Vitamins and Minerals

The Digestive System

Food can be used by the body only after it has been broken down into small molecules. The process by which food is changed into a form the body can use is digestion. Digestion takes place in a continous tubelike passageway that extends from the mouth to the anus. This passageway is known as the alimentary canal, or digestive tract. The alimentary canal and other organs associated with digestion make up the digestive system.

The digestive system serves two major functions. The first, of course, is digestion – the breaking down of food into molecules the body can use. The nutrient molecules must then get to the cells where they are needed, Therefore, the second major function of the digestive system is absorption. Absorption is the movement of the nutrient molecules into the blood vessels or other vessels. The blood carries these nutrients to the cells, which use the nutrients for energy, growth, and repair.

Digestion takes two forms – mechanical and chemical. Mechanical digestion is the physical tearing and grinding of food into smaller pieces. Mechanical digestion thus increases the amount of surface area of food exposed to the action of digestive enzymes. These enzymes help bring about the second form of digestion. Chemical digestion changes food particles into molecules the body can use.

The Mouth

Mechanical and chemical digestion both start in the mouth, or oral cavity. Food is bitten, cut, and torn by the incisors, the sharp teeth at the front of the mouth, and the teeth next to them, the canines. Strong muscles of the jaws and tongue move the food into position for chewing. Food is then crushed and ground by the broad, flat surfaces of the premolars, flat surfaces of the premolars and the molars at the rear of the mouth.

While in the mouth, food is moistened by saliva, a mixture of mucus and digestive enzyme called ptyalin, or salivary amylase. Saliva is produced by the three pairs of salivary glands. The largest of these are the parotid glands located in the cheek region. The sublingual glands are in the floor of the mouth under the tip of the tongue, and the submaxillary glands are along the lower jaw.

Saliva lubricates food so that it moves smoothly through the digestive tract. Saliva may also kill some bacteria in the mouth. Ptylian starts the breakdown of starches to glucose. However, because food remains in the mouth for such a short time, ptylian acts on less than 5 percent of the starches. The food and saliva eventually form a moist, soft ball called a bolus.

When you swallow, your tongue presses against the hard palate, the bony plate in the roof of the mouth. The pressure forces the bolus to muscle tissue called the soft palate.

The Pharynx and Esophagus

From the soft palate area, the bolus moves into the pharynx, a commom passageway for food and air. As the bolus is forced to the back of the mouth, the soft palate moves up and closes off the nasal cavities. At the same time, a flap of tissue called the epiglotis seals off the trachea, or wind-pipe. The larynx, or voice box, at the bottom of the epiglottis. Food can then pass quickly through the pharynx, across the trachea, and into the stomach. If a person attempts to breathe while swallowing, food gets into the trachea.

Until food enters the pharynx, voluntary muscles control the process of mechanical digestion. Once a swallow has started, however, it cannot be stopped because involuntary muscles take over the pharynx. A strong contraction of a muscle around the pharynx propels food into the esophagus. This contraction starts a wavelike motion called peristalsis, which moves food along. Peristalsis results from the action of the two layers of involuntary muscles that form the walls of most of the digestive tract. One layer of muscles wraps around the tract, and the second layer runs along its length. While the circular muscles squeeze, the parallel ones relax. The squeezing and contraction of the two sets of muscles always occurs above the bolus or liquid in the digestive tract.

The Stomach

At the end of the esophagus is a muscular valve called the cardiac sphincter. A sphinkter is a muscle that controls a circular opening of the body. This valve prevents food from reentering the esophagus. Food passes through the valve into the stomach, a J-shaped, baglike organ with a capacity of 2 or 4 L (2.1 to 4.2 qt.). Both mechanical and chemical digestion continue in the stomach. In addition to the layers of involuntary muscle, the stomach has a third, diagonal layer of muscle. Through the action of these three muscle layersa, the stomach can actually grind food.

Chemical digestion of protein begins in the stomach. The stomach contains about 35 million glands that produce mucus and gastric secretions. The chief gastric secretions produced by the stomach are hydrochloric acid and an enzyme called pepsin. Pepsin is active only in a highly acidic environment. This enzyme splits protein into smaller groups of amino acids called polypeptides. Hydrochloric acid also dissolves minerals and kills bacteria. There is a muscular valve that controls the passage of food out of the stomach. This valve is known as the pyloric sphincter.

What prevents the stomach from digesting itself? Mucussecreting cells line the surface of the stomach. The mucus helps protect the stomach lining hydrochloric acid. About 500,000 cells of the stomach lining are shed every minute. As a result of this process, the cell layer replaces itself every three days. Occasionally too little mucus or too much acid exists in the stomach. An open, painful sore called an ulcer may than form in the stomach lining. Ulcers may sometimes bleed severely. They can also eat completely through the stomach wall, leading to much more serious conditions.

