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.
Some very small animals of simple structure require no blood; every part catches its own food and gives off its own wastes to the air or water in which the creature lives. When, however, an animal is larger and more complex, made up of many organs, some of which are far away from the surface of its body, this is impossible; some organs are therefore set apart to catch food, and arrangements made to carry some of this food to the others.
In our own bodies many parts lie faraway from the stomach and intestines which receive, digest, and absorb our food, and from the lungs which take oxygen gas out of the air we breathe; yet every part, bone and muscle, brain and nerve, skin and gland, needs a steady supply of both of these things to keep it alive. The division of labor, in accordance with which some organs are especially set apart for the purpose of receiving substances from the outside world to build up, nourish, and repair the body, necessitates an arrangement by which the matters received shall be distributed to other parts. This distribution is accomplished by the bloody which flows into every organ from the crown of the head to the sole of the foot. Being pumped round and round, from place to place, by the heart, the blood picks up nourishing things in its course through the walls of the alimentary canal, and oxygen as it flows through the lungs; it then carries them to all other parts of the body.
What kind of animals do not need blood? How are their wants supplied and their wastes removed? Why do we find special receptive organs in larger animals? Illustrate from the human body. What arrangement is necessitated by the fact that special organs are set apart in the body for receiving food and oxygen? How is the distribution effected?
The rapidly flowing blood not only conveys a supply of nutritive material for all the organs, but is a sort of sewage stream that drain3 off their wastes (p. 127), and carries them to the excretory organs, by which they are sent entirely out of the body.
The blood is a middleman: on the one hand, between the receiving organs (lungs and alimentary canal) and all the rest; and on the other hand, between the excretory organs and all the others. Each part is thus kept in a well-fed and healthy state, though it may lie far distant from all places where new materials first enter the body, and from those where refuse and deleterious substances are finally passed from it.