Materials which are suitable for foods must be changed from their natural state into forms that can be absorbed and used by the tissues of the body. It is very seldom that any portion of our food can be absorbed without being changed. The process by which this change is brought about is called digestion.
Organs of the Digestive System. In the simple forms of life the individual cell, possessed of ameboid movements, wraps itself about a particle of food, digests what it needs, then by unfolding itself discards any residue which remains undigested. In the higher forms of life the process is more complex, and special cells have taken on the function of dissolving and absorbing particles of food. Groups of such cells united in masses are known as organs. Each of the digestive organs contributes a definite part to the process of digestion. However, the function of each organ is more or less over-lapped by that of another, so that should one be injured or prevented from acting normally, its part in the digestive scheme can be covered to a large degree by other organs. Along the course of the digestive tract we note the mouth and teeth, the esophagus, stomach, small intestine, liver and pancreas which are glands emptying into the first portion of the small intestine, the colon, and rectum.
The digestive organs as a whole form a tube, with a few outpocketings where special glands are located. In the walls of this tube are muscular fibers which encircle and others which run lengthwise with the lumen. It is by the co-ordinated relaxation and contraction of these fibers that the act of swallowing is accomplished. This form of wave-like motion is called peristalsis. It also brings about the mixing of the food mass and its movement through the stomach and intestines.
What Happens in the Mouth and Esophagus. Most of the food eaten must be broken up or ground so that it may be swallowed, and also that it may be in suitable form to be acted upon by the digestive juices. While it is being crushed by the teeth, several glands about the mouth pour out increased quantities of the watery fluid which keeps the mouth moist, commonly known as saliva. The total daily production is probably about three pints. This serves to soften food and to lubricate its movements in the mouth and throat. Saliva contains an enzyme called Ptyalin which partially digests starches. The digestion is begun in the mouth and continued after the food reaches the stomach until the mass has become acidified by the gastric juice. We may think of starch as being “cracked” or “split” by this early digestive process, because the form to which it is changed is preliminary to complete digestion in the small intestine.
Saliva eases the passage of food through the esophagus. The presence of saliva in the lower esophagus promotes the reflex response of the nerves by which the act of swallowing is largely controlled. Food in the lower esophagus causes it to relax, which with an accompanying relaxation of the first part of the stomach forms a space which is an aid in swallowing. Washing food down with fluids is not a good habit to form. The food is received into the stomach as if into a temporary storage, and is passed on at intervals into the small intestine.
Digestion in the Stomach. The secretions from the wall of the stomach, referred to as gastric juice, contain hydrochloric acid in a concentration of about 0.4 per cent, pep-sin, rennin, often a small amount of a fat-splitting enzyme, lipase, which likely regurgitates from the duodenum, and an unnamed substance which is important in the building of blood. Other substances of undetermined significance may be found at times.
The flow of gastric juice in response to food may be divided into two periods. That produced in the first is the result of nervous impressions arising from seeing, smelling, and tasting foods. It is sometimes referred to as the “psychical” or “appetite” juice. It appears several minutes after beginning to eat a meal, and is larger in amount as the food is more desirable. The juice produced during the second period follows the stimulation of the stomach mucous membrane by partly digested proteins. This portion is less in volume, but more active in chemical and biological ingredients. In animals that have been starved a juice which is rich in digestive agents is produced as soon as they begin to eat, but in man the stimulation from starvation is rarely observed, and the juice is quite constant in its concentration, digestive properties, and hydrochloric acid content.
The giving of acids by mouth increases the production of gastric juice. On the other hand, alkalis in limited quantities may produce a temporary increase, but if used freely for a period of time they actually lead to a depression of acid secretion, with a depression of secretion from the other digestive glands along the course of the allmentary tract. Even when the stomach contains no food, a small amount of gastric juice is formed in it as if the glands of the walls were always functioning to some degree.
Hydrochloric acid is formed by cells in the first portion of the stomach. The chlorine of the acid is believed to come from table salt, the other constituent of which (sodium) is used by the body in maintaining important chemical states in the blood. One action of the hydrochloric acid is the softening of protein substances. As it combines with the proteins water is taken up resulting in a soft spongy or gelatinous mass being formed. In protein foods which remain in contact with hydrochloric acid for ten or twelve hours, digestion proceeds to a certain fractional point and no further. Other acids such as sulphuric, phosphoric, lactic (in sour milk), or citric (in citrus fruits) will act on proteins essentially in the same manner.
The second important function of hydrochloric acid is its activation of pepsin, an enzyme in the gastric juice which is effective only in the presence of an acid. The effect of acid on pepsin is largely independent of the actual concentration of acid present. Pepsin, when activated, dissolves the proteins already softened, and as it continues in contact with these substances, various pro-ducts of digestion and hydrolysis are produced. Proteins are broken into fractional parts in each of which only a few amino acids are present. Rennin, another ferment in the gastric juice, acts in the formation of the curd of milk.