The Small Intestine

Food leaves the stomach as a semifluid mass called chyme. Chyme enters the small intestine, a tube about 7 m (23.1 ft.) long and 3 cm (1.2 in.) in diameter.

The small intestine has three sections. The uppermost section, the duodenum is about 25 cm (1.2 ft.) long. The next section, the jejunum,is about 4m (13.1 ft.) long. The last 2.5 m (8.2 ft.) form the ileum. The majority of chemical digestion and absorption takes place in the small intestine.

The Pancreas and the Liver

The pancreas and the liver secrete digestive juices into the small intestine and so play important roles in digestion. The pancreas, located behind the stomach, has many small lobes that secrete enzymes and sodium bicarbonate. The sodium bicarbonate neutralizes the acidity of the chyme leaving the stomach. The chief enzymes secreted by the pancreas are pancreatic amylase, pancreatic lipase, trypsin, and chymotrypsin. Pancreatic amylase continues the chemical digestion of starch that began in the mouth. It converts starches into maltose. Pancreatic lipase breaks down fats into their component molecules, fatty acids and glycerol. Trypsin and chymotrypsin break down the proteins by splitting them into smaller chains of amino acids called peptides.

The liver is the largest internal organ, weighing about 1.5 kg. It produces bile, which is a salt solution, not an enzyme. Bile emulsifies fat – that is, it breaks down large fat globules into tiny droplets. This process greatly increases the surface area of fat particles. Lipase can than act on the fat more effectively. This process is referred to as the detergent effect of bile, because detergent does the same thing to fat. Bile generally enters the duodenum from the gall bladder, a small sac where bile is stored.

Digestion in the Small Intestine

Most chemical digestion occurs in the duodenum. A heavy layer of mucus protects the first few centimeters of the duodenum from the acidic chyme released by the stomach. If the mucus protection is not sufficient, the high acid level can cause duodenal ulcers, which are even more common than stomach ulcers.

Enzymes produced by the small intestine include peptidases, maltase, lactase, sucrase, and intestinal lipase. Various peptidases break down peptides into amino acids. Maltase, lactase, and sucrase convert disaccharides into monosaccharides. Intestinal lipase, like pancreatic lipase splits fats into fatty acids and glycerol.

Absorption in the Small Intestine

Amino acids, monosacharides, fatty acids, glycerol, water, and minerals are all absorbed in the small intestine. Absorption occurs quickly in the small intestine because of its lining. The mucuous lining consists of folds covered with millions of tiny projections called villi. Each villus has a brush border composed of approximately 600 microvilli, which are extensions of the epithelial tissue covering the villi. Intestinal enzymes are not released into the cavity of the small intestine. Instead, the enzymes remain in the brush border where they act upon the food molecules. The molecules are then absorbed into the bloodstream through the microvilli walls. The lining folds, the villi, and the microvilli together increase the surface are of the small intestine 600 times. The result is an absorptive are equal in size to the area of a tennis court.

Absorption occurs through the processes of diffusion and active transport. Each villus contains tiny blood vessels called capillaries, through which monosaccharide and amino acid molecules enter the bloodstream. The blood carries these nutrient to the body’s tissues. The liver converts excess glucose into glycogen, a form of starch, and stores it as a future energy source. The cells of the villi resynthesize fatty acids and glycerol into the fats. The villi contain tiny vessels called lacteals, which absorb the fats. These fats eventually pass from the lacteals into the bloodstream.

The large Intestine

Minerals, water, and undigested foods enter the last part of the digestive tract, called the large intestine. The large intestine is also known as the colon. Absorption of water, minerals, and vitamins is completed in the large intestine.

Approximately 9 L of water pass through the digestive tract in one day. Some of the water is transported by osmosis into capillaries lining the walls of the large intestine. The solid waste material that remains is called feces. It stays in the body until it is eliminated through the anus. The last part of the large intestine long enough for the water to be absorbed. Constipation is the opposite condition in which the feces do not remain in the large intestine long enough for the water to be absorbed. Constipation results when the feces remain in the colon too long. As a result too much water is absorbed. Diarrhea may be caused by bacteria or viruses, emotional stress, or eating certain foods. Prolonged diarrhea can result in dehydration and even death. Constipation may be caused by insufficient fiber and water in the diet. It results in hard, dry feces that can make defecation, or elimination of fecesm painful.

Near the beginning of the large intestine is a small, finger-like projection called the appendix. The appendix is a blind, saclike stucture. The appendix has generally been considered a useless stucture in humans. However, some scientists now believe it helps produce antibodies. Sometimes the appendix becomes infected and must be immediately removed. This painful condition is called appendicitis.

Date: 2014-12-22; view: 1288