The course taken by the blood in its way to the various parts of the body is called the circulation, on account of its having to make repeatedly the circuit of vessels leading to and from the heart. The heart is the great motor power which drives the blood through all the vessels, of which there is one set belonging to the circulation of the organs of the system generally, and another leading to and from the lungs.

Diagram of Circulation, showing right (r. h.) and left (l. h.) hearts.

Fig. 110. Diagram of Circulation, showing right (r. h.) and left (l. h.) hearts, and the pulmonary (p) and systemic (s) sets of capillary vessels.

Capillary Network of the Choroid of Child of a few months old.

Fig. 111. Capillary Network of the Choroid of Child of a few months old. (Cadiat ) (a) Artery. (b) Vein, and capillary network intervening.

Anatomists speak of two circulations - the greater or systemic, and the lesser ox pulmonary. However, if we follow the course of the blood, we see that both these sets of vessels really belong to the one circulation, and in fact form but one circuit. In all the higher animals the heart forms a single organ, but it really is composed of two muscular pumps which are anatomically united though distinct in function. These functionally distinct hearts work at different parts of the circuit traversed by the blood. The blood on its way through the lungs and systemic vessels visits the heart twice, in order to acquire the force necessary to overcome the resistance of these two sets of vessels. The right heart is visited before the pulmonary vessels, and is the agent for pumping the blood through the lungs. The left heart is placed before the systemic vessels and pumps the blood through the body generally. Thus anatomically there appear to be two circulations and but one heart; physiologically there is one circulation and two hearts; or two separate points of resistance and a distinct pumping organ to drive the blood through each.

The circulation might then be represented by a simple diagram (Fig. no) in which the direction of the current is indicated by the arrows. L H shows the position of the left or systemic pump, and S the resistance in the systemic vessels. R H represents the pulmonary pump and P the second obstacle in the circuit, viz., the vessels of the lungs. This functional distinction must be kept in view in studying the dynamics of the circulation, although the two pumping organs are fused into one viscus, with two distinct and separate channels for the passage of the blood.

In each system of blood vessels we have the same general arrangement for the distribution and re-collection of the blood.

In passing from either the right or left side of the heart the blood flows into tubes called arteries, which divide and subdivide until the branches become microscopical in size.. From the very minute arteries the blood passes into the capillaries, which cannot be said to branch, but form a network of delicate tubes with meshes of varying closeness according to the tissue.

Connected with the meshes of the capillaries are the small veinlets which collect the blood from the networks (Fig. in). These unite, gradually forming larger vessels, which again are but the tributaries of the large veins which convey the blood back to the heart.

About three hundred years ago the true course of the blood current through the systemic and pulmonary heart, arteries and veins, so as to form one circle, was demonstrated by Harvey. Before his time only the so-called "lesser" or pulmonary circuit was known. The magnifying glasses at his disposal did not enable him to see the capillaries, which were first described by Malpighi some fifty years later.

In the hope of making their different functions appear more striking, the various parts of the circulatory apparatus may be enumerated as follows, and roughly illustrated by a diagram: -

1. The left (systemic) heart (L H) pumps the blood into the systemic arteries, and thus keeps these vessels over filled.

2. The larger systemic arteries (A), by their elasticity, exert continuous pressure on the blood with which they are distended.

3. The smaller systemic arterioles (A'), by their vital contractility, check and regulate the amount of blood flowing out of the larger arteries into the capillaries, and thus keep up a high pressure in the larger arteries.

4. In the systemic capillaries (S C), the essential operations of the blood are carried out, viz., the chemical interchanges between it and the tissues.

5. The wide systemic veins (V) are the passive channels conveying the impure blood to the pulmonary heart.

6. The right (pulmonary) heart (R H) pumps the blood into the pulmonary arteries and distends them.

Diagram ot the Circulation of the Blood and the absorbent vessels.

Fig. 112. Diagram ot the Circulation of the Blood and the absorbent vessels. For details, see text.

7. The pulmonary arteries (PA) press steadily upon the blood and force it through -

8. The small pulmonary arterioles (P a), regulate the flow into the capillaries of the lungs.

9. In the pulmonary capillaries (P C), the blood is exposed to the air, and undergoes active gas interchange.

10. The pulmonary veins (P V) carry the blood to the left heart, and thus complete the circuit.

"L/i indicates the lymphatics, which drain the tissues, and L,c the lacteals, which absorb from the stomach and intestines (I).

Although the blood enters the arteries by jerks, its motion through the capillaries is even, because the arteries constantly press on the blood they contain, their elastic walls being distended by the pumping of the heart, which fills the aorta and arteries more quickly than they can empty themselves, unless the adequate pressure has been attained. The contracting arterioles are the chief agents in resisting the outflow and keeping up the arterial pressure.