The Circulation : Part 2

Hygienic Physiology: with Special Reference to the Use of Alcoholic Drinks and Narcotics
by Joel Dorman Steele, Ph.D.

(Page 6 of 17)

These Flaps Are Strengthened like sails by slender cords, which prevent their being pressed back through the opening. If the cords were attached directly to the walls of the heart, they would be loosened in the systole, and so become useless when most needed. They are, therefore, fastened to little muscular pillars projecting from the sides of the ventricle; when that contracts, the pillars contract also, an. Therefore the cords are held tight.

The Semi lunar Valves. - In the passages outward from the ventricles, are valves, called from their peculiar half-moon shape semi lunar valves (semi, half; Luna, Moon). Each consists of three little pocket-shaped folds of membrane, with their openings in the direction which the blood is to take. When it sets back, they fill, and, swelling out, close the passage.

The Arteries are the tube-like canals which convey the blood from the heart. They carry the red blood. They are composed of an elastic tissue, which yields at every throb of the heart, and then slowly contracting again, keeps up the motion of the blood until the next systole. The elasticity of the arteries acts like the air chamber of a fire engine, which converts the intermittent jerks of the brakes or pump into the steady stream of the hose nozzle.

The arteries sometimes communicate by means of branches or by meshes of loops, so that if the blood be blocked in one, it can pass round through another, and so get by the obstacle. When an artery penetrates a muscle, it is often protected by a sheath or by fibrous rings, which prevent its being pulled out of place or compressed by the play of the muscles.

The arteries are generally located as far as possible beneath the surface, out of harm's way, and hence are found closely hugging the bones or creeping through safe passages provided for them. They are generally nearly straight, and take the shortest routes to the parts which they are to supply with blood.

The Arterial System starts from the left ventricle by a single trunk - the aorta - which, after giving off branches to the head, sweeps back of the chest with a bold curve - the arch of the aorta - and thence runs downward, dividing and subdividing, like a tree, into numberless branches, which, at last, penetrate every nook and corner of the body.

The Pulse. - At the wrist (radial artery) and on the temple (temporal artery) we can feel the expansion of the artery by each little wave of blood set in motion by the contraction of the heart. In health, there are about seventy-two pulsations per minute. They increase with excitement or inflammation, weaken with loss of vigor, and are modified by nearly every disease. The physician, therefore, finds the pulse a good index of the state of the system and the character of the disorder.

The Veins are the tube-like canals which convey the blood to the heart. They carry the dark or venous blood. As they do not receive the direct impulse of the heart, their walls are made much thinner and less elastic than those of the arteries. At first small, they increase in size and diminish in number as they gradually pour into one another, like tiny rills collecting to form two rivers, the vena cava ascending and the vena cava descending, which empty into the right auricle.

Some of the veins creep along under the skin, where they can be seen, as in the back of the hand; while others accompany the arteries, some of which have two or more of these companions.

Valves similar in construction to those already described (the semilunar valves of the heart, page 114) are placed at convenient intervals, in order to guide the blood in its course, and prevent its setting backward. We can easily examine the working of these valves. On baring the arm, blue veins may be seen running along the arm toward the hand. Their diameter is tolerably even, and they gradually decrease in size. If now the finger be pressed on the upper part of one of these veins, and then passed downward so as to drive its blood backward, swellings like little knots will make their appearance. Each of these marks the location of a valve, which is closed by the blood we push before our finger. Remove the pressure, and the valve will swing open, the blood set forward, and the vein collapse to its former size.

The Capillaries (capillus, a hair) form a fine network of tubes, connecting the ends of the arteries with the veins. They blend, however, with the extremities of these two systems, so that it is not easy to tell just where an artery ends and a vein begins. So closely are they placed, that we can not prick the flesh with a needle without injuring, perhaps, hundreds of them. The air cells of the blood deposit there their oxygen, and receive carbonic acid, while in the delicate capillaries of the lungs they give up their load of carbonic acid in exchange for oxygen.

If, by means of a microscope, we examine the transparent web of a frog's foot, we can trace the route of the blood. It is an experiment of wonderful interest. The crimson stream, propelled by the heart, rushes through the arteries, until it reaches the intricate meshes of the capillaries. Here it breaks into a thousand tiny rills. We can see the disks winding in single file through the devious passages, darting hither and thither, now pausing, swaying to and fro with an uncertain motion, and anon dashing ahead, until, at last, gathered in the veins, the blood sets steadily back on its return to the heart.

The Circulation consists of two parts - the lesser, and the greater.

1. The Lesser Circulation. - The dark blood from the veins collects in the right auricle, and, going through the tricuspid valve, empties into the right ventricle. Thence it is driven past the semilunar valves, through the pulmonary artery, to the lungs. After circulating through the fine capillaries of the air cells contained in the lungs, it is returned, bright and red, through the four pulmonary veins, to the left auricle.

2. The Greater Circulation. - From the left auricle, the blood is forced past the bicuspid valve to the left ventricle; thence it is driven through the semilunar valves into the great aorta, the main trunk of the arterial system. Passing through the arteries, capillaries, and veins, it returns through the vene cave, ascending and descending, gathers again in the right auricle, and so completes the "grand round" of the body. Both these circulations are going on constantly, as the two auricles contract, and the two ventricles expand simultaneously, and vice versa.