Food and digestion

The digestive system is a biological mechanism that dismantles foods into their chemical components some destined to form muscle, bone, blood, skin, or other tissue; some producing energy to power the processes of life.

The main part of the digestive system is the gut, or alimentary canal, a convoluted muscular tube measuring about 30 feet (9 meters) if extended, with an opening at each end. One opening, the mouth, admits unprocessed food. The other opening, the anus, releases food wastes. Between these two lie specialized organs like the teeth, stomach, and pancreas. These break down proteins, fats, and carbohydrates into molecules small enough to filter from the gut into the blood supply. In this way, nutrients are absorbed into the body.

The process is principally one of chemical reactions speeded up by biological catalysts called enzymes. During digestion, starches and complex sugars break down into simple sugars. Fats become fatty acids and glycerol. Proteins break down into amino acids. But water, minerals, and vitamins enter body tissues undigested.

The digestive system begins with the lips, teeth, mouth, and tongue, which form a chamber in which food is moistened and crushed before it starts its passage through the stomach and intestines. Most absorption occurs in the long, narrow, “small” intestine.

Inside the mouth

The first stages of digestion take place inside the mouth. Each mouthful of solid food is cut up and crushed by the teeth. An adult normally has 32 teeth of four kinds, designed for different purposes. Two sets of four somewhat chisel-shaped incisors at the front slice food with a scissorlike action. Four strong canines flanking the incisors have a pointed chewing surface and help to tear up large chunks of food into smaller pieces. Next come eight premolars, each with two cusps (pointed chewing surfaces) that crush food into still smaller pieces. The twelve molars at the back of the jaws are strong, broad-crowned teeth that grind food into small particles.

While teeth are masticating food, the tongue and cheek muscles push the food around so that all of it is subject to the chopping, grinding process. Reflex action ensures that the tongue itself does not get trapped between the teeth and bitten.

Meanwhile, saliva is entering the mouth cavity from three glands on each side of the face. These are the sublingual gland below the tongue, the submandibular gland below and behind the sublingual gland, and the parotid gland in front of and below the ear. Saliva is rich in ptyalin (salivary amylase), an enzyme that starts the conversion of starches into simple sugars. The lubricating, mixing action of saliva helps to shape each mouthful of food into a ball (bolus) that can be swallowed easily and will not stick in the throat.

Swallowing involves coordinated reflexes that stop food from entering the nasal cavity or windpipe. As the tongue throws the bolus back into the pharynx, the soft palate automatically rises to protect the nasal cavity, and the epiglottis a flap of cartilage and membrane-helps to shut the windpipe. Meanwhile, the top of the esophagus, or gullet, relaxes and receives the bolus, which now starts its journey through the gut.

An X ray of the stomach taken after a radiopaque substance—called a “barium meal”—has been swallowed shows most of the barium in the fundus of the stomach. Some remains in the esophagus, however, and some is beginning to move through the rest of the stomach as a result of the stomach’s muscular action.

Inside esophagus and stomach

The esophagus is a short length of tube between the lower pharynx (throat) and the stomach. Like other sections of the gut, its inner surface is lubricated by mucus. The tube’s thin walls consist of both skeletal and smooth muscle, which contract in waves (peristalsis) that move down through the esophagus, each wave taking about ten seconds from top to bottom.

The lower end of the esophagus is usually kept closed by a ring of muscle called the cardiac sphincter and by external pressure. The sphincter opens to release food into the stomach, then shuts again, preventing food from escaping back up through the esophagus.

The stomach is an enlarged section of the gut shaped rather like a letter J and closed by sphincters at the top and bottom. Three muscle layers, each with fibers aligned in a different direction, and a mucous membrane form the stomach walls.

The stomach serves partly as a place to store swallowed food before this passes on into the small intestine. As the stomach fills, its elastic walls relax and allow it to expand.

The stomach also acts to mix the swallowed food. Its muscles contract in a coordinated fashion that sends waves sweeping through the stomach, churning up its contents. This mixes the food with gastric juices secreted by glands and cells in pits in the mucous lining of the stomach wall.
Each day, the stomach yields about 6 pints (3 liters) of secretions. Among these is the stomach’s chief digestive enzyme, pepsin, which starts to break down proteins. Pepsin needs acid if it is to work, and the stomach obliges by manufacturing hydrochloric acid. This also helps to kill bacteria that might otherwise cause intestinal infections. Stomach acid is strong enough to burn skin, but the stomach’s walls are protected by mucous coating.

Food is mixed with and partly broken down by gastric juices to form a homogeneous mixture known as chyme. From this mixture, water, glucose, salts, and alcohol can pass directly to the bloodstream.

Every minute, however, about one per cent of the stomach’s contents spurts out through the pyloric sphincter, the stomach’s lower opening, into the small intestine, where the last all-important stages of digestion and absorption occur.

Digestion is accomplished by the actions of digestive juices, which are secreted by glands into different parts of the alimentary canal. The table shows some of the main glands and the enzymes they produce, in the left column; the part of the alimentary canal and the medium in which they act, in the center column; and their effect, in terms of the main chemicals they act on and produce, in the right column.

The intestines

Below the stomach food passes into the convoluted intestines: the long, narrow small intestine and the shorter, broader large intestine. The small intestine is supplied with juices that help complete the breakdown and absorption of most proteins, fats, and carbohydrates. The large intestine turns undigested wastes into feces for expulsion from the body.

