THE object of digestion is to break down the constituents of the food into the simplest parts, so that the food is more readily absorbed. Also the proteins must be broken down first, so that they may be built up later into proteins characteristic of the animal that uses them. Mutton protein as such would be useless to man. It must be absorbed in pieces so that he can make his own peculiar protein for himself.
Digestion is accomplished by means of certain juices which break up the constituents of the food. These juices are secreted by glands. A typical gland is composed of a number of cells arranged to form a flask or retort, from the opening of which a duct leads. The cells have the power of taking substances from the blood and converting them into juices or secretions which are carried down the duct to the place where they are used. Each gland is supplied with nerves which control its working.
When food is taken into the mouth, it is broken up into small fragments and intimately mixed with the saliva which is secreted by certain glands in the neck. This saliva has the power of breaking down the starches and polysaccharides into disaccharides. The stimulus which makes the glands secrete is not only the presence of food in the mouth, but even the sight or smell of food. It is a well-known fact that the ‘mouth will water ‘even at the sight of some tasty morsel.
When the food is swallowed, it reaches the stomach, and here it comes under the powerful influence of the gastric juice, which breaks down the proteins into proteoses and peptones and has some small action on the sugars. The mechanism of
the secretion of gastric juice has been well shown by Pavlov’s experiments with dogs. He cut the gullet in the neck and brought each end to the surface, so that food swallowed by the dog did not go into the stomach but was discharged from the cut end. He also brought a small part of the stomach to the surface of the body, so that he could directly watch how much juice was secreted.
He found that the sight or smell of food, even if it was not given to the dog, would start the stomach secreting. If food was swallowed, the stomach would secrete, even if food did not reach it. Also if food was introduced into the stomach without the knowledge of the dog, as could easily be done, the stomach would secrete, but the amount of secretion depended on the character of the food. Meat was more potent in producing a flow than bread, for example. Here, laid before us, is the whole mechanism of glandular secretion. First comes the psychical secretion at the sight of food, then the mere presence of food in the stomach from its mechanical action, and thirdly the chemical stimulus from the nature of the food.
HOW THE FUEL IS BROKEN UP
AFTER the food leaves the stomach it enters the duodenum, which is the next part of the bowel. Into this open two ducts, one the bile duct from the liver and the other the pancreatic duct from the pancreas. The juice from the pancreas contains several ferments which act in conjunction with the bile. This, though it contains no ferments itself, is a powerful assistant to the pancreatic juice. Between them these two juices break up the remains of the proteins into polypeptides and amino acids, the fats into fatty acids and glycerine, and the sugars and starches into monosaccharides.
Some few of the food constituents will have escaped the breaking-down process, and these are acted upon by a juice, the succus entericus, which is secreted by the small intestine itself. The result is that the contents of the intestine now consist of a fluid mass containing amino acids, fatty acids, glycerine and monosaccharides in such a condition that they are ready for absorption.
HOW FOOD GETS INTO THE BLOOD
THE intestine is well supplied by blood vessels and lymphatics into which the products of digestion can be taken. The mechanism by which this is affected is by no means certain,
and the probability of its exact working being discovered is remote, for the process is an obscure and essentially vital one that would require a knowledge of the deepest secrets of the cells themselves. It may be stated that during absorption work is performed by the cells, for they have to work against a definite pressure. The contents of the intestine are more concentrated than the blood, and therefore water will tend to pass out of the tissues into the bowel.
This tendency to equalise concentration on both sides of a membrane is known as osmotic pressure, and the cells have to work against it in absorbing material into the blood. One thing, however, we do know, and that is that the finger-like processes known as villi, which cover the inside of the intestine, are contractile elements. Their function is to increase the effective area over which the absorption can take place and also to assist it mechanically.
A villus is a minute structure, like the frond of a sea-anemone, covered on the outside with absorbing cells and filled with soft, connective tissue. Running up the centre is a lymphatic vessel and blood vessels into which the products are absorbed. There is also a muscle which can increase and decrease the volume of the villus, so that suction on the contents of the bowel can be applied. This suction sucks the bowel contents through the cells and not through any opening. Under certain circumstances the villi can be seen contracting rhythmically under the microscope, proving the action. This is all that we know of the mechanism of absorption, and it is by no means the whole story.
The fats, which have been re-synthesised in the villi after their splitting in the bowel, are absorbed into the lymphatics, and later enter the blood stream. The sugars and amino acids are absorbed into branches of the portal vein, and pass from there to the liver, as we know from our anatomy.
THE LIVER : CHEMICAL FACTORY OF THE BODY w
HEN the sugars reach the liver, they are converted by the activity of the liver cells into glycogen, which, as we have mentioned before, is a complex starch made up of many monosaccharide molecules. The glycogen is stored in the liver and later, as required, split down again into glucose and passed into the circulation, where it is used directly by the tissues. The liver, thus, is a storehouse for sugar and prevents its too rapid use.
Of the amino acids absorbed, some are burnt up in the same way as the sugars are, in order to provide warmth and energy, and the liver plays a large part in this process. Before they can be burnt, they must be converted into sugars, and this action is performed by the liver, which removes the nitrogen from their molecules so that it can be excreted by the kidneys. Those amino acids which are not burnt up for energy escape the gauntlet of the liver and pass to the body cells in general, where they are built up into complex human proteins.
This is the part of our food which replaces the wear and tear of the body and is so essential for the well-being of an adult and the growth of a child. Wc require actually only sufficient protein in our diet to enable us to replace our wear and tear. Sugar and fat can supply the energy, but they can never be of use in this particular way. This, too, is why growing children require so much more protein than adults.
The fats also eventually reach the liver by way of the general blood stream. Some of them after absorption are taken up by the fat cells under the skin and in other parts of the body, and stored there until they are wanted for the production of energy. Before they can be burnt, however, they must pass through the liver, wliich changes them slightly and so makes them more readily combustible.
The liver is one of the important workshops in which sugar is converted into fat. That this is possible in the body is interesting, for it is the cause of fatness in many people to-day. It was for long considered that excessive eating of sugar could lead to obesity, and it was proved conclusively by feeding two pigs of the same litter, one with and one without sugar. The weight of fat in the pig fed with sugar greatly exceeded the fat in the body of the one who had gone without. The majority of people to-day who are unduly fat could reduce comfortably by cutting down their diet, especially that part containing sugar.
INSULIN: THE MATCH THAT LIGHTS OUR FIRES THE story of insulin is a fascinating one, but we can deal with it only very briefly here. Our warmth, energy and muscular activity depend upon the amount of fat and sugar which is burnt inside our bodies. Before fat can be burnt, sugar must be burnt, too—the fat is, as it were, burnt up in the heat generated by the carbohydrate fires. If sugar is not burnt at the same time as fat, the fat is incompletely burnt
and the products of its disintegration are poisonous. They are known as ketone bodies, owing to their chemical composition. This unhappy result takes place either when there is no sugar to burn with the fat, as during a prolonged starvation, or when the sugar, though present in abundance, cannot for some reason be burnt itself.
Insulin is a substance secreted into the blood stream by the pancreas, which makes it possible for the body to burn sugar. Without insulin no sugar can be burnt nor, of course, can the fat be completely used up. The result of this is that sugar accumulates in the blood and overflows into the urine, while poisonous products of fat metabolism, the ketone bodies, appear in the body and may eventually cause death. The cure for this condition which, of course is diabetes, is the injection of insulin under the skin. The insulin puts a match, as it were, to the fires of carbohydrate metabolism in which the fats are completely burnt without leaving a trace of poisonous ash.