We know now that the blood is the vehicle by which oxygen is carried into all parts of the body, for the purpose of burning up or oxidizing the worn-out waste tissues. Those waste matters could not be removed from the system in their solid form. Oxygen, the powerful agent of combustion, burns them up, and converts them into soluble substances, which are readily absorbed and carried away in the stream of the blood.
But the blood is not merely the oxygen carrier. It has other duties beyond this. It brings material for building up again those parts of the body which have been worn out by their work. There is not only constant wear and waste going on, but constant repair as well. This explains why a person in good health varies so little in size and weight year after year.
All the materials, whether for making bone, brain, muscle, or any part of the substance of the body, are brought by the blood to the spot where they are wanted, and the blood gets them all from the food which we eat.
When we have not taken food for some time we begin to be conscious of a peculiar emptiness in the stomach;we feel weary and faint, and unfit for either work or play; we feel a great desire to eat-we are hungry. We go to our meal, and come away with renewed vigour, ready to resume our ordinary occupations.
When a person is in a healthy condition, this desire for food returns several times a day. He is said to have a good appetite. Indeed, if our appetite falls off, we know that something is wrong; we have to consult the doctor. If a person were kept without food, his body would shrink and lose weight, and he would gradually become unable to do any work with either mind or body.
So then the body feeds on the blood, and the blood itself feeds on the food which we eat. In other words, our food supplies new materials to the blood, and the blood carries them to rebuild just those identical tissues that have been destroyed with their work.
The food has actually to be converted into blood. This is the whole work of digestion. Digestion has to change the food we eat into blood fit to feed the body. The bread and butter, meat, pudding, potatoes, are solid substances, and quite useless for the purpose in that form. It is the work of digestion to change the nature of the food, so as to make it easily soluble. In this state, and in this state only, can it ever be absorbed into the blood.
The full force of this would be readily seen by putting some powdered starch and sugar into separate glasses of water. If the two were stirred with a spoon, the sugar inthe one would entirely disappear, but the starch in the other would still be seen, and would, if left to itself, sink to the bottom. In the one case the water holds the sugar in solution; it has become, for the time, inseparable from the water. We could not pour out the water and leave the sugar behind. Not so the starch. This settles at the bottom, and we may easily pour off the water and leave the starch.
But in addition to this, if we tried our osmosis experiment with the two substances, there would be a marked difference. The dissolved sugar would readily pass through the membrane; it would be impossible for the water to pass and leave the sugar behind, and this is equally true of every dissolved substance. On the other hand, no amount of force or coaxing would send any of the starch through the membrane. This must be clearly understood now, as it will have an important bearing on the lessons which are to follow. The whole of our food must be dissolved, for it can only pass into the blood by osmosis. This dissolving of the food is the work of digestion.