Organs and Processes of Digestion : Part 4

Physiology and Hygiene for Secondary Schools
by Francis M. Walters

(Page 13 of 31)

The mucous membrane of the small intestine is richly supplied with blood vessels and contains glands that secrete a digestive fluid known as the intestinal juice. The membrane is thrown into many transverse, or circular, folds which increase its surface and also prevent materials from passing too rapidly through the intestine. One important respect in which the small intestine differs from all other portions of the food canal is that its surface is covered with great numbers of minute elevations known as the villi. The purpose of these is to aid in the absorption of the nutrients as they become dissolved.

The muscular coat of the small intestine is made up of two distinct layers - the inner layer consisting of circular fibers and the outer of longitudinal fibers. These muscles keep the food materials mixed with the juices of the small intestine, but their main purpose is to force the materials undergoing digestion through this long and much-coiled tube.

The outer, or serous, coat of the small intestine, like that of the stomach, is an extension from the general lining of the abdominal cavity, or peritoneum. In fact, the intestine lies in a fold of the peritoneum, somewhat as an arm in a sling, while the peritoneum, by connecting with the back wall of the abdominal cavity, holds this great coil of digestive tubing in place. The portion of the peritoneum which attaches the intestine to the wall of the abdomen is called the mesentery.

Most of the liquid acting on the food in the small intestine is supplied by two large glands, the liver and the pancreas, that connect with it by ducts.

The Liver is situated immediately below the diaphragm, on the right side and is the largest gland in the body. It weighs about four pounds and is separated into two main divisions, or lobes. It is complex in structure and differs from the other glands in several particularys. It receives blood from two distinct sources - the portal vein and the hepatic artery. The portal vein collects the blood from the stomach, intestines, and spleen, and passes it to the liver. This blood is loaded with food materials, but contains little or no oxygen. The hepatic artery, which branches from the aorta, carries to the liver blood rich in oxygen. In the liver the portal vein and the hepatic artery divide and subdivide, and finally empty their blood into a single system of capillaries surrounding the liver cells. These capillaries in turn empty into a single system of veins which, uniting to form the hepatic veins (two or three in number), pass the blood into the inferior vena cava.

The liver secretes daily from one to two pounds of a liquid called bile. A reservoir for the bile is provided by a small, membranous sack, called the gall bladder, located on the underside of the liver. The bile passes from the gall bladder, and from the right and left lobes of the liver, by three separate ducts. These unite to form a common tube which, uniting with the duct from the pancreas, empties into the duodenum. Though usually described as a digestive gland, the liver has other functions of equal or greater importance.

The Bile is a golden yellow liquid, having a slightly alkaline reaction and a very bitter taste. It consists, on the average, of about 97 percent of water and 3 percent of solids. The solids include bile pigments, bile salts, a substance called cholesterol, and mineral salts. The pigments (coloring matter) of the bile are derived from the hemoglobin of broken-down red corpuscles.

Much about the composition of the bile is not understood. It is known, however, to be necessary to digestion, its chief use being to aid in the digestion and absorption of fats. It is claimed also that the bile aids the digestive processes in some general ways - counteracting the acid of the gastric juice, preventing the decomposition of food in the intestines, and stimulating muscular action in the intestinal walls. No enzymes have been discovered in the bile.

The Pancreas is a tapering and somewhat wedge-shaped gland, and is so situated that its larger extremity, or head, is encircled by the duodenum. From here the more slender portion extends across the abdominal cavity nearly parallel to and behind the lower part of the stomach. It has a length of six or eight inches and weighs from two to three and one half ounces. Its secretion, the pancreatic juice, is emptied into the duodenum by a duct which, as a rule, unites with the duct from the liver.

The Pancreatic Juice is a colorless and rather viscid liquid, having an alkaline reaction. It consists of about 97.6 percent of water and 2.4 percent of solids. The solids include mineral salts (the chief of which is sodium carbonate) and four different chemical agents, or enzymes, - trypsin, amylopsin, steapsin, and a milk-curding enzyme. These active constituents make of the pancreatic juice the most important of the digestive fluids. It acts with vigor on all of the nutrients insoluble in water, producing the following changes:

1. It converts the starch into maltose, completing the work begun by the saliva. This action is due to the amylopsin, which is similar to ptyalin but is more vigorous.

2. It changes proteins into peptones and proteoses, completing the work begun by the gastric juice. This is accomplished by the trypsin, which is similar to, but more active than, the pepsin.

3. It digests fat. In this work the active agent is the steapsin.

The necessity of a milk-curding enzyme, somewhat similar to the rennin of the gastric juice, is not understood.

Digestion of Fat. - Several theories have been proposed at different times regarding the digestion and absorption of fat. Among these, what is known as the "solution theory" seems to have the greatest amount of evidence in its favor. According to this theory, the fat, under the influence of the steapsin, absorbs water and splits into two substances, recognized as glycerin and fatty acid. This finishes the process so far as the glycerin is concerned, as this is soluble in water; but the fatty acid, which (from certain fats) is insoluble in water,(62) requires further treatment. The fatty acid is now supposed to be acted on in one, or both, of the following ways: 1. To be dissolved as fatty acid by the action of the bile (since bile is capable of dissolving it under certain conditions). 2. To be converted by the sodium carbonate into a form of soap which is soluble in water.

D.C. Heath and Co. - Publishers
Original copyright 1909