The ileum, where most absorption of nutrients occurs, is a narrow tube formed by layers of smooth (involuntary) muscle around numerous fingerlike villi projecting into the lumen of the tube. These villi give the ileum an enormous surface area and so maximize the quantity of dissolved nutrients that can be absorbed through the epithelial cells. Amino acids, glucose, and some fatty acids enter the blood capillaries and are carried through the hepatic portal vein to the liver. Other fats pass into the lymphatic system via the lacteals, and ultimately return to the circulation and reach the liver by this route. Digestive enzymes are produced by the enzyme-secreting cells at the base of the villi. Mucus-secreting cells on the villi help lubricate the ileum’s contents.

The small intestine

This longest section of the gut forms closely-packed loops through which waves of muscular contraction force chyme food mixed with gastric juices. Anatomists divide the small intestine into three sections: duodenum, jejunum, and ileum.

The duodenum, a curved, short length of tubing, leads from the stomach and receives ducts from the pancreas and liver. This makes the duodenum chemically very active, for digestive pancreatic juice flows in from the pancreas, while the liver yields bile, a digestive juice made up of salts and pigments produced by chemical breakdown. Much of the bile reaches the small intestine via a storage depot, the gall bladder. The flow of bile and pancreatic juice varies with hormone output that is stimulated by chyme arriving in the duodenum from the stomach.

The alkaline pancreatic juice and bile counteract gastric acid from the stomach and make the duodenum strongly alkaline, which helps to activate its digestive enzymes and neutralize digestive juices from the stomach. When duodenal ulcers form, this happens at the duodenum’s upper, stomach end, where unneutrai-ized acid chyme makes contact with the duodenum wall.
Inside the duodenum, carbohydrates, fats, and proteins are broken down. Bile salts and the churning action of the duodenal walls break large fat droplets into smaller ones. These offer a large surface area to be attacked by lipase a fat-splitting enzyme that indirectly helps bile salts to create still smaller droplets called micelles. Meanwhile, the pancreatic enzyme amylase is breaking down carbohydrates to simpler compounds, the sugars dextrose and maltose. The enzymes trypsin and chymotrypsin found in pancreatic juice break down proteins into their component amino acids.

From the duodenum, chemically dismembered particles of food continue through the jejunum the first two-fifths of the remainder of the small intestine. The jejunum is a major transfer station where nutrients from digested food are absorbed into the bloodstream. This absorption is facilitated by the structure of the small intestine’s inner surface, pocked with pits and lined with millions of tiny, fingerlike projections known as villi, each supplied with a network of capillaries around a central lymph channel. Some villi in their turn form bases for millions of even smaller microvilli. The mass of villi and microvilli produce a surface like fine velvet pile: its total area may exceed that of the complete body surface by five times. This enormous area permits the mass transfer of digested particles of food from the small intestine into the bloodstream. Digestion is carried out by glands at the bases of the villi, which release enzymes that complete the breakdown of fats, proteins, and carbohydrates into units small enough to be absorbed.

The ileum has an especially thick lining of villi that complete this stage of food absorption.

The colon, like the ileum, has—beneath its outer covering—a layer of longitudinal smooth muscle and, inside this, a layer of circular smooth muscle. These act together to move digested food from the cecum, into which food comes from the ileum, to the rectum, from which it is expelled as feces through the anus. Between the layer of circular muscle and the lumen is the mucous membrane. In contrast to the small intestine, the colon has no villi. The epithelium contains mucus-secreting cells and numerous absorptive cells, through which salts, other materials, and water are reabsorbed.

The large intestine

Shorter and less convoluted than the small intestine, but much broader, the large intestine, or colon, starts in the right lower abdomen with the cecum, a short blind passage leading downward. The human cecum is the vestigial equivalent of a relatively much larger structure found in such herbivorous mammals as rabbits, where it is important for cellulose digestion.

Another vestigial structure, the vermiform (worm-shaped) appendix is a slim tubular cul-de-sac about 3.5 inches (9 centimeters) long, projecting from the otherwise blind end of the cecum. Lined with lymphatic cells, the appendix has little known value to the body and can even prove a liability. Bacterial infection may inflame it, producing the condition called appendicitis. Inflammation can even cut off blood flow to the appendix, killing tissue and causing gangrene. Or an infected appendix may burst, spreading infection to surrounding organs. Appendicitis can thus cause peritonitis or inflammation of the peritoneum, the thin membrane lining the abdominal cavity. Swift, simple removal of an inflamed appendix usually prevents such complications. Removal of the appendix leaves the work of the intestines unimpaired.

By the time swallowed substances reach the large intestine, the work of digestion and absorption is almost complete. What remains is mainly indigestible roughage, salts, dead cells from the lining of the gut, bile pigments, and water. All these enter the large intestine where the ileum joins it just above the cecum.

Peristalsis pushes undigested debris upward through the ascending colon, then horizontally through the transverse colon, which lies across the upper abdomen. Then comes a sharp change of direction, with the descending colon, which plunges down in the left side of the abdomen. Lastly, the load of waste material travels through a sharp bend, the sigmoid flexure, down through the rectum, a straight, short length of tube, and out through the anus—a hole closed and opened by a ring of muscle, the anal sphincter.

Considerable changes happen to the wastes during their passage through the colon. Bacteria feed on these wastes and help convert them into feces. The bacteria also produce valuable vitamins and enzymes that help digest some fibrous vegetable matter. These useful products pass through the colon wall into the body, together with much water—the colon is the principal site of water reabsorption and some salts.

Food consists principally of proteins, carbohydrates, fats, and liquids, which in turn provide the essential vitamins and minerals that the body requires. The digestive tract also needs a proportion of indigestible matter, called roughage, which is usually provided in adequate quantities by a varied diet. This table lists the most common sources of the different kinds of food, indicates the importance of the major vitamins, and the average adult daily requirements of them (expressed in milligrams—1 ounce equals 28,350 milligrams), and lists the main minerals and trace elements.