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.
Those organs of the body whose function it is to gather new material from outside for its use are known as receptive organs. There are two chief sets of these—one to receive oxidizable things, and the other to receive oxygen. The first set is represented by the alimentary canal, consisting of mouth, gullet,* stomach, and intestines. It takes in food and drink. The second set consists of the lungs, with the air passages leading to them ; their business, as receptive organs, is to absorb oxygen.
What facts make it clear that a man must daily give off several pounds weight of matter from his body? Does a child's increase in weight equal the weight of the food it has eaten? What is meant by the "wastes" of the body? How do most foods differ from wastes in regard to oxidation ? Why must wastes be removed from the body ? Name the chief wastes of the body.
What is meant by the receptive organs?
The organs whose duty it is to get rid of waste materials formed in the body are called excretory organs. The three most important excretory organs are the lungs, the kidneys, and the skin; the lungs pass carbon dioxide gas out to the air, and also water; the kidneys get rid of urea and water; and the skin, of water and a little urea.
Between the taking of oxidizable substances into our mouths and oxygen into our lungs, on the one hand, and the return of oxidized matters from our bodies to the surrounding world on the other, a great many intermediate steps take place. The alimentary canal (see Fig. 1) is a tube which runs through the body but nowhere opens into it; so long as food lies in this tube it therefore does not really form a part of the body, and is of no use to it: it resembles coals in the tender of a locomotive, waiting to be transferred to the furnace. In our bodies the furnace is everywhere; wherever there is a living tissue things are burned to enable it to work; and the food or fuel must be brought therefore, to every corner of our frames.
What are the functions of the two chief sets? Name those concerned in receiving oxidizable things. Those whose business it is to absorb oxygen.
What is meant by the excretory organs? Name the most important. What does each get rid of?
Why is food in the stomach not really a part of the body? To what may we liken it? Where is the furnace of the body? Why must food be carried all over the body?
* The technical name for the gullet is œsophagus.
A great part of our food is solid, and could not of itself get outside of the alimentary canal. To render it available it must be dissolved so that it can soak through the walls of the stomach and intestines. For this purpose we find a set of digestive organs which make solvent juices and pour them on the food which we swallow, and so get it into a liquid state in which it can be absorbed.
The solution containing our digested food if it simply soaked through the walls of the alimentary canal, would be of no use to distant parts, as the brain, or the muscles of the limbs. We find, therefore, in the body a set of tubes containing blood, and called blood-vessels: the blood is driven through these by a pump, the heart. Much of the dissolved food passes into the blood-vessels of the alimentary canal, and from them is carried by other blood-vessels to every organ, no matter how remote. As the blood flows unceasingly, round and round in its vessels, from part to part, the organs concerned in moving and conveying it are called circulatory organs, and the blood-flow itself is known as the circulation.
Some of the dissolved food is taken up into another set of tubes in the walls of the alimentary canal; these tubes carry it afterwards into the blood-vessels. They are called the absorbents.
Why must many foods be dissolved? What is accomplished by the digestive organs?
What are the blood-vessels? What enters those of the alimentary canal? How are organs distant from the alimentary canal nourished? Why are the organs which keep up the blood-flow called circulatory organs? What is meant by the circulation?
What are the absorbents? Where do they convey the foods which they take up in the walls of the alimentary canal?
The blood in its course flows through the lungs. It is necessary not merely that food, but oxygen also should be carried to every part of the body. As the blood traverses the lungs it picks up oxygen from the air in them; this air is then renewed by taking a fresh breath, and so on. The organs concerned in renewing the air in the lungs are the respiratory organs, and the act of renewal is respiration.
As each organ works it oxidizes; some of its substance is broken down by combination with oxygen brought to it by the blood, and is thus converted into burnt waste matter. The blood, as we have seen, brings, however, not merely oxygen, but also food matters in solution. These ooze through the walls of the blood-vessels, and are taken up by the living tissues and built into new tissues like themselves, to replace the part which has been used up and destroyed. This building and repair of tissues and organs from the dissolved food obtained from the blood is known as assimilation,—in plain English, "a making alike." Each living tissue takes from the blood foods which are not like itself, and builds them up into a form of matter like its own, The converse process, which accompanies all vital action, the breaking down into wastes of a living tissue when it works, is called dissimilation, or "a making unlike".