The Organs Of Circulation

The Organs Of Circulation are the heart and the blood vessels; the blood-vessels are of three kinds, arteries, capillaries, and veins. The arteries carry blood from the heart to the capillaries; the veins collect it from the capillaries and return it to the heart. There are two distinct sets of blood-vessels in the body, both connected with the heart; one set carries blood to, through, and from the lungs, the other guides its flow through all the remaining organs; the former are known as the pulmonary, the latter as the systemic blood-vessels.

General Statement

During life the pumping of the heart keeps the blood flowing rapidly through the paths marked out for it by the blood-vessels; these paths it never leaves except in cases of disease or injury.

The blood-vessels form a continuous system of closed tubes comparable in a certain way to the water-mains of a city. These tubes begin at the heart, and are very much branched except close to it, just as the water pipes are single only where the main aqueduct leaves the reservoir, and more and more divided the further one follows them from that point, until, in the various houses, they end in numerous but very much smaller tubes. The course of the blood differs, however, essentially from that of the water supplied to a city, for the water does not return to the reservoir, whereas the blood is carried back to the heart: instead of having a large supply of liquid stored up as in a reservoir, there is at any one time only quite a small amount in the heart, but this is steadily replaced by the inflow through the veins as fast as it is carried off by outflow through the arteries.

Name the organs of circulation. Name the kinds of blood-vessels. What is the function of the arteries? Of the veins? How many sets of blood-vessels are there in the body? What does each set do? What are they called?

What is brought about by the beat of the heart? Under what circumstances may blood leave a blood channel ?

What do the blood-vessels form? Illustrate their function by comparison with the water-mains of a city.