attach to conical-shaped papillary muscles which are extensions of the inner wall of the ventricles. The atrioventricular valves are pushed open when the atria contract but, when the ventricles contract, the flaps of each valve press tightly closed so preventing the return of blood to the atria. At the same time the papillary muscles contract so tightening the fibrous cords. This prevents the valves from being turned inside out. Semi lunar valves are found at the points where the pulmonary artery and aorta leave the heart. These prevent blood from getting back into the ventricles. Just beyond the aortic valve are the openings of the two coronary arteries. These are the only blood vessels which supply oxygenated blood to the walls of the heart. The walls of the heart are composed of cardiac muscle fibres, connective tissue and tiny blood vessels. Each muscle fibre possesses one or two nuclei and many mitochondria. Each fibre is made up of many myofibrils. These contain actin and myosin filaments which bring about contraction. They account for the striped appearance of the muscle fibres. The dark bands known as intercalated discs are cell surface membranes separating individual muscle cells. The structure of the membranes is modified to allow ions to diffuse rapidly across them. This allows rapid spread of excitation through the muscle. When one cell becomes excited, the action potential spreads quickly to all the others, so that the whole mass of fibres behaves as one unit. The fibres branch and cross-connect with each other to form a complex net-like arrangement. No neurons are present in the wall of the heart. The cardiac cycle refers to the sequence of events which take place during the completion of one heartbeat. It involves repeated contraction and relaxation of the heart muscle. Contraction is called systole and relaxation is called diastole. Atrial diastole: during the time when the atria and heart ventricles are both relaxed, blood returning to the heart under low pressure in the veins enters the two atria. Oxygenated blood enters the left atrium and deoxygenated blood enters the right atrium. At first the bicuspid and tricuspid valves are closed but, as the atria fill with blood, pre...