The Circulation : Part 1

Physiology and Hygiene for Secondary Schools
by Francis M. Walters

(Page 5 of 29)

A Carrier must move. To enable the blood to carry food and oxygen to the cells and waste materials from the cells, and also to distribute heat, it is necessary to keep it moving, or circulating, in all parts of the body. So closely related to the welfare of the body is the circulation of the blood, that its stoppage for only a brief interval of time results in death.

Discovery of the Circulation. - The discovery of the circulation of the blood was made about 1616 by an English physician named Harvey. In 1619 he announced it in his public lectures and in 1628 he published a treatise in Latin on the circulation. The chief arguments advanced in support of his views were the presence of valves in the heart and veins, the continuous movement of the blood in the same direction through the blood vessels, and the fact that the blood comes from a cut artery in jets, or spurts, that correspond to the contractions of the heart.

No other single discovery with reference to the human body has proved of such great importance. A knowledge of the nature and purpose of the circulation was the necessary first step in understanding the plan of the body and the method of maintaining life, and physiology as a science dates from the time of Harvey's discovery.

Organs of Circulation. - The organs of circulation, or blood vessels, are of four kinds, named the heart, the arteries, the capillaries, and the veins. They serve as contrivances both for holding the blood and for keeping it in motion through the body. The heart, which is the chief organ for propelling the blood, acts as a force pump, while the arteries and veins serve as tubes for conveying the blood from place to place. Moreover, the blood vessels are so connected that the blood moves through them in a regular order, performing two well-defined circuits.

The Heart. - The human heart, roughly speaking, is about the size of the clenched fist of the individual owner. It is situated very near the center of the thoracic cavity and is almost completely surrounded by the lungs. It is cone-shaped and is so suspended that the small end hangs downward, forward, and a little to the left. When from excitement, or other cause, one becomes conscious of the movements of the heart, these appear to be in the left portion of the chest, a fact which accounts for the erroneous impression that the heart is on the left side. The position of the heart in the cavity of the chest is shown in Fig. 13.

The Pericardium. - Surrounding the heart is a protective covering, called the pericardium. This consists of a closed membranous sac so arranged as to form a double covering around the heart. The heart does not lie inside of the pericardial sac, as seems at first glance to be the case, but its relation to this space is like that of the hand to the inside of an empty sack which is laid around it. The inner layer of the pericardium is closely attached to the heart muscle, forming for it an outside covering. The outer layer hangs loosely around the heart and is continuous with the inner layer at the top. The outer layer also connects at certain places with the membranes surrounding the lungs and is attached below to the diaphragm. Between the two layers of the pericardium is secreted a liquid which prevents friction from the movements of the heart.

Cavities of the Heart. - The heart is a hollow, muscular organ which has its interior divided by partitions into four distinct cavities. The main partition extends from top to bottom and divides the heart into two similar portions, named from their positions the right side and the left side. On each side are two cavities, the one being directly above the other. The upper cavities are called auricles and the lower ones ventricles. To distinguish these cavities further, they are named from their positions the right auricle and the left auricle, and the right ventricle and the left ventricle. The auricles on each side communicate with the ventricles below; but after birth there is no communication between the cavities on the opposite sides of the heart. All the cavities of the heart are lined with a smooth, delicate membrane, called the endocardium.

Valves of the Heart. - Located at suitable places in the heart are four gate-like contrivances, called valves. The purpose of these is to give the blood a definite direction in its movements. They consist of tough, inelastic sheets of connective tissue, and are so placed that pressure on one side causes them to come together and shut up the passageway, while pressure on the opposite side causes them to open. A valve is found at the opening of each auricle into the ventricle, and at the opening of each ventricle into the artery with which it is connected.

The valve between the right auricle and the right ventricle is called the tricuspid valve. It is suspended from a thin ring of connective tissue which surrounds the opening, and its free margins extend into the ventricle. It consists of three parts, as its name implies, which are thrown together in closing the opening. Joined to the free edges of this valve are many small, tedious cords which connect at their lower ends with muscular pillars in the walls of the ventricle. These are known as the chorde tendinee, or heart tendons. Their purpose is to serve as valve stops, to prevent the valve from being thrown, by the force of the blood stream, back into the auricle.

The mitral, or bicuspid, valve is suspended around the opening between the left auricle and the left ventricle, with the free margins extending into the ventricle. It is exactly similar in structure and arrangement to the tricuspid valve, except that it is stronger and is composed of two parts instead of three.

The right semi lunar valve is situated around the opening of the right ventricle into the pulmonary artery. It consists of three pocket-shaped strips of connective tissue which hang loosely from the walls when there is no pressure from above; but upon receiving pressure, the pockets fill and project into the opening, closing it completely. The left semi lunar valve is around the opening of the left ventricle into the aorta, and is similar in all respects to the right semi lunar valve.

D.C. Heath and Co. - Publishers
Original copyright 1909