Chapter VI Digestion and Absorption
The gastrointestinal system transfers organic nutrients, minerals, and water from the external environment to the internal environment.
Mechanical digestion and its regulation
Digestion can be divided into three phases, depending on where food stimuli are acting. The cephalic phase begins even before food ingestion, when the idea or stimuli arising from the presence of food are detected. The gastric phase of digestion begins as soon as food enters the stomach, and the intestinal phase begins as soon as food enters the intestine. The three phases therefore have considerable overlap in time. Stimuli arising from the physical and chemical presence of food in the mouth, the stomach, and the intestine regulate the movement of material through the gastrointestinal tract and the rate of secretion of enzymes. Neural reflexes are mediated by short reflexes in the enteric nervous system and by long reflexes involving afferent and efferent neurons to and from the central nervous system.
Chew and swallow Chewing breaks up food into smaller particles suitable for swallowing. Food moved into the pharynx by the tongue initiates swallowing, which is coordinated by the swallowing center in the medulla. Food is prevented from entering the trachea by inhibition of respiration and by closure of the glottis. The upper esophageal sphincter relaxes as food is moved into the esophagus and then closes. Food is moved through the esophagus toward the stomach by peristaltic waves. The lower esophageal sphincter remains open throughout swallowing. If food does not reach the stomach with the first peristaltic wave, distension of the esophagus will initiate secondary peristalsis.
Digestion of stomach The basic electrical rhythm generated by stomach smooth muscle determines peristaltic wave frequency. Contraction strength depends on the frequency of action potentials triggered by the slow waves. Peristaltic waves sweeping over the stomach become stronger in the antrum, where most mixing occurs. With each wave, only a small portion of the stomach contents are expelled into the small intestine through the pyloric sphincter. The larger a meal, the faster the stomach empties. Distension of the small intestine and fat, acid, or hypertonic solutions in the intestinal lumen inhibit gastric emptying.
Digestion of small intestine Intestinal motility is coordinated by the enteric nervous system and modified by long and short reflexes and hormones. During and shortly after a meal, the intestinal contents are mixed by segmenting movements of the intestinal wall. After most of the food has been digested and absorbed, segmentation is replaced by the migrating motility complex that sweeps the undigested material into the large intestine by a series of peristaltic waves. The muscular contractions that are responsible for mixing and moving the food from one region to another are the result of smooth muscle membrane depolarization and reflex pathways of the enteric neural plexuses.
Function of large intestine The primary function of the large intestine is to store and concentrate fecal matter prior to defecation. Three to four times a day, mass movements in the large intestine move its contents into the rectum. Distension of the rectum initiates defecation, which is assisted by a forced expiration against closed glottis. Defecation can be voluntarily controlled through somatic nerves to the skeletal muscles of the external anal sphincter.
Chemical digestion and absorption
The organization of the gastrointestinal system allows sequential processing of the food. In the mouth, food particles are reduced in size and mixed with saliva. In the stomach, the food is acidified, protein digestion begins, and food is stored until the chyme produced by mixing food with secretions is delivered to the small intestine. In the small intestine, the contents are neutralized by bicarbonate secretions, and digestion and absorption of proteins, carbohydrates, and lipids occur. The role of the large intestine is to store unabsorbed material.
Functions of digestive juices It is present in saliva, along with fluid and a-amylase. In the stomach, HCI and pepsin are secreted. In the small intestine, HCO3, and enzymes for the digestion of carbohydrates, proteins, and lipids are added from the pancreas. The bile secreted by the liver enters the duodenum along with the pancreatic secretions. Acid and bicarbonate secretion is the result of active transport and facilitated transport processes. Blood leaving the stomach is more alkaline than blood in the rest of the circulation, especially after a meal. Blood leaving the intestine and pancreas, the sites of bicarbonate secretion, is more acidic than arterial blood. Over time, the secretion of acid and base tend to cancel one another. Bile salts produced in the liver and released with bile into the duodenum are essential for fat digestion and absorption because of their emulsifying effect.
Most proteolytic enzymes, including trypsin, are secreted by the pancreas in inactive forms. Trypsin is activated by enterokinase located on the luminal membranes of the small intestine and in turn activates the other inactive enzymes. The hormone secretin, released from the small intestine in response to increased luminal acidity, stimulates bicarbonate secretion, while CCK released from the small intestine in response to the products of fat and protein digestion stimulates pancreatic enzyme secretion. The major ingredients in bile are bile salts, cholesterol, lecithin, bicarbonate ions, and bile pigments. Small amounts of salt, bicarbonate, and water are secreted by the small intestine.
Site of absorption The area available for absorption in the small intestine is greatly increased by the folding of the intestinal wall, the presence of villi, and the microvilli on the surface of the epithelial cells. New cells arising by mitosis continuously replace the epithelial cells lining the intestinal tract. The venous blood from the small intestine, containing absorbed nutrients other than fats, passes to the liver via the hepatic portal vein before returning to the heart. Fat is absorbed into the lymphatic vessels (lacteals) in the middle of each villus.
Mechanisms of absorption The movement of water, amino acids, and sugars into the body from the alimentary canal depends largely on the movement of Na+.Na+ diffuse from the lumen into the epithelial cells in response to a gradient created by active transport ofNa+ out of the cells into the interstitial fluid. As Na+ moves out of the lumen, water follows by osmosis. The gradient for Na+ also drives the carrier-mediated transport of most amino acids and sugars into the epithelial cells. The carriers recognize particular classes of sugars or amino acids.
Carbohydrate Starch is digested by amylases secreted by the salivary glands and pancreas, and the resulting products, as well as ingested disaccharide, are digested to monosaccharides by enzymes in the luminal membranes of epithelial cells in the small intestine. The monosaccharides are absorbed by secondary active transport. Some polysaccharides, such as cellulose, cannot be digested and therefore pass to the large intestine, where they are metabolized by bacteria.
Protein Proteins are broken down into small peptides and amino acids, which are absorbed by secondary active transport in the small intestine. The breakdown step to peptides is catalyzed by pepsin in the stomach and by the pancreatic enzyme trypsin and chymotrypsin in the small intestine. In the small intestine, peptides are broken down into amino acids by reactions catalyzed by pancreatic carboxypeptidase and intestinal aminopetidase.
Fat The digestion and absorption of fat by the small intestine requires mechanisms that solubilize the fat and its digestion products. Large fat globules leaving the stomach are emulsified in the small intestine by bile salts secreted by the liver. Lipase from the pancreas digests fat at the surface of the emulsion droplets forming fatty acids and monoglycerides. These water-insoluble products of lipase action, when combined with bile salts form micelles that are in equilibrium with the free molecules. Free fatty acids and monoglycerides diffuse across the luminal membranes of epithelial cells, within which they are enzymatically recombined to form triacylglycerol, which is released as chylomicrons from the blood side of the cell by exocytosis. The released chylomicrons enter lacteals in the intestinal villi and pass, by way of the lymphatic system, to the cardiovascular system.
Vitamins Fat-soluble vitamins are absorbed by the same pathway used for fat absorption. Most water-soluble vitamins are absorbed in the small intestine by diffusion or carrier-mediated transport. Vitamin B12 is absorbed in the ileum after combining with intrinsic factor secreted by the stomach. Water is absorbed from the small intestine by osmosis following the active absorption of solutes, primarily sodium chloride.
Iron Iron absorption is regulated by the secretion of the carrier molecule, transferrin, into the lumen. Most iron in the body is recycled between old red blood cells and the cells in the bone marrow that produce new red blood cells. The liver remove iron from heme and the iron are released into the blood, where it is carried by transferrin to the bone marrow.
