Monary Circulation

MONARY CIRCULATION Course of the Circulation in Mammals.—In mammals and birds, the heart has become entirely divided into two halves, right and left, which have no direct communication with one another (fig. 1, C). The right auricle receives the venous blood from all parts of the body. From the right auricle, the blood passes to the right ventricle, and from here it is forced into the lungs along the pulmonary artery. In the lungs it takes up oxygen and becomes arterial, and is returned by the pulmonary veins to the left auricle and to the ventricle. The left ventricle forces the blood into the aorta, whence by the branching arteries it is carried to all parts of the body.

There are thus two circulations—the one pulmonary, from the right side of the heart to the pulmonary artery and the capillaries of the lungs and to the left heart by the pulmonary veins ; the other systemic, from the left side of the heart, by the aorta, to the arteries and capillaries of the body tissues and organs, from which by the veins to the right side of the heart. A schematic representation of the circulatory system is given in fig. 2. The muscular walls of the right ventricle are much thinner than those of the left ventricle. This is so because the energy required of the left ventricle must exceed that of the right ventricle, as resistance in the systemic circuit exceeds that in the pulmonary circuit.

The

heart becomes filled with venous blood during its relaxa tion or diastole, and forces the blood into the arteries during its contraction or systole. The large arteries are of less capacity than the corresponding veins, and their walls are essentially ex tensile and elastic. The small arteries and arterioles are essen tially muscular tubes and can vary considerably in diameter. The arterioles open into the capillaries, and these are so numerous that each organ may be regarded as a sponge full of blood. The skeletal muscles and the muscular walls of the viscera at each contraction express the blood within them and materially influence the circulation. The whole muscular system must therefore be regarded as an accessory pump to the vascular system. The veins are of larger calibre than the corresponding arteries, and have tough and inextensile walls. The veins are not, as a rule, dis tended with blood to their full potential capacity. The latter is

so great that the whole blood of the body can collect within the veins.

The heart and lungs are placed within the thoracic cavity, the floor of which is formed by the muscular diaphragm ; the heart is itself enclosed in a tough inextensile bag, the pericardium, the function of which is to check over-dilatation of the heart. Below the heart, the pericardium is fixed to the central tendinous part of the diaphragm; above, it is suspended by the mediastinum. On account of this fixed position of the pericardium, the heart is prevented from oscillating.

The Valves of the Heart.

As regards the valves of the heart (fig. 3), the tricuspid guards the right auriculo-ventricular opening, and consists of three flaps of fibrous tissue covered, like all the internal surfaces of the heart, with the smooth shining membrane, the endocardium. The flaps are continuous at their base, forming an annular membrane surrounding the opening. The bicuspid or mitral valve consists of two cusps, and guards the left auriculo-ventricular opening. The under surface and free edge of each cusp of these valves are attached by chordae ten dinae to two papillary muscles ; these are pillars of muscle which rise up from the inner surface of the ventricles.

The papillary muscles and chordae tendinae pull down the diaphragm formed by the closed valves (the floor of the auri cles), thus expanding the auricles and enabling the valvular as well as the muscular parts of the wall of the ventricles to ap proach together and force out the blood. The ventricles are never completely emptied, for some blood remains in contact with the auriculo-ventricular valves up to the end of systole and ensures their closure. The aortic and pulmonary valves consist of three semilunar, pocket-shaped cusps. A fibrous nodule is placed centrally in the free edge of each cusp, whence numerous tendinous fibres radiate to the attached borders of the cusp. Opposite the cusps are bulgings of the aortic walls—the sinuses of Valsalva. From the anterior one arises the right coronary artery, and from the left posterior the left coronary artery; these vessels supply the substance of the heart with blood.