The Mechanics of Circulation and Respiration.—In order to understand the mechanics of circulation and respiration, it is necessary to appreciate four fundamental facts—that the thorax is a distensible, air-tight cage; that it contains and is filled by the heart, lungs, and great blood-vessels; that the exchange of gases in the blood takes place in the lungs; and that, on the development of the chest and respiratory muscles, depend the development of the lungs and the force of the circulation.
Fig. 7.—The bony thorax, anterior view (Ingals).
The Thorax.—The chest or thorax is a cone-shaped, distensible cage, formed of bones, elastic cartilage, and muscles. The spinal column forms the fixed part of this living cage, and the ribs are attached to this in such a way as to allow of their being raised in inspiration, thus increasing the anteroposterior and the lateral diameters of the chest.
The thorax is converted into an air-tight cavity by means of muscles. The base is made up of one huge muscle, the diaphragm. This is attached at its border to the ribs and posteriorly to the backbone. It is the diaphragm which separates the cavity of the thorax from that of the abdomen. When the diaphragm is relaxed, it has a concavoconvex form, the convexity being directed toward the chest, and the heart and lungs rest directly on it, while the concave surface covers or rests on the liver.
The Circulatory Apparatus.—This consists of a central force and suction-pump, the heart, and a series of elastic tubes that grow smaller the further from the heart they are situated; they divide and subdivide, like the branches of a tree. The smallest arteries, called capillaries, from their hair-like size, are so minute that they only allow the passage of a single corpuscle at a time and their walls are transparent.
The heart is a somewhat cone-shaped organ, placed between the two lungs; it is situated more or less obliquely in the chest, immediately back of the breast-bone. Roughly speaking, the base of the heart corresponds to the right edge of the sternum, while the apex lies a little below and to the right of the left nipple.
The heart is divided into a right and left side. The left side forms the force pump, whose motive power is supplied by the contraction of its own muscle-fibers. The bright red blood, with its fresh supply of oxygen—hence its color—flows from the lungs into the left heart, which then contracts automatically and forces the blood into the arteries of the body.
The arteries consist of a series of elastic tubings; hence, the smaller the tubing, the greater the resistance which has to be overcome by the force of the heart’s beat, so that during violent exercise, when the contraction of the muscles causes a pressure on the minute arteries and capillaries situated in them, the more forcible must be the beat of the heart to overcome this additional resistance. Likewise, when the surface of the body is suddenly chilled, as by a plunge into cold water, all the vessels situated here contract, and, again, more work is thrown on the heart.
The three chief factors in the mechanics of the circulation are the force and frequency of the heart’s beat, the peripheral resistance, and the elasticity of the arterial walls. Any disturbance between these relations brings about abnormal conditions.