The digestive system is a group of organs responsible for the conversion of food into absorbable chemicals that are then used to provide energy for growth and repair. A number of other names also describe the digestive system. Some of those names include the gut, digestive tube, alimentary canal, gastrointestinal (GI) tract, intestinal tract, and intestinal tube. The digestive system consists of the mouth, esophagus, stomach, and small and large intestines, along with several glands, such as the salivary glands, liver, gall bladder, and pancreas. These glands secrete digestive juices containing enzymes that break down the food chemically into smaller, more absorbable molecules. In addition to providing the body with the nutrients and energy it needs to function, the digestive system also separates and disposes of waste products ingested with the food.
Food is moved through the alimentary canal by a wavelike muscular motion known as peristalsis, which consists of the alternate contraction and relaxation of the smooth muscles lining the tract. In this way, food is passed through the gut in much the same manner as toothpaste is squeezed from a tube. Churning is another type of movement that takes place in the stomach and small intestine, which mixes the food so that the digestive enzymes can break down the food molecules.
Food in the human diet consists of carbohydrates, proteins, fats, vitamins, and minerals. The remainder of the food is fiber and water. The majority of minerals and vitamins pass through to the bloodstream without the need for further digestive changes, but other nutrient molecules must be broken down to simpler substances before they can be absorbed and used.
Food taken into the mouth is first prepared for digestion in a two step process known as mastication. In the first stage, the teeth tear and break down food into smaller pieces. In the second stage, the tongue rolls these pieces into balls (boluses). Sensory receptors on the tongue (taste buds) detect taste sensations of sweet, salt, bitter, and sour, or cause the rejection of bad-testing food. The olfactory nerves contribute to the sensation of taste by picking up the aroma of the food and passing the sensation of smell on to the brain.
The sight of the food also stimulates the salivary glands. Altogether, the sensations of sight, taste, and smell cause the salivary glands, located in the mouth, to produce saliva, which then pours into the mouth to soften the food. An enzyme in the saliva called amylase begins the break down of carbohydrates (starch) into simple sugars, such as maltose. Ptyalin is one of the main amylase enzymes found in the mouth; ptyalin is also secreted by the pancreas.
The bolus of food, which is now a battered, moistened, and partially digested ball of food, is swallowed, moving to the throat at the back of the mouth (pharynx). In the throat, rings of muscles force the food into the esophagus, the first part of the upper digestive tube. The esophagus extends from the bottom part of the throat to the upper part of the stomach.
The esophagus does not take part in digestion. Its job is to get the bolus into the stomach. There is a powerful muscle (the esophageal sphincter), at the junction of the esophagus and stomach, which acts as a valve to keep food, stomach acids, and bile from flowing back into the esophagus and mouth.
Digestion in the stomach
Chemical digestion begins in the stomach. The stomach, a large, hollow, pouched-shaped muscular organ, is shaped like a lima bean. When empty, the stomach becomes elongated; when filled, it balloons out.
Food in the stomach is broken down by the action of the gastric juice containing hydrochloric acid and a protein-digesting enzyme called pepsin. Gastric juice is secreted from the linings of the stomach walls, along with mucus, which helps to protect the stomach lining from the action of the acid. The three layers of powerful stomach muscles churn the food into a fine semi-liquid paste called chyme. From time to time, the chyme is passed through an opening (the pyloric sphincter), which controls the passage of chyme between the stomach and the beginning of the small intestine.
There are several mechanisms responsible for the secretion of gastric juice in the stomach. The stomach begins its production of gastric juice while the food is still in the mouth. Nerves from the cheeks and tongue are stimulated and send messages to the brain. The brain in turn sends messages to nerves in the stomach wall, stimulating the secretion of gastric juice before the arrival of the food. The second signal for gastric juice production occurs when the food arrives in the stomach and touches the lining. This mechanism provides for only a moderate addition to the amount of gastric juice that was secreted when the food was in the mouth.
Gastric juice is needed mainly for the digestion of protein by pepsin. If a hamburger and bun reach the stomach, there is no need for extra gastric juice for the bun (carbohydrate), but the hamburger (protein) will require a much greater supply of gastric juice. The gastric juice already present will begin the break down of the large protein molecules of the hamburger into smaller molecules: polypeptides and peptides. These smaller molecules in turn stimulate the cells of the stomach lining to release the hormone gastrin into the bloodstream.
