While the work of the heart is to pump the blood and keep it in circulation, the work of the arteries is to carry the blood with its oxygen and other nutrient materials to every cell and part of the body.
The arteries are composed of soft, elastic muscle fibers and are powerful enough to take the full impact of the pumping heart. They expand to receive the blood that the heart pumps into them and contract to force it onward.
Since the arteries are composed of living tissue they also require food and oxygen, but, just as in the case of the heart, they obtain their nutrients not from the blood that flows through them, but from the blood brought to them by their own special arteries, the vaso vasorum. An intricate system of nerves plays an important part in regulating their functions.
Starting with the main artery, the aorta, which emerges from the lower left chamber of the heart, the arterial system branches into a vast network of large and small arteries, reaching out in every direction and carrying blood with its oxygen and other nutrient elements to every cell and part of the body. The large arteries subdivide first into smaller arteries, and then into still smaller arteries, and finally into the minute, hairlike blood vessels kn‚wn as the capillaries.
The Capillaries The actual interchange between the blood and the tissue cells takes place not in the larger arteries or their smaller subdivisions but in their smallest subdivisions, the capillaries. The capillaries are the minute, thin-walled hairlike channels stretching many thousands of miles, that reach into every nook and corner of the body, that transport food and oxygen to all the cells of the body and remove their waste products.
This interchange between the capillaries and the tissue cells takes place through the very thin walls of the capillaries and proceeds at billions of different points simultaneously.
This process of interchange is known as osmosis, and may be compared to the transposition that takes place when two liquids, one containing salt, the other sugar, are separated from one another by a thin, permeable membrane. A reciprocal exchange of the contents of both solutions takes place and the contents of both solutions become equalised. This is how the exchange between the cells and the blood takes place. The cells take the oxygen and other essential elements out of the blood, and in turn give up their waste products.
It is not always easy for the human mind to visualise the minuteness of a capillary. Kahn3 states that each capillary is fifty times finer than the finest human hair, and points out that it is so minute that about seven hundred capillaries could be packed into the space occupied by the thickness of a pin.
Through these microscopic blood vessels the blood corpuscles pass steadily in single file, carrying oxygen and other nutritive elements to the cells, and carrying off their toxins.
The capillaries, the smallest subdivision of the arteries, also form the beginning of the veins. Neighbouring capillaries merge and form small veins, venulea, out of which by fusion the larger veins are formed.