The digestive system is that part of the body which specializes in the digestion and absorption of nutrients in the body. It is composed of several segments that each has a special function that is related to the process of digestion. The process of digestion in the gastrointestinal tract includes not only the digestion of macromolecules into its constituents amino acids and fatty acids and glucose but also it involves other functions such as the expulsion of food residues from the body.
Accessory organs in the body that help in the digestion process include the gallbladder and the pancreas. The digestive system has two openings in which foreign pathogens can enter through it to the body. These are: the mouth and the anal canal. The gastrointestinal system is for this reason vulnerable to injury by chemicals and pathogens that can enter the body by ingestion of food in the mouth.
The gastrointestinal system has its own defenses against foreign invaders. An example is the acidic environment in the stomach which is offensive for pathogens, thus killing them. In addition, the gastrointestinal system has a special form of lymphatic system that is called mucosa associated lymphatic tissue or MALT. This lymphatic system helps in protecting the body against toxic chemicals and pathogens.
The digestion of food occurs mainly in the small intestine. This is usually a relatively long tube in comparison with the large intestine which is shorter in length. The epithelium of the small intestine is specialized for the process of digestion and absorption of nutrients. Cells of the small intestine are metabolically very active in the process of absorption of food into the blood stream. The blood supply to the small intestine is extensive. This is so to suffice the supply of enough energy and oxygen to these cells. The epithelial cells of the small intestine have special structures that are called microvilli that help in the absorption process by increasing the surface area of the cells for absorption.
The accessory organs in the digestive system are the pancreas and the gallbladder. The gallbladder is connected with the duodenum of the small intestine through the ampulla of vater which is a joint opening for the gallbladder and the pancreas to the duodenum. The gallbladder secretes fluid that is called bile into the duodenum. This fluid is basic in pH due to its content of bicarbonate ions. It also contains bile acids and the pigment bilirubin. Bile acids help to solubilize lipids that are found in the small intestine as part of the food.
Lack of bile for any reason can lead to malabsorption of lipids with the developement of constipation. This is true especially if there is also pancreatic insufficiency. Bile can help solubilize lipids by emulsifying it. The pancreas is the other accessory organ in the digestive system. Its main function is the secretion of digestive enzymes into the duodenum of the small intestine. Its secretion is also basic in pH due to the presence of bicarbonate ions. The secreted enzymes are specific for the digestion of the various types of food such as proteins and carbohydrates in addition to lipids.
Impairment of pancreatic secretion as occurs in pancreatitis or pancreatic insufficiency can lead to malabsorption of food in the intestine that can cause constipation and accumulation of lipids in the stool. After the absorption process takes place in several parts of the gastrointestinal tract including the mouth and the stomach the food residues enter into the large intestine where it solidifies due to the absorption of water and electrolytes by the cells of the colon. This is its last form before it is expelled from the body by the defecation reflex.
The colon or large intestine is the last part of the gastrointestinal tract. It has little role in the absorption process of nutrients in the body to the blood circulation except for electrolytes and water absorption. Its main role is as a reservoir of feces which are eventually propelled out of the body by a peristaltic movement of the muscles of the colon. The colon has similar function to that of the esophagus in the sense that both secrete mucus that lubricates its inner surface, thus making easy the propulsion of food along these two structures.
The cells that secrete this mucus are called Goblet cells and are present also in addition to the esophagus and the colon between the epithelial lung cells. The colon has a special form of nervous system that is called the enteric nervous system. This nervous system has connections with both the sympathetic and parasympathetic nervous systems. Peristaltic motion of the colon muscles are governed by neurons in the vagus nerve which synapse with neurons of the enteric nervous system, thus increasing peristalsis.