This section is from the book "The Human Body: An Elementary Text-Book Of Anatomy, Physiology, And Hygiene", by H. Newell Martin. Also available from Amazon: The Human Body.
The blood vascular system is quite closed except at two points, one on each side of the neck, where lymph vessels pour the excess of lymph back into the veins. Valves at these two points let lymph flow into the blood-vessels, but will not allow blood to pass the other way. Accordingly everything which leaves the blood must do so by oozing through the walls of the blood-vessels, and everything which enters it must do the same, except matters conveyed in by the lymph at the points above mentioned. This interchange through the walls of the vessels takes place only in the capillaries. In India rain falls only at certain seasons, and is stored in huge tanks ; during subsequent dry months the water is distributed over the fields by a set of small ditches and channels, cut through them in all directions, and from which the liquid soaks through the surrounding soil; this is known as irrigation, and the capillaries may be said to form an irrigation system for the body. In a certain sense also they may be compared to the water spigots of a house, which lie at the end of the supply tubes, the arteries; but, to make the comparison more accurate, we would have to imagine instead of ordinary spigots bags of very fine muslin through which the water oozed when the tap was turned on. The capillaries, though far the smallest tubes in the vascular system, are the really important parts; the heart, arteries, and veins are all merely arrangements for keeping the capillaries full, and renewing the blood within them. It is while flowing through these and soaking through their walls that the blood does its physiological work.
How does the blood flow differ from that of water in the mains of a city? How is the supply of blood in the heart kept up?
Where is the blood vascular system not closed ? What occurs at its openings? What is the function of the valves at these openings? How must substances leave the blood ? How enter it? In what vessels does interchange through the walls of the blood-vessels occur? Illustrate the function of the capillaries by comparison with an irrigation system.