2. With the left auricle, the four pulmonary veins. These receive blood from the lungs and empty it into the left auricle.
3. With the right ventricle, the pulmonary artery. This receives blood from the heart and by its branches distributes it to all parts of the lungs.
4. With the left ventricle, the aorta. The aorta receives blood from the heart and through its branches delivers it to all parts of the body.
How the Heart does its Work.—The heart is a muscular pump[18] and does its work through the contracting and[pg 046] relaxing of its walls. During contraction the cavities are closed and the blood is forced out of them. During relaxation the cavities open and are refilled. The valves direct the flow of the blood, being so arranged as to keep it moving always in the same direction (Fig. 17).
The heart, however, is not a single or a simple pump, but consists in reality of four pumps which correspond to its different cavities. These connect with each other and with the blood vessels over the body in such a manner that each aids in the general movement of the blood.
Fig. 18—Diagram illustrating the "cardiac cycle."
Work of Auricles and Ventricles Compared.—In the work of the heart the two auricles contract at the same time—their contraction being followed immediately by the contraction of both ventricles. After the contraction of the ventricles comes a period of rest, or relaxation, about equal in time to the period of contraction of both the auricles and the ventricles.[19] On account of the work which they perform, the auricles have been called the "feed pumps" of the heart; and the ventricles, the "force pumps."[20] It is the function of the auricles to collect the blood from the veins, to let this run slowly into the ventricles when both the[pg 047] auricles and ventricles are relaxed, and finally, by contracting, to force an excess of blood into the ventricles, thereby distending their walls. The ventricles, having in this way been fully charged by the auricles, now contract and force their contents into the large arteries.
Sounds of the Heart.—Two distinct sounds are given out by the heart as it pumps the blood. One of them is a dull and rather heavy sound, while the other is a short, sharp sound. The short sound follows quickly after the dull sound and the two are fairly imitated by the words "lūbb, dŭp." While the cause of the first sound is not fully understood, most authorities believe it to be due to the contraction of the heart muscle and the sudden tension on the valve flaps. The second sound is due to the closing of the semilunar valves. These sounds are easily heard by placing an ear against the chest wall. They are of great value to the physician in determining the condition of the heart.
Arteries and Veins.—These form two systems of tubes which reach from the heart to all parts of the body. The arteries receive blood from the heart and distribute it to the capillaries. The veins receive the blood from the capillaries and return it to the heart. The arteries and veins are similar in structure, both having the form of tubes and both having three distinct layers, or coats, in their walls. The corresponding coats in the arteries and veins are made up of similar materials, as follows: