The Digestive System

Digestion is the breakdown and conversion of food into materials that can be absorbed and assimilated into the body.

As food passes through the digestive tract, it is broken down mechanically and chemically into smaller and smaller particles. The food is acted upon by digestive secretions from the salivary glands, stomach, liver, pancreas, and small intestine.

Unabsorbed residues pass through the small intestine into the large intestine, where they are stored for variable periods before being eliminated from the body. Most of the digestive process is controlled by a complex series of reflexes and by hormones produced in various parts of the gastrointestinal tract.

The chemical phase of digestion involves a series of hydrolytic reactions (all catalyzed by specific enzymes) in which the nutrient molecules are split with water. Water is an important part of the digestive process. It acts as a solvent as well as being one of the main reactants. As a result of digestion, proteins are broken down into amino acids, carbohydrates are broken down into glucose, and fats into fatty acids and glycerol. These comparatively simple substances are used by the body for energy, for replacement of cells, and for growth.

Image credits: p d

Digestion begins with chewing, which softens the food and exposes it to the digestive enzyme produced by the salivary glands. The food is also mixed with saliva, which increases the water content and facilitates transport of the food mass through the esophagus.

Saliva is produced by three pairs of salivary glands: the parotid glands, which lie in front and below the ears; the submaxillary glands, which are under the jaw; and the sublingual glands, which lie under the forepart of the floor of the mouth.

A watery secretion is produced by the parotid glands and a mucous secretion by the submaxillary and sublingual glands. The total amount of saliva produced daily in man ranges from 1,000 to 1,500ml (30-45 oz).

The most important digestive component of saliva is the enzyme amylase. Salivary amylase, or ptyalin, breaks down starch and glycogen molecules, first to dextrins, which are complex poly-saccharides, and finally to a mixture of maltose and glucose. Saliva also flushes the mouth and makes possible the tasting of food.

Food that is mixed with saliva and chewed becomes a bolus (a soft ball of food) ready to be swallowed. The food is moved backward in the mouth by the tongue toward the pharynx (the back of the throat). As soon as the bolus of food reaches the back wall of the pharynx, neuromuscular reflexes cause the muscles of the pharynx to contract; the contraction propels the food bolus into the esophagus.

During the process of swallowing, breathing stops because a flap of tissue- the epiglottis-covers the upper opening of the windpipe, or trachea, and thus prevents food from entering the lungs.

The esophagus is a simple tubular structure consisting of two layers of muscular tissue and extending from the pharynx to the stomach. The upper third of the esophagus has striated muscle similar to the voluntary muscles of the extremities; the lower portion consists of smooth muscle. Both ends of the esophagus are closed by sphincter muscles (rings of muscle). The upper esophageal sphincter is located at the level of the larynx, while the lower sphincter is at the junction of the esophagus and the stomach.

Food is propelled down the esophagus by a wave of contraction. Passage through the esophagus takes approximately 8 seconds. Early in the act of swallowing, the sphincter between the esophagus and the stomach relaxes so that the bolus of food can pass from the esophagus into the stomach. After the food enters the stomach, the sphincter closes, thereby preventing the food from reentering the esophagus.

The stomach is a muscular J-shaped organ that is connected to the esophagus and the small intestine. It is separated from the small intestine by the pyloric sphincter muscle.

The stomach has both motor and secretory functions. The upper portion of the stomach serves as a storage organ, permitting further digestion of carbohydrates by salivary amylase. The motor functions of the stomach include division of the food into smaller particles and propulsion of the contents from the stomach into the small intestine. Little motor activity occurs in the upper part of the stomach. However, in the lower half the contractions exert powerful pressures on the contents of the stomach, both kneading them and mixing them.

The lower, or antral, section of the stomach has a pumping mechanism that allows a small amount of the gastric contents to be discharged into the first portion of the small intestine, with each contraction. After consumption of a meal it takes several hours for the stomach to empty completely.

The tissues of the stomach lining secrete gastric juice, which consists of water, hydrochloric acid, and pepsin and other enzymes; it also contains mucus, which forms a protective layer over the cells of the gastric mucosa. The glands that secrete mucus are in the upper part of the stomach. The pyloric glands, which are in the lower part of the stomach, secrete a hormone called gastrin that stimulates gastric secretion. The fundic glands, which are found in the mid-portion of the stomach, consist of parietal cells, chief cells, and mucous neck cells. Parietal cells secrete hydrochloric acid; the chief cells produce pepsinogen, a precursor, or inactive form, of the digestive enzyme pepsin. Pepsinogen is activated into pepsin by hydrochloric acid. Pepsin attacks proteins and breaks them down into smaller units called peptides.

Gastric secretion is controlled by the central nervous system and by the hormone gastrin. Gastrin is a polypeptide that is secreted into the bloodstream, from which it is returned to the stomach and stimulates the secretion of acid. Gastrin formation is inhibited when the concentration of acid in the lower part of the stomach becomes sufficiently great. The secretion of gastric juice is inhibited by the hormone secretin, which is secreted by the cells of the lining of the small intestine.

The control of the central nervous system over the secretions of the stomach is through the vagus nerve (one of the cranial nerves originating in the brain). Secretion can be initiated by the thought of eating, but normally there is little or no secretion between meals. However, secretion is markedly increased in response to the entrance of food into the stomach. When the products of gastric digestion are in the proper form, they are discharged into the small intestine. Food in this stage of digestion is called chyme. Chyme is highly acid because of the presence of the hydrochloric acid of gastric juice.

The small intestine is the most important part of the digestive tract. Digestion is completed in the intestine and the nutrients are absorbed into the cells of the intestinal lining; from these cells the nutrients are absorbed into the bloodstream and lymphatic system and carried to all the cells of the body.

The small intestine is divided into three parts. The uppermost section, the duodenum, is the shortest and widest part of the small intestine and is from 20 to 25 cm (8 to 10 inches) long. The jejunum, or middle section, and the ileum, or terminal section, have a combined length of from 7 to 9 meters (21 to 27 feet). There are both anatomic differences and physiologic differences between the various parts of the small intestine.

The inner surface of the small intestine, particularly in the duodenum and upper jejunum, is covered with fingerlike projections called villi, which greatly increase the absorptive surface of the intestinal lining. There are about 5 million villi in the intestine of man. The villi are covered by a layer of epithelial cells. The border of these cells resembles a brush and is made up of hundreds of microvilli. These structures also increase the absorptive surface of the small intestine.

In the depressions between the villi are the openings of small glands called the crypts of Lieberkuhn. These structures are the site of much mitotic activity, and it is thought that they produce new cells to replace those that are worn off or shed from the villi. The duodenum also contains glandular structures called Brunner's glands, which secrete mucus. These glands are located around the area where the pancreatic juice and bile enter the duodenum, and the large amount of mucus they produce protects the intestinal wall from the action of the digestive juices.

The duodenum is the site for the mixing of food from the stomach with secretions from the liver and pancreas. Both the duodenum and jejunum are characterized by circular folds of tissue that extend into the intestinal lumen. These folds, which are known as valvulae conniventes, are covered with villi. In addition to increasing the surface area of the intestine these structures form a transverse barrier that slows down the passage of food through the small intestine.

The jejunum is the main site for the homogenization and mixing of the intestinal contents and the formation of intestinal enzymes. It is also an important site for absorption of nutrients, including sugars, amino acids, and fats.

The ileum is the site of absorption of vitamin 612, bile acids, and remaining portions of amino acids and fats. Both the jejunum and ileum are responsible for the absorption of water and electrolytes.

The hormones secretin and pancreozymin, both of which stimulate pancreatic secretion, are produced in the small intestine, along with Cholecystokinin, which stimulates the gallbladder. In addition to the hormones, the intestinal wall is the site of formation and storage of histamine and serotonin. Histamine is produced in the mast cells of the gastrointestinal tract and may play a role in gastric secretion. Serotonin comes from the argentaffine cells. It stimulates intestinal motility, and it may be a regulator of the transit of food through the intestine.

The cells lining the small intestine contain the intestinal enzymes. Some of the cells are shed into the intestinal lumen, or cavity, and their enzymes become mixed with the contents of the intestine, thereby adding to the digestive capacities of the intestinal juice. Some digestion, particularly of carbohydrates, occurs with the cells of the intestinal lining after the nutrients have been absorbed from the lumen. The intra-cellular enzymes include peptidases, lipases, and disaccharidases. The peptidases are responsible for splitting of peptides into absorbable amino acids; the lipases attack long-chain fatty acids; and the disaccharidases act on various types of sugars.

The motor activities of the small intestine mix the food and facilitate action by the digestive enzymes; slow the downward movement of chyme; and facilitate intestinal absorption. The most characteristic form of motor activity of the small intestine is that of segmentation, in which adjacent segments alternately contract and relax, thereby kneading and mixing the chyme. The movement of chyme through the small intestine is very slow; it takes about 4 hours for a meal to pass through.

The pancreas secretes important enzymes and electrolytes into the small intestine. It is connected to the duodenum by the main pancreatic duct; a sphincter muscle separates the two parts. Pancreatic secretion is controlled primarily by hormonal stimulation. The two hormones that effect the pancreas are secretin and pancreozymin, which are secreted by the small intestine. Secretin is a polypeptide formed in the tissues of the duodenal lining when the acid contents of the stomach enter the small intestine. The hormone is then absorbed into the bloodstream and carried to the pancreas, where it stimulates the pancreas to produce a watery juice that is high in bicarbonate but low in enzyme content. The high content of bicarbonate is of great importance for neutralizing the acidity of the gastric juice in the food from the stomach. Pancreozymin, another hormone formed in the duodenum, stimulates the release of enzymes from the pancreas. Pancreatic secretion is also stimulated by the passage through the duodenum of the breakdown products of protein. Gastrin also increases the output of enzymes from the pancreas. The average daily production of pancreatic juice is about 2,000 ml (60 oz).

