Fig. 8.—Front view of heart and lungs, showing relations to other thoracic organs (Ingals).
The average frequency of the heart’s beat, or the pulse, is 72 times a minute. It is increased by exercise; it is quicker in the standing than in the sitting posture. It is quickened by meals, and, on the whole, it is quicker in the evening than in the early morning hours. Independent of muscular exertion, it is quickened by great altitudes. It is said to be quicker in summer than in winter. Its rate is profoundly influenced by mental conditions.
The whole of the blood of the body passes through the heart in 32 beats—that is, in less than half a minute. The greatest part of this time is spent in the capillaries. There the tissues are obtaining their fresh supplies of food and discharging their waste matter into it.
Fig. 9.—Relation of heart and great vessels to the wall of the thorax. The collapsed lungs are drawn slightly aside (after Heath).
The heart, great blood-vessels, and the lungs are placed in the air-tight cavity of the thorax, and are subjected to the pumping action of the respiratory movements. The inspiratory muscles elevate the ribs, at the same time that the diaphragm, by its contraction, pushes the contents of the abdomen downward. The cavity of the chest, so enlarged, causes the pressure around the heart and the great blood-vessels within the chest to be less than that on the blood-vessels outside the chest; hence, during each inspiration the venous blood is sucked back into the right side of the heart.
The tissues deprive the blood of its oxygen, so that which flows back to the heart in the veins is blue. The right heart then sends this blue blood to the lungs, that it may get rid of its carbonic acid, which is not only not needed, but is actually injurious to the body, and to receive a fresh supply of oxygen, which has been carried into the lungs in breathing.
The blood is the great medium of exchange between all parts of the body. It is, at the same time, the nourisher and the scavenger of all the tissues. After the food has been liquefied and converted into new substances in the digestive system it is poured into the blood. From the blood all the tissues draw material to renew their own worn-out parts and other material which they store up as latent force, which, when it unites with the oxygen of the blood, becomes active force, such as heat and motion.
