THE CIRCULATORY MECHAN ism. The anatomical mechanism for the circulation of the blood and lymph consists of the heart, the blood-vessels, and the lymph-vessels. With regard to the blood we have the central fact that the powerful Inns eulature of the heart serves as a force-pump driving the blood out from the ventricles through the circuit of the blood-vessels and back to the heart. The system of arteries branching from the heart resembles a tree with its central trunk and its vast number of branches of different sizes. As one goes outward from the heart toward the periphery, the arteries distributed to each organ become smaller and smaller until they end in Minute capillaries. The combined area of these branches increases, however, toward the periphery, so that the sum of the areas of all the capillaries arising from the aorta is several hundred times as great as that of the aorta itself, just as the combined diameters of all of the twigs of a tree would much exceed that of the trunk. The quantity of blood being prac tically the same at all times, it becomes dis tributed as it goes out from the heart over a wider and wider area, o•, to use a convenient figure, flows through a wider and wider bed. As a result of this fact the velocity of the blood-flow becomes smaller as we approach the capillaries. While in the aorta the blood may flow with a velocity of 300 millimeters per second, in the capillaries the velocity becomes reduced to millimeter. In the venous system the same general fact bolds true. The capillaries unite into larger veins, and these into still larger ones, until finally all of the blood in the aortic sys tem is collected into two large veins, the superior and the inferior vena cava. which open into the auricles. As a consequence the blood, as it flows back to the heart, passes through a path that be comes narrower and narrower, and its velocity increases proportionately. With regard to the velocity of the blood-flow, therefore, we can state that it is greatest in the large arteries and veins and least in the capillaries.
In the arteries, moreover, the blood is under a much greater pressure than in the capillaries and veins. From a cut artery the blood spurts to some distance, while from a cut vein the blood flows out quickly, but with little force. The cause of this difference is easily understood. In flowing through the vessels the blood encoun ters considerable resistance. Naturally this re sistance is greatest in the capillaries and in the small arteries and veins eommunicating with them. On account of the great resistance in the capillary region, known technically as the periph eral resistance, the blood is dammed up, retarded on the arterial side, stretching the elastic walls of the arteries and putting the blood under a considirable tension. In the capillaries th:t
pressure of the blood is much less, and in the veins it becomes smaller and smaller as we ap proach the heart. Physiologically the most im portant part of the blood-circuit is in the capil laries. While flowing through these thin-walled and very minute vessels the blood fulfills its function of nourishing the tissues. At this point the blood, while still under some pressure, is flowing at its slowest rate, and the liquid part of it, the blood plasma, transudes through the thin walls and comes into direct contact with the tissues, thus forming the tissue-lymph. The oxy gen carried by the red corpuscles of the blood is liberated in the capillaries and diffuses from the blood to the tissues, being transported to the latter while held in solution in the lymph. The lymph gives up its food materials and oxygen to the tissue cells and at the same time receives from them the waste products and carbon dioxide formed during their nutritive activity. The lymph is prevented from accumulating in the tissues by the fact that it is continually drained off by the system of lymphatic vessels. These vessels form a system parallel in course and structure to that of the veins. They begin in the tissues by small capillaries which unite to larger and larger vessels. forming eventually two main trunks that open into the veins of the neck. Py this arrangement the excess of lymph is con tinually drained off and returned to the blood. The means by which the supply of lymph is regulated forms one of the most interesting and difficult subjects of study in modern physiology.
The heart and blood-vessels are not a fixed and rigid system. On the contrary, they are supplied with nerves through which the heat of the heart and the capacity and resistance of the blood vessels may he reflexly adapted to the differeot conditions of the body as a whole, or the i” dividual needs of its separate parts. The heart receives two sets of nerve fibres; one, the in hibitory fibres. are capable of slowing the heart beat; the other, the accelerative fibres. quicken the rate, The small arteries, on the other hand, receive also two sets of nerve fibres; one. the. vasoconstrictors, causes a diminution in size of the blood-vessels, while the other, the vaso dilators, brings about an increase in size. The numerous conditions under Nvhieli these regulat ing nerve fibres act are too complex to lie de serihed here. The mechanism is so adjusted as to control automatically the supply of blood to different organs under varying conditions of rest and activity.