Gastrin then circulates throughout the body, and eventually reaches the stomach, where it stimulates the cells of the stomach lining to produce more gastric juice. The more protein there is in the stomach, the more gastrin will be produced, and the greater the production of gastric juice. The secretion of more gastric juice by the increased amount of protein in the stomach represents the third mechanism of gastric juice secretion.
Alexis St. Martin's stomach
An understanding of the complex mechanisms of gastric juice secretion began with American army doctor William Beaumont (1785–1853). He was able to directly observe the process of digestion in the stomach from the wound of a soldier named Alexis St. Martin.
In 1822, Beaumont treated the soldier for an accidental gunshot wound. This wound left a large hole in the left side of St. Martin's body, tearing away parts of the ribs, muscles, and stomach wall. When the wound healed, the stomach wall had grown to the outer body wall, leaving a permanent hole from the outer body to the interior of the stomach. When St. Martin ate, bandages were needed to keep the food in place. For the first time in medical history, a physician was able to study the inner workings of the stomach. Beaumont's observations and experiments on St. Martin's stomach extended over 11 years.
In that time, he observed the secretion of gastric juice and placed the fluid from St. Martin's stomach on a piece of meat. There he could observe the digestion of protein. He was also able to observe the churning movements of the stomach when food entered it. Beaumont's investigation of St. Martin's stomach laid the groundwork for later investigations into the complexities of the digestive process.
Digestion and absorption in the small intestine
While digestion continues in the small intestine, it also becomes a major site for the process of absorption, that is, the passage of digested food into the bloodstream, and its transport to the rest of the body.
The small intestine is a long, narrow tube, about 20 ft (6 m) long, running from the stomach to the large intestine. The small intestine occupies the area of the abdomen between the diaphragm and hips, and is greatly coiled and twisted. The small intestine is lined with muscles that move the chyme toward the large intestine. The mucosa, which lines the entire small intestine, contains millions of glands that aid in the digestive and absorptive processes of the digestive system.
The small intestine, or small bowel, is sub-divided by anatomists into three sections, the duodenum, the jejunum, and the ileum. The duodenum is about 1 ft (0.3 m) long and connects with the lower portion of the stomach. When fluid food reaches the duodenum it undergoes further enzymatic digestion and is subjected to pancreatic juice, intestinal juice, and bile.
The pancreas is a large gland located below the stomach that secretes pancreatic juice into the duodenum via the pancreatic duct. Three enzymes in pancreatic juice digest carbohydrates, lipids, and proteins. Amylase (the enzyme found in saliva) breaks down starch into simple sugars such as maltose. The enzyme maltase in intestinal juice completes the break down of maltose into glucose.
Lipases in pancreatic juice break down fats into fatty acids and glycerol, while proteinases continue the break down of proteins into amino acids. The gall bladder, located next to the liver, secretes bile into the duodenum. While bile does not contain enzymes, it contains bile salts and other substances that help to emulsify (dissolve) fats, which are otherwise insoluble in water. Breaking the fat down into small globules allows the lipase enzymes a greater surface area for their action.
Chyme passing from the duodenum next reaches the jejunum of the small intestine, which is about 3 ft (0.91 m) long. Here, in the jejunum, the digested breakdown products of carbohydrates, fats, proteins, and most of the vitamins, minerals, and iron are absorbed. The inner lining of the small intestine is composed of up to five million tiny, fingerlike projections called villi. The villi increase the rate of absorption of the nutrients into the bloodstream by extending the surface of the small intestine to about five times that of the surface area of the skin.
There are two transport systems that pick up the nutrients from the small intestine. Simple sugars, amino acids, glycerol, and some vitamins and salts are conveyed to the liver in the bloodstream. Fatty acids and vitamins are absorbed and then transported through the lymphatic system, the network of vessels that carry lymph and white blood cells throughout the body. Lymph eventually drains back into the bloodstream and circulates throughout the body.
The last section of the small intestine is the ileum. It is smaller and thinner-walled than the jejunum, and it is the preferred site for vitamin B12 absorption and bile acids derived from the bile juice.
