Oral Cavity Both physical and chemical digestion begin in the mouth or oral cavity, which is the point of entry of food into the digestive system. The food is broken into smaller particles by mastication, the chewing action of the teeth.
All mammals have teeth and can chew their food to begin the process of physically breaking it down into smaller particles. The chemical process of digestion begins during chewing as food mixes with saliva, produced by the salivary glands Figure Saliva contains mucus that moistens food and buffers the pH of the food. Saliva also contains lysozyme, which has antibacterial action. It also contains an enzyme called salivary amylase that begins the process of converting starches in the food into a disaccharide called maltose.
Another enzyme called lipase is produced by cells in the tongue to break down fats. The chewing and wetting action provided by the teeth and saliva prepare the food into a mass called the bolus for swallowing. The tongue helps in swallowing—moving the bolus from the mouth into the pharynx.
The pharynx opens to two passageways: the esophagus and the trachea. The esophagus leads to the stomach and the trachea leads to the lungs.
The epiglottis is a flap of tissue that covers the tracheal opening during swallowing to prevent food from entering the lungs. Esophagus The esophagus is a tubular organ that connects the mouth to the stomach. The chewed and softened food passes through the esophagus after being swallowed.
The smooth muscles of the esophagus undergo peristalsis that pushes the food toward the stomach. The peristaltic wave is unidirectional—it moves food from the mouth to the stomach, and reverse movement is not possible, except in the case of the vomit reflex. The peristaltic movement of the esophagus is an involuntary reflex; it takes place in response to the act of swallowing. Ring-like muscles called sphincters form valves in the digestive system. The gastro-esophageal sphincter or cardiac sphincter is located at the stomach end of the esophagus.
In response to swallowing and the pressure exerted by the bolus of food, this sphincter opens, and the bolus enters the stomach. When there is no swallowing action, this sphincter is shut and prevents the contents of the stomach from traveling up the esophagus. A large part of protein digestion occurs in the stomach Figure The stomach is a saclike organ that secretes gastric digestive juices. Protein digestion is carried out by an enzyme called pepsin in the stomach chamber.
The highly acidic environment kills many microorganisms in the food and, combined with the action of the enzyme pepsin, results in the catabolism of protein in the food. Chemical digestion is facilitated by the churning action of the stomach caused by contraction and relaxation of smooth muscles. The partially digested food and gastric juice mixture is called chyme. Gastric emptying occurs within two to six hours after a meal. Only a small amount of chyme is released into the small intestine at a time.
The movement of chyme from the stomach into the small intestine is regulated by hormones, stomach distension and muscular reflexes that influence the pyloric sphincter. The stomach lining is unaffected by pepsin and the acidity because pepsin is released in an inactive form and the stomach has a thick mucus lining that protects the underlying tissue. Chyme moves from the stomach to the small intestine. The small intestine is the organ where the digestion of protein, fats, and carbohydrates is completed.
The small intestine is a long tube-like organ with a highly folded surface containing finger-like projections called the villi. The top surface of each villus has many microscopic projections called microvilli. The epithelial cells of these structures absorb nutrients from the digested food and release them to the bloodstream on the other side.
The villi and microvilli, with their many folds, increase the surface area of the small intestine and increase absorption efficiency of the nutrients. The human small intestine is over 6 m The duodenum is separated from the stomach by the pyloric sphincter. The chyme is mixed with pancreatic juices, an alkaline solution rich in bicarbonate that neutralizes the acidity of chyme from the stomach. Pancreatic juices contain several digestive enzymes that break down starches, disaccharides, proteins, and fats.
Bile is produced in the liver and stored and concentrated in the gallbladder; it enters the duodenum through the bile duct. Bile contains bile salts, which make lipids accessible to the water-soluble enzymes.
The monosaccharides, amino acids, bile salts, vitamins, and other nutrients are absorbed by the cells of the intestinal lining. The undigested food is sent to the colon from the ileum via peristaltic movements.
The ileum ends and the large intestine begins at the ileocecal valve. The appendix of humans has a minor role in immunity. The large intestine reabsorbs the water from indigestible food material and processes the waste material Figure The human large intestine is much smaller in length compared to the small intestine but larger in diameter.
It has three parts: the cecum, the colon, and the rectum. The cecum joins the ileum to the colon and is the receiving pouch for the waste matter. The colon has four regions, the ascending colon, the transverse colon, the descending colon and the sigmoid colon. The main functions of the colon are to extract the water and mineral salts from undigested food, and to store waste material.
The rectum Figure The feces are propelled using peristaltic movements during elimination. The anus is an opening at the far-end of the digestive tract and is the exit point for the waste material. Two sphincters regulate the exit of feces, the inner sphincter is involuntary and the outer sphincter is voluntary. The organs discussed above are the organs of the digestive tract through which food passes. Accessory organs add secretions and enzymes that break down food into nutrients.
Accessory organs include the salivary glands, the liver, the pancreas, and the gall bladder. The secretions of the liver, pancreas, and gallbladder are regulated by hormones in response to food consumption. The liver is the largest internal organ in humans and it plays an important role in digestion of fats and detoxifying blood.
The liver produces bile, a digestive juice that is required for the breakdown of fats in the duodenum. The liver also processes the absorbed vitamins and fatty acids and synthesizes many plasma proteins.
The gallbladder is a small organ that aids the liver by storing bile and concentrating bile salts. The pancreas secretes bicarbonate that neutralizes the acidic chyme and a variety of enzymes for the digestion of protein and carbohydrates. Nutrition The following video is primarily about water soluble vitamins such as vitamin B and C their roles, especially in energy metabolism.
Some of the more common and obscure minerals found in vitamins are also identified. And the next video is an introduction to another category of vitamins, the fat soluble group such as vitamin E, D and K. The human diet should be well balanced to provide nutrients required for bodily function and the minerals and vitamins required for maintaining structure and regulation necessary for good health and reproductive capability Figure Explore this interactive United States Department of Agriculture website to learn more about each food group and the recommended daily amounts.
The organic molecules required for building cellular material and tissues must come from food. During digestion, digestible carbohydrates are ultimately broken down into glucose and used to provide energy within the cells of the body.
Complex carbohydrates, including polysaccharides, can be broken down into glucose through biochemical modification; however, humans do not produce the enzyme necessary to digest cellulose fiber. Then, a variety of proteases and peptidases in the intestine finish the job. Pepsin is an enzyme produced in the stomach that is essential for the digestive process. Its job is to break down proteins.
Because pepsin usually resides in the stomach, it is active at a low acidic pH. Pepsin works its best in strong hydrochloric acid. But the similarity with the other enzymes pictured here refers to a second type of acid. The active site of the acid proteases rely on two acidic aspartate amino acids, which activate a water molecule and use it to cleave protein chains. Pepsin top and pepsinogen bottom. During the holiday season, we often place greater demands on our digestive enzymes than at other times of the year.
Our digestive system contains a host of tough, stable enzymes designed to seek out those rich holiday treats and break them into small pieces. Begin typing your search term above and press enter to search. Press ESC to cancel. Skip to content Home Popular articles What organ is pepsin active?
Popular articles. Esther Fleming August 10, Table of Contents. Previous Article How many letters can you have on a Wisconsin license plate? Back To Top.
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