CIRCULATION
As water in the plant is the carrier of plant food throughout the plant, so is blood the carrier and distributor of food in the animal. When food is absorbed, it either passes into the lymphatic system or into the capillaries of the blood system. If in the former, it is carried to the thoracic duct, which extends along the spinal column and enters one of the main blood vessels. If collected by the capillary system, it is carried to the portable vein, thence to the liver and finally to the heart, where it meets with the blue blood collected from all parts of the body.
At this point, the blood contains both the nutriment and the waste matter of the body. Before it can be sent through the body again the waste material must be thrown out of the system by means of the lungs. This is accomplished by the heart forcing to the lungs the impure blood with its impurities collected from all parts of the body and also the nutriment collected from the digestive tract.
The chief organs, therefore, of the circulatory system are the blood and lymphatic vessels containing respectively blood and lymph. The only difference between these two materials is in the fact that lymph is blood without the red-blood corpuscles. The body, after all, really depends upon this lymph for nourishment, since it wanders to all parts of the body, surrounds all the cells in all of the tissues and in this way carries to the cells the very kinds of food that they need.
Lymph Passes Through Cell Walls.
—The blood vessels have no openings into the body at all. In this respect the blood system is like the digestive system; it is separate and distinct in itself. The blood, however, does creep through the walls of the blood vessels. In so doing the blood corpuscles are left behind and lymph is the result.
HOW THE BLOOD CIRCULATES THROUGH THE BODY
The center of the blood system is the heart. It is the engine of the body. Going out from it is the great aorta, which subdivides into arteries and farther away further subdivides until there is a great network of little arteries; these in turn become very tiny and take the name of capillaries. Thus the red blood, by means of arteries and capillaries, is carried to all parts of the body. This plan of distribution would not be complete unless some way were provided for the return of the blood to the heart and lungs for purification. And just such an arrangement has been provided. Another kind of network collects this scattered blood at the extremities into separate vessels, which gradually increase in size and finally empty their possessions into the heart. These are the veins of the body, and have to do with the impure blood of the body.
How the Heart Does Its Work.
—The power back of blood distribution is the heart. It is an automatic pump, as it were, that sends blood to the lungs and through the arteries to all parts of the body. The heart is divided into four divisions: the left and right ventricles and the right and left auricles. The right auricle receives the blood from the upper half of the body through a large vein and the lower half of the body through another large vein, and the blood from both lungs empties into the left auricle through two left and two right pulmonary veins. The large arteries of the heart which carry the blood from the heart to the different organs arise from the ventricle.
The blood always flows in the same direction. It goes into the auricle from the veins, and from this into the ventricle. It then passes into the arteries, then to the veins and then to the capillaries.
The action of the heart is very much like a force pump; the dark blood flows into the right auricle, which contracts; when this is done, the blood is forced into the right ventricle; this in turn contracts and forces the blood into the lungs, where oxygen is taken on and carbonic acid gas and other impurities are thrown off. From the lungs the blood, now red and pure, passes into the left auricle and thence into the left ventricle, from which it is forced into the aorta to be distributed to all parts of the body.
We now see the close connection existing between the digestive system and the circulatory system. The digested food in the intestines is gathered in by villi cells. The question can now be asked, What do these cells do with this nutriment or digested food? They pour it into the absorbent vessels or lymphs, as they are called; these in turn empty the assimilated stores of food into larger and still larger vessels, which continues until the whole of the nutritive fluid is collected into one great duct or tube, which pours its contents into the large veins at the base of the neck, from whence it is carried into the circulatory system, the very basis of which is the blood.