Digestive+System

= Digestive System = The digestive system is one of the largest organ systems in the human body.It is responsible for processing of ingested food and liquids. The cells of the human body all require a wide array of chemicals to support their metabolic activities, from organic nutrients used as fuel to the water that sustains life at the cellular level. The digestive system not only chemically reduces the compounds in food into their fundamental building blocks effectively, but also acts to retain water and excrete undigested materials. The functions of the digestive system can thus be summarized as below:
 * 1) Ingestion - eating food
 * 2) Digestion - breakdown of food
 * 3) Absorption - extraction of nutrients from the food
 * 4) Defecation - removal of waste products
 * 5) Builds and replaces cells and tissues that are constantly dying

Follow the link below to view the overall processing of the food by the digestive system: []

= Digestive Organs  = The digestive system consists of a group of organs that form a closed tube like structure called the gastrointestinal tract(GI tract) or the alimentary canal.For convenience, the GI tract is divided into upper GI tract and lower GI tract. =Upper Gatrointestinal Tract(GI) = It consists of :
 * Mouth or Oral Cavity
 * Esophagus
 * Stomach

__**THE ORAL CAVITY**__ The human mouth also called the oral cavity or the buccal cavity, is the entry point into the human digestive system. Not only are nutrients initially processed in this location, but also serves as the connecting point between the respiratory system and the outside environment. Salivary Glands : There are thre pairs of accessory glands called the salivary glands in the oral cavity.Saliva, the chemical secretion of the salivary glands, is actually a complex mixture that performs a variety of functions for the digestive system namely:
 * 1) It contains a small amount of an enzyme called lysozyme, which inhibits, but does not eliminate , the formation of bacterial colonies in the oral cavity.
 * 2) Mucus, a watery mixture of complex polysaccharides, helps lubricate and protect the oral cavity.
 * 3)  Saliva contains another enzyