Digestion and Absorption in the Gastrointestinal Tract

GIT SYSTEM

Lecture 9
Dr. Noor Jawad

Digestion and Absorption in the Gastrointestinal Tract

DIGESTION OF CARBOHYDRATES
Only three major sources of carbohydrates exist in the normal human diet. They are sucrose, which is the disaccharide known popularly as cane sugar; lactose, which is a disaccharide found in milk; and starches, which are large polysaccharides present in almost all non-animal foods, particularly in potatoes and different types of grains.

Digestion of Carbohydrates Begins in the Mouth and Stomach.

When food is chewed, it is mixed with saliva, which contains the digestive enzyme ptyalin (an α-amylase) secreted mainly by the parotid glands. This enzyme hydrolyzes starch into the disaccharide maltose and other small polymers of glucose that contain three to nine glucose molecules.

However, the food remains in the mouth only a short time, so probably not more than 5 percent of all the starches will have become hydrolyzed by the time the food is swallowed.
Starch digestion sometimes continues in the body and fundus of the stomach for as long as 1 hour before the food becomes mixed with the stomach secretions.

Activity of the salivary amylase is then blocked by acid of the gastric secretions because the amylase is essentially inactive as an enzyme once the pH of the medium falls below about 4.0. Nevertheless, on average, before food and its accompanying saliva become completely mixed with the gastric secretions, as much as 30 to 40 percent of the starches will have been hydrolyzed, mainly to form maltose.

Digestion by Pancreatic Amylase.

Pancreatic secretion, like saliva, contains a large quantity of α-amylase that is almost identical in its function to the α-amylase of saliva but is several times as powerful. Therefore, within 15 to 30 minutes after the chyme empties from the stomach into the duodenum and mixes with pancreatic juice, virtually all the carbohydrates will have become digested.


DIGESTION OF PROTEINS
Dietary proteins are chemically long chains of amino acids bound together by peptide linkages.

Digestion of Proteins in the Stomach. Pepsin, an important peptic enzyme of the stomach, is most active at a pH of 2.0 to 3.0 and is inactive at a pH above about 5.0. Consequently, for this enzyme to cause digestion of protein, the stomach juices must be acidic, the gastric glands secrete a large quantity of hydrochloric acid. This hydrochloric acid is secreted by the parietal (oxyntic) cells in the glands at a pH of about 0.8, but by the time it is mixed with the stomach contents and with secretions from the non-oxyntic glandular cells of the stomach, the pH then averages around 2.0 to 3.0, a highly favorable range of acidity for pepsin activity.

One of the important features of pepsin digestion is its ability to digest the protein collagen, an albuminoid type of protein that is affected little by other digestive enzymes. Collagen is a major constituent of the intercellular connective tissue of meats; therefore, for the digestive enzymes to penetrate meats and digest the other meat proteins, it is necessary that the collagen fibers be digested. Consequently, in persons who lack pepsin in the stomach juices, the ingested meats are less well penetrated by the other digestive enzymes and, therefore, may be poorly digested.

Pepsin only initiates the process of protein digestion, usually providing only 10 to 20 percent of the total protein digestion to convert the protein to proteoses, peptones, and a few polypeptides. This splitting of proteins occurs as a result of hydrolysis at the peptide linkages between amino acids.

DIGESTION OF FATS

Digestion of Fats Occurs Mainly in the Small Intestine.
A small amount of triglycerides is digested in the stomach by lingual lipase secreted by lingual glands in the mouth and swallowed with the saliva. This amount of digestion is less than 10 percent and is generally unimportant. Instead, essentially all fat digestion occurs in the small intestine.

ANATOMICAL BASIS OF ABSORPTION

The total quantity of fluid that must be absorbed each day by the intestines is equal to the ingested fluid (about 1.5 liters) plus that secreted in the various gastrointestinal secretions (about 7 liters), which comes to a total of 8 to 9 liters. All but about 1.5 liters of this fluid is absorbed in the small intestine, leaving only 1.5 liters to pass through the ileocecal valve into the colon each day.

The stomach is a poor absorptive area of the gastrointestinal tract because it lacks the typical villus type of absorptive membrane, and also because the junctions between the epithelial cells are tight junctions. Only a few highly lipid-soluble substances, such as alcohol and some drugs like aspirin, can be absorbed in small quantities.

ABSORPTION IN THE SMALL INTESTINE

Absorption from the small intestine each day consists of several hundred grams of carbohydrates, 100 or more grams of fat, 50 to 100 grams of amino acids, 50 to 100 grams of ions, and 7 to 8 liters of water. The absorptive capacity of the normal small intestine is far greater than this; each day as much as several kilograms of carbohydrates, 500 grams of fat, 500 to 700 grams of proteins, and 20 or more liters of water can be absorbed. The large intestine can absorb still more water and ions, although it can absorb very few nutrients.


ABSORPTION IN THE LARGE INTESTINE: FORMATION OF FECES
About 1500 milliliters of chyme normally pass through the ileocecal valve into the large intestine each day. Most of the water and electrolytes in this chyme are absorbed in the colon, usually leaving less than 100 milliliters of fluid to be excreted in the feces. Also, essentially all the ions are absorbed, leaving only 1 to 5 mEq each of sodium and chloride ions to be lost in the feces.

Most of the absorption in the large intestine occurs in the proximal one half of the colon, giving this portion the name absorbing colon, whereas the distal colon functions principally for feces storage until a propitious time for feces excretion and is therefore called the storage colon.