Pancreatic enzymes break down proteins, fats, and carbohydrates. The pancreas secretes trypsinogen and chymotrypsinogen, which are the inactive forms of the enzymes, trypsin and chymotrypsin. Trypsinogen is activated to trypsin in the small intestine by the action of the enzyme enterokinase, which is produced by the small intestine. The trypsin itself then acts as a catalyst and activates both more trypsinogen and also chymotrypsinogen. Both of these enzymes split proteins into small units. There are other pancreatic enzymes, such as elastase and procar-boxypeptidase, that are activated in the small intestine and act to break down peptides and similar groups to amino acids. Pancreatic lipase breaks down fats. Lipase is produced in a relatively inactive form and is activated in the small intestine by bile acids and salts. The breakdown products of fat include diglycerides and mono-glycerides and a small amount of glycerol and fatty acids. Pancreatic amylase, or amylopsin, acts on starch, glycogen, and dextrins, splitting them into sugars.

Bile is formed by the cells of the liver and flows through a duct into the gallbladder, where it is concentrated five or ten times and stored. The hepatic ducts together with the cystic ducts from the gallbladder form the common bile duct, which opens into the duodenum. In the absence of the gallbladder, bile is discharged continuously from the liver into the gastrointestinal tract. However, in the presence of an intact gallbladder, emptying occurs only after eating. The gallbladder is stimulated to contract by cholecystokinin, a hormone formed in the wall of the duodenum in response to the presence of fats.

Bile is a complex solution containing about 3% solids and 97% water (before it is concentrated). The yellow-green color of bile is the result of its content of bilirubin, which is a breakdown product of red blood cells.

The major digestive components of the bile are the bile salts. The bile salts include salts of four bile acids, of which glycocholic acid is the most important in man. The physiologic functions of bile acids include activation of lipases; emulsification of fats by decreasing their surface tension; promotion of absorption of fatty acids; stimulation of bile formation and flow; and stimulation of motor activity of the small intestine.

The bile salts are secreted with the bile into the small intestine where they combine with fats to form micelles, which are absorbed in the lower part of the small intestine. After absorption, bile salts are released from the cells of the intestinal lining and circulate in the blood; they are taken up in the liver, which again secretes them into the bile.

Absorption of the breakdown products of digestion is the prime function of the small intestine. Absorption may be divided into three stages: transport of material from the lumen of the intestine into the cells of the intestinal lining; metabolism of the nutrients within the cells; and removal of the material from the intestinal cells into the bloodstream or lymphatic system.

There are marked differences in the rate of absorption of various substances from the intestinal lumen. Water, glucose, amino acids, and certain fats are absorbed in large quantities, while only small amounts of iron and vitamin Bi2 are absorbed.

In man about 8.5 liters (8.9 quarts) of fluid enter the gastrointestinal tract per day. Of this about 1.5 liters (1.57 quarts) are taken in through the mouth, while the rest is secreted into the tract from other parts of the body. Since only about 150 ml of water are lost in the solid waste daily, more than 8 liters (8.4 quarts) of fluid are absorbed from the gastrointestinal tract.

There are two types of transport across the membrane of the intestinal lining. The first is by simple diffusion, or passive transport. In this case transport of material across the membrane is based upon the physical-chemical processes of diffusion and osmosis and does not proceed against electrochemical gradients and does not utilize energy. Active transport, on the other hand, proceeds against an electrical or chemical gradient and requires the expenditure of energy.

Water, chloride, ascorbic acid, pyridoxine, and riboflavin move mostly or exclusively by passive transport from the lumen of the intestine into the cells of the intestinal lining. On the other hand, most nutrients, including sugars, fats, amino acids, bile acids, vitamin B11, thiamin, and calcium, are absorbed by an active transport mechanism.

Active transport may occur through the use of a "membrane carrier system," in which the molecules of nutrient combine with a "carrier." The carrier then reacts with the cell membrane at special reactive sites, thereby enabling the nutrient-carrier complex to enter the cell.

Different sugars are absorbed at different rates, with glucose and galactose being absorbed most rapidly. These sugars are actively absorbed through the cell membranes, particularly in the upper portion of the small intestine. Intact proteins are not absorbed from the intestinal lumen but instead are broken down to their constituent amino acids, which are then absorbed. The protein found in the small intestine is from the food in the diet and endogenous protein, which is derived from the enzymes of digestive secretions and from the tissue shed from the lining of the gastrointestinal tract. The dietary intake of protein is about 50 grams a day, while endogenous protein is at least three times as great. The absorption of amino acids is also thought to depend on a carrier mechanism.

Fats are emulsified in the intestinal lumen. The more complex fats are hydrolyzed by pancreatic lipase to their component parts. These substances, along with fatty acids, form fat micelles. Because the micelles are fat soluble, the fatty acids penetrate the lipid-containing cellular membrane of the intestinal absorbing cells. The fats are resynthesized within the cells of the intestinal lining. Fats leave the cells in the form of chylomicrons, which are small fat droplets in conjunction with phospholipids and proteins.

The large intestine does not have a digestive function; it serves to store undigested food residues and to convert the liquid contents from the small intestine into a solid mass. In the process, between 250 and 350 ml (7.5-10.5 oz) of water are absorbed daily. There is also an exchange of sodium chloride for bicarbonates and a loss of potassium into the lumenal contents.