Absorption and elimination in the large intestine
The large intestine, or colon, is wider and heavier then the small intestine, but much shorter—only about 4 ft (1.2 m) long. It rises up on one side of the body (the ascending colon), crosses over to the other side (the transverse colon), descends (the descending colon), forms an s-shape (the sigmoid colon), reaches the rectum, and anus, from which the waste products of digestion (feces or stool), are passed out, along with gas. The muscular rectum, about 5 in (13 cm) long, expels the feces through the anus, which has a large muscular sphincter that controls the passage of waste matter.
The large intestine extracts water from the waste products of digestion and returns some of it to the bloodstream, along with some salts. Fecal matter contains undigested food, bacteria, and cells from the walls of the digestive tract. Researchers publishing in the June issue of the journal JAMA Pediatrics showed evidence that newborn and infant gut bacterial loads were heavily influenced by breast feeding and bacteria found in mother's milk and around the areolar skin. Certain types of bacteria of the large intestine help to synthesize the vitamins needed by the body. These vitamins find their way to the bloodstream along with the water absorbed from the colon, while excess fluids are passed out with the feces.
The liver is the largest organ in the body and plays a number of vital roles, including metabolizing the breakdown products of digestion, and detoxifying substances that are harmful to the body. The liver also provides a quick source of energy when the need arises and it produces new proteins. Along with the regulation of stored fats, the liver also stores vitamins, minerals, and sugars. The liver controls the excretion and production of cholesterol and metabolizes alcohol into a mild toxin. The liver also stores iron, maintains the hormone balance, produces immune factors to fight infections, regulates blood clotting, and produces bile.
The most common liver disorder in the United States and other developed countries is cirrhosis of the liver. The main cause for this disease is alcoholism. Cirrhosis is characterized by the replacement of healthy liver cells by fibrous tissue. The replacement process is gradual and extends over a period of two to ten years to complete. There is no cure for the disease. Symptoms may not be noticed in its early development, but in its advanced stages there are a number of symptoms and the condition can lead to coma. Close medical attention is required to treat the disease.
Another common liver disorder is hepatitis. It is an inflammation of the liver caused by viruses. The most noticeable symptom of this disease is jaundice, that is, the skin, eyes, and urine turn yellow. The nine viruses known to cause hepatitis include Hepatitis A, B, C, D, and E; the recently discovered F and G viruses; and two herpes viruses (Epstein-Barr and cytomegalovirus).
The gallbladder lies under the liver and is connected by various ducts to the liver and the duodenum. The gallbladder is a small hollow organ resembling a money pouch. Its main function is to store bile until it is concentrated enough to be used by the small intestine. The gall bladder can store about 2 oz (57 g) of bile. Bile consists of bile salts, bile acids, and bile pigments. In addition, bile contains cholesterol dissolved in the bile acids. If the amount of cholesterol in the bile acids increases or the amount of acid decreases, then some of the cholesterol will settle out of the acid to form gallstones that accumulate and block the ducts to the gallbladder.
Infection in the gallbladder can be another cause for gallstones. Gallstones may be in the gallbladder for years without giving any signs of the condition, but when they obstruct the bile duct they cause considerable pain and inflammation. Infection and blockage of the bile flow may follow. Surgical removal of the gallbladder may be necessary to treat this condition. Since the liver both produces and stores sufficient amounts of bile, the loss of the gallbladder does not interfere with the digestive process provided fat intake in the diet is regulated.
If the gallstones contain mainly cholesterol, drug treatment for gallstones may be possible. Nevertheless, if there is too much other material in the gallstones, surgery may still be necessary. Even after drugs and diet have treated the condition successfully, the condition can return. The drug treatment takes years to dissolve the gallstones.
The appendix is a hollow finger-like projection that hangs from the occum at the junction between the small intestine and the large intestine. The appendix does not function in humans; however, in some animals, such as rabbits, the appendix is rather large and helps in the digestion of cellulose from bark and wood, which rabbits eat. The appendix in humans is therefore a vestigial organ, which may have had uses for earlier types of ancestral human digestive processes before the evolution of Homo sapiens.