It is believed that certain types of bacteria and amoeba are destroyed by the normal acid of the stomach. Accompanying some forms of parasitic infection in the bowels the absence of hydrochloric acid in the stomach is commonly observed. Some who have investigated the subject claim that virulent organisms such as streptococcus-hemolyticus are destroyed largely in the stomach.
A rhythmical wave motion known as peristalsis, which has been explained above, begins in the wall of the stomach when it contains food. These movements involve the stomach wall and pass along the small intestine and end in the first part of the colon. The muscular outlet of the stomach, called the pylorus, relaxes rhythmically and small portions of food are passed into the duodenum. The acid concentration of the stomach contents has an influence on the rate of emptying. When the acid is high the emptying is prolonged, while it is markedly shortened when there is little or no acid in the stomach. As a portion passes it is alkalinized by the duodenal and pancreatic juices, and as the pylorus relaxes in peristaltic rhythm another portion enters from the stomach. The activity of the muscles is constant while food is present. An average mixed meal may be emptied from the stomach in from two to five hours.
Digestion in the Small Intestine. When the acidified food begins to pass from the stomach into the duodenum or first portion of the small intestine, a hormone, secretin, is formed in the wall of the duodenum which is carried by the blood to the pancreas producing a marked increase in the secretions of this gland. The formation of secretin is enhanced when bile and bile salts are present in the duodenum, which is natural at this stage of the digestive process when the liver and gall bladder function normally.
The first portion of the small intestine is the most important portion of the whole digestive tract, if we can think of one portion being more important than another. We have mentioned its response to the entry of food. The digestive fluids are added to the food here more rapidly and in more potent concentration than in any other similar section. Among these are included the secretions from the wall of the duodenum and the jejunum (the second portion of the small intestine), the pancreas, and liver. Not only does this mean that a large amount of fluid is added to the food, which aids in digestion, but from each of these sources come chemical agents or enzymes that are necessary in the digestive process. In both the pancreatic and intestinal secretions several digestive agents are found.
Were one digestive fluid to be considered as more important than another it would be that produced by the pancreas. In it are enzymes for digesting (respectively) proteins, carbohydrates, and fats. Probably a pint is produced during the digestion of an average meal. If any of these enzymes are deficient or absent, then the digestion of the particular food concerned is impaired, as it is doubtful whether adequate digestion of common foods is accomplished without the pancreatic secretion. The starch-splitting enzyme found here can change starches into a sugar that is quickly absorbed. The proteins, to a degree, are broken into their fundamental amino acids or into substances made up of much smaller groups of amino acids than any previously formed, and the fat-splitting enzyme prepares fats for absorption. If the pancreatic juice is lacking to a marked degree, ill health quickly appears, followed by death from inanition.
Bile is often thought of as a waste substance. But it has several functions in digestion that make us feel that it is quite important. A small quantity stored in the gall bladder is normally emptied into the duodenum as soon as a meal begins to leave the stomach, and serves to hasten the production of secretin which stimulates the flow of juices from the pancreas and intestinal walls. The activity of the starch-splitting and fat-splitting enzymes in the pancreatic juice is increased a number of times in the presence of bile. This favorable effect seems to be shared by nearly all the enzymes found in the intestine. Some of the elements in bile are necessary in absorbing digested fats. If it is kept from reaching the intestinal tract the unabsorbed digested fats appear in the stool. In normal states free bile and bile salts are not found in the stool, possibly because of having served as a vehicle in absorbing fats.
The final steps in digestion are brought about in the small intestine under the action of the intestinal juice or succus entericus. The complete splitting of proteins to amino acids is accomplished by one of its enzymes, erepsin. The more complex sugars are acted upon by enzymes which change them into simple forms, chiefly glucose, so that they can be absorbed into the blood stream. It is thought by some that these ferments are found in the mucous membrane lining the intestines, and that they may exert their action largely while the several sugars are passing through the cellular wall of the bowel. How-ever, it is believed more generally, that they are present in the secretions from the membranes, acting upon the sugars while they are in the hollow space of the intestine. Digestion is practically completed in the small intestine, the food reaching its junction with the colon in approximately twelve to eighteen hours.
Digestion in the Colon. In the colon very little actual digestion takes place. Residues of digestive juices already secreted continue to act on any undigested particles of food. Salts and glucose may be absorbed in some degree. Water, however, is absorbed in large amounts. The residue which at first is watery or pasty becomes firmer until a semi-solid consistency is attained. In general the movements of the colon are slow. Series of contractions and relaxations occur, but not with the close sequence seen in peristalsis. These movements plus the constant entering of food from above result in pushing the drier residue forward until it collects in the descending portion of the colon on the left side of the abdomen. Mucus is secreted from its walls, increasing in amount as we follow its course, making the passage of feces easier. The function of the lower colon in part at least is to serve as a storage space. When it finally expels the undigested residue, the colon completes its part in the digestive process.