The Digestive system is the biological system which performs digestion (the process by which food and drink are broken down and absorbed). While some forms of digestion are comparatively simple, vertebrates accomplish the process through a complicated series of highly orchestrated events involving numerous independent organs. Most digestive systems have a similar order of events, organs, enzymes, as well as hormones that are involved with the regulation of the digestive process.
There are three different phases of digestion. They are known as cephalic, gastric, and intestinal. The cephalic phase is where the food enters the stomach. The Gastric phase is stimulated by the rise of pH levels and also distension of the stomach. In this phase it releases more gastric juices. Finally the intestinal phase, which is where partially digested food fills the duodenum. This is also where the release of gastrin takes place.
Almost all animals digest their food extracellularly, meaning that they take in food through a body cavity that is continuous with the outside of their environment. The body cavity of most animals secrete digestive enzymes. One of these digestive systems that are not so complicated is the gastrovascular cavities. The gastrovascular cavity connects to the outside world through only one opening. One example of an animal, which has one opening are the cnidarians. The cnidarians capture their prey using a stinging method. They then take their prey into their gastrovascular cavity with the aid of their tentacles. The enzymes within their cavity will digest the prey.
Most animals take in food through the mouth. The ingested material travels into the stomach, the small intestine, then the large, getting progressively more dense and less nutritionally valuable as vitamins and minerals are absorbed. The solid waste that comes through the gut is then excreted through the anus. At the head of the animal is the mouth or mouth cavity. The different animals use different mechanisms to break down the food they have taken in. Some vertebrates use teeth, snails use the radula,insects use mandibles. The gizzard is a muscular portion of the gut that can be found in earthworms and most birds. The gizzard grinds the food within the gut using stones. Many reptiles, notably snakes, can digest large amounts of food with little trouble. The stomach and crop are the storage chambers that are used to ingest large amounts of food. Within these storage spaces food can be further broken up. Depending on the species of animal digestion may not occur in the stomach and crop. The midgut is the next part of the gut, which the food passes through. In the midgut the food is further mixed, it is also the site where nutrients and water are absorbed. The midgut is specialized to secrete some digestive enzymes and the gut wall then secretes other enzymes. The hindgut restores water and stores undigested waste. This undigested waste is also known as faeces, which is released into the environment at the appropriate time. The rectum is a muscle that is located near the anus and assists the anus in secreting the faeces. In most animals the part of the gut that absorbs the nutrients is the part the takes up the most surface area of the animal.
- Main Article: Alimentary canal
- Main Article: Mouth
The mouth (oral cavity) is where digestion begins. It is where the food is chewed and saliva is secreted. The saliva comes from the partoid, submandibular, and the submaxillary glands. The saliva is mixed with the food, which is churned by the tongue. The saliva is used for a few things; mainly to help the food move to the esophagus more easily, and it also contains enzymes, which breaks down different chemicals. Swallowing is what transfers the food into the esophagus.
- Main Article: Esophagus
After the food has traveled through the oral cavity it travels to the food tube, which is the esophagus. Once the food has reached the esophagus, peristalsis takes over. This is known as a smooth muscle contraction, which pushes the food toward the stomach. When swallowing a bolus, which is a chunk of chewed food, it causes the esophagus to be stretched. By the stretching of the esophagus, it causes a wave that moves from the gut to the anus. The food travels from the esophagus to the stomach. There is a sphincter, which prevents food from entering the stomach. This is called the lower esophageal sphincter, which is a thick ring of circular smooth muscle at the point between the esophagus and the stomach. The sphincter is usually closed, but at times it has waves of relaxation , which allows the food to pass into the stomach.
- Main Article: Stomach
The purpose of the stomach is to hold food until it can be digested. While the food is being held with in the stomach it kills microorganisms that may have entered into the body with the food that has been eaten. It also starts to digest any proteins that are in the food that was eaten. One of the most important enzymes that is produced by the stomach is pepsin. There are gastric glands that are within the walls of the stomach. There are other materials that are secreted by the stomach. Mucus, which is secreted by the stomach, is used to protect the walls of the stomach. The stomach churns its contents in order to mix it with other contents in the stomach. All of these contents mixed together are known as chyme. The stomach contents are then pushed into the small intestines. A contraction known as peristaltic-contraction pushes the chyme from the stomach and as the stomach relaxes it sends squirts of chyme into the small intestines.
The pH levels when the food enters the body is usually around 6.8, which is only slightly acidic. Once the food has entered the stomach, secretions lower it to about 3. The pH in the stomach is acidic and makes it so that the carbohydrates can not be broken down. This is good for some things though it serves in order to denature proteins for further digestion.
- Main Article: Small intestine
The chyme from the stomach enters the small intestine in small amounts making it easier for the small intestines to effectively digest and absorb. The small intestine is where most of the absorption of the nutrients from the foods are absorbed. The villi, which are structured on the walls of the small intestines, are used the grab the nutrients.
- Main Article: Colon
The food from the small intestines the enters into the large intestine, which has three parts; the cecum, colon, and the rectum. The leftover nutrients which did not get absorbed before can now be absorbed and stored in the large intestine. The food that is left after the storage is then called the faeces, which will be secreted by the anus.
- Main Article: Liver
The liver is one of the most important organs in the body, playing many roles, including filtering harmful substances from the blood, storing the sugar that your body uses for energy, and producing bile for the digestive system that aids in the digestion of fats from food. Attached to the side of the liver is a small sac-like structure called the gall bladder. The gallbladder is an important structure in the biliary system. During periods of time when bile is not flowing into the intestine, the excess bile gets dehydrated and stored in the gallbladder.  The major components of bile include cholesterol, phospholipids, bilirubin (a metabolite of red blood cell hemoglobin), and bile salts. Bile salts act as "detergents" that help digest and absorb dietary fats. Damage to the liver or an obstruction of some kind in the bile duct (called a gallstone) can lead to serious problems such as cholestasis, steatorrhea, or jaundice.
- Main Article: Appendix
The appendix is an important component of the digestive system also known as the vermiform appendix or cecal appendix. It is a blind-ended tube connected to the cecum (a pouch-like structure of the colon), which assists with the fermentation of cellulose (plant fiber). Many cite the human appendix as a vestigial organ - a remnant of an intestinal pouch. The appendix in orangutans and grazing animals is much larger. For that reason a number of critics of creationism and of intelligent design have claimed the appendix as a counterexample of intelligent design.
- Main Article: Enzymes
The digestive glands secrete juices that contain acid, enzymes, and salts to aid in digestion. The salivary glands in the mouth are the first to secrete digestive juices. Saliva produced by these glands contains an enzyme that begins to digest the starch from food into smaller molecules. An enzyme is a substance that speeds up chemical reactions in the body. The next set of digestive glands are in the stomach lining. They produce stomach acid and an enzyme that digests protein. A thick mucus layer coats the mucosa and helps keep the acidic digestive juice from dissolving the tissue of the stomach itself. In most people, the stomach mucosa is able to resist the juice, although food and other tissues of the body cannot.
After the stomach empties the food and juice mixture into the small intestine, the juices of two other digestive organs mix with the food. One of these organs, the pancreas, produces a juice that contains a wide array of enzymes to break down the carbohydrate, fat, and protein in food. Other enzymes that are active in the process come from glands in the wall of the intestine.
The second organ, the liver, produces yet another digestive juice—bile. Bile is stored between meals in the gallbladder. At mealtime, it is squeezed out of the gallbladder, through the bile ducts, and into the intestine to mix with the fat in food. The bile acids dissolve fat into the watery contents of the intestine, much like detergents that dissolve grease from a frying pan. After fat is dissolved, it is digested by enzymes from the pancreas and the lining of the intestine.
- Main Article: Hormone
Secretin was the first hormone to be discovered of the digestive hormones. This hormone came from the duodenum. It causes the pancreas to secrete digestive juices. There are a few hormones that control the secretion of the pancreas. The main job of the secretin is to stimulate the pancreas so that it secretes a solution, which has a strong presence of bicarbonate ions.
Cholecystokinin is a hormone that is secreted by the small intestines. This hormone is used to stimulate the gallbladder to release bile. It also stimulates the pancreas, which then releases digestive enzymes. Together with secretin it slows the movement of the stomach.
Gastrin is hormone that is secreted from the stomach. When the gastrin is released from the stomach it is then combined with the blood. The gastrin is released when the food in the stomach stimulates it. Gastrin starts off in the lower region of the stomach. It the travels up through the stomach and when it finally reaches the upper region of the stomach it then stimulates movement within the stomach. When the contents of the stomach become acidic the gastrin is released.
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- Your Digestive System and How It Works by the National Digestive Diseases Information Clearinghouse
- Purves, p.978