If food gets trapped in the appendix, an irritation of its membranes may occur leading to swelling and inflammation, a condition known as appendicitis. If the condition becomes serious, removal of the appendix is necessary to avoid a life-threatening condition if it were to rupture.
When food reaches the small intestine, the pancreas secretes pancreatic juices. When there is no food in the small intestine, the pancreas does not secrete its juices. The economy of this process puzzled researchers who wondered what the mechanism for this control might be. In 1902, Sir William Bayliss (1860–1924) and Ernest Starling (1866–1927), two British physiologists, conducted experiments to find the answer. They reasoned that the same mechanism that initiated gastric juices when food first enters the mouth might be the same mechanism for releasing the flow of pancreatic juices.
These researchers made an extract from the lining of the small intestine and injected it into an experimental animal. The extract caused the animal to secrete large amounts of pancreatic juice. They concluded that the extract from the intestinal lining must have some substance responsible for the flow, which they named secretin. The experiment gave the first real proof for the existence of hormones, substances secreted by one group of cells that travel around the body which target other groups of cells.
Insulin is another important hormone secreted by a group of cells within the pancreas called the islets of Langerhans, which are part of the endocrine system rather than the digestive system. Insulin released into the bloodstream targets liver and muscle cells. It allows them to take excess sugar from the blood and store it in the form of glycogen. When the pancreas does not produce sufficient insulin to store dietary sugar, the blood and urine levels of sugar reach dangerous levels. Diabetes mellitus is the resultant disease. Mild cases can be controlled by a properly regulated diet, but severe cases require the regular injection of insulin.
Disorders of the digestive system
Several disorders of the esophagus are esophagitis, esophageal spasm, and esophageal cancer. Esophagitis (heartburn) is an inflammation of the esophagus usually caused by the reflux of gastric acids into the esophagus and is treated with (alkalis) antacid. Esophageal spasm is also caused by acid reflux and is sometimes treated with nitroglycerine placed under the tongue. Esophageal cancer can be caused by smoking and is generally fatal.
Disorders of the stomach include hiatal hernia, ulcers, and gastric cancer. A hiatal hernia occurs when a portion of the stomach extends upwards into the thorax through a large opening in the diaphragm. People over the age of 50 years often contract the illness. Stomach ulcers are sores that form in the lining of the stomach. They may vary in size from a small sore to a deep cavity, surrounded by an inflamed area, sometimes called ulcer craters. Stomach ulcers and ulcers that form in the esophagus and in the lining of the duodenum are called peptic ulcers because they need stomach acid and the enzyme pepsin to form. Duodenal ulcers are the most common type. They tend to be smaller than stomach ulcers and heal more quickly. Ulcers that form in the stomach lining are called gastric ulcers. As of 2010, about six million people in the United States have ulcers and 20 percent of those have gastric ulcers. About 10 percent of all adults in the United States will have an ulcer at some point in their lives. Those people who are at most risk for ulcers are those who smoke, middle-age and older men, chronic users of alcohol, and those who take anti-inflammatory drugs, such as aspirin and ibuprofen.
Until 1993, the general belief in the medical community concerning the cause of stomach ulcers was that there were multiple factors responsible for their development. By 1993 there was mounting evidence that an S-shaped bacterium, Helicobacter pylori, could be one of the factors causing ulcers. Helicobacter pylori live in the mucous lining of the stomach near the surface cells and may go undetected for years. Researchers argued that irritation to the stomach caused by the bacteria weakened the lining, making it more susceptible to damage by acid and resulting in the formation of ulcers.
Barry James Marshall (1951–), an Australian gastroenterologist, was the chief proponent of the theory that stomach ulcers are caused by H. pylori infections, rather than a multiple factor explanation, such as stress or poor diet. Although Marshall was discouraged by his colleagues from pursuing this line of research, he demonstrated his hypothesis by swallowing a mixture containing H. pylori. Marshall soon developed gastritis, which is the precursor condition to ulcers.
The treatment of ulcers has undergone a radical change with Marshall's discovery that stomach ulcers are caused by H. pylori infections. Ulcer patients today are being treated with antibiotics and antacids rather than special diets or expensive medicines. It is believed that about 80 percent of stomach ulcers may be caused by the bacterial infection, while about 20 percent may be from other causes, such as the use of anti-inflammatory medicines.