* Digestion Journey in Humans, Mouth to Anus * Mechanical and Chemical Breakdown of Food * Nutrient Absorption
*Digestion Journey in Humans, Mouth to Anus 1. MOUTH The first stages of digestion take place in the mouth, where the teeth tear food into fragments (mechanical digestion) and amylase, an enzyme secreted by one of the salivary glands, begins the breakdown of carbohydrates (chemical digestion). 2. ESOPHAGUS This mixture of food and saliva is then pushed into the pharynx and esophagus. The esophagus is a muscular tube whose muscular contractions (peristalsis) propel food to the stomach.
3. STOMACH Gastric juice in the stomach contains agents such as hydrochloric acid and some enzymes, including pepsin, rennin, and traces of lipase. Pepsin breaks proteins into peptones and proteases. Rennin separates milk into liquid and solid portions; lipase acts on fat. Another function of stomach digestion is gradually to release materials into the upper small intestine, where digestion is completed. 4. SMALL INTESTINE Predigested material supplied by the stomach is subjected to the action of three powerful digestive
5. LARGE INTESTINE Cells lining the large intestine reabsorb water and salts fluids: pancreatic fluid, intestinal juice, and bile. These fluids neutralize the gastric acid, ending the gastric phase of digestion. This is where the final stages of chemical enzymatic digestion occur and where almost all nutrients are absorbed. from food material undigested by the small intestine. 6. RECTUM The rectum is a muscular tube about 5 in.(12.7 cm)
long that connects the large intestine to the anus. 7. ANUS The anus is the opening at the end of the large intestine that leads out of the body. Stool or bowel movements pass out of the body through the anus.
*Mechanical and Chemical Breakdown of Food Mechanical processes - include chewing to reduce food to small particles, the churning action of the stomach, and intestinal peristaltic action. Chemical processes Three chemical reactions take place: 1. conversion of carbohydrates into such simple sugars as glucose 2. breaking down of protein into such amino acids as alanine 3. conversion of fats into fatty acids and glycerol *These processes are accomplished by specific enzymes
*Nutrient Absorption CARBOHYDRATES These are broken into simpler molecules by enzymes in the saliva, in juice produced by the pancreas, and in the lining of the small intestine. Starch is digested in two steps: First, an enzyme in the saliva and pancreatic juice breaks the starch into molecules called maltose; then an enzyme in the lining of the small intestine (maltase) splits the maltose into glucose molecules that can be absorbed into the blood. Glucose is carried through the bloodstream to the liver, where it is stored or used to provide energy for the work of the body.
PROTEIN An enzyme in the juice of the stomach starts the digestion of swallowed protein. Further digestion of the protein is completed in the small intestine. Here, several enzymes from the pancreatic juice and the lining of the intestine carry out the breakdown of huge protein molecules into small molecules called amino acids. These small molecules can be absorbed from the hollow of the small intestine into the blood and then be carried to all parts of the body to build the walls and other parts of cells. FATS The first step in digestion of a fat is to dissolve it into the watery content of the intestinal cavity. The bile
acids produced by the liver act as natural detergents to dissolve fat in water and allow the enzymes to break the large fat molecules into smaller molecules, some of which are fatty acids and cholesterol. The bile acids combine with the fatty acids and cholesterol and help these molecules to move into the cells of the mucosa. In these cells the small molecules are formed back into large molecules, most of which pass into vessels (called lymphatics) near the intestine. These small vessels carry the reformed fat to the veins of the chest, and the blood carries the fat to storage depots in different parts of the body. VITAMINS Another vital part of our food that is absorbed from the small intestine is the class of chemicals we call vitamins
The two different types of vitamins are classified by the fluid in which they can be dissolved: water-soluble vitamins (all the B vitamins and vitamin C) and fat-soluble vitamins (vitamins A, D, and K). WATER AND SALT Most of the material absorbed from the cavity of the small intestine is water in which salt is dissolved. The salt and water come from the food and liquid we swallow and the juices secreted by the many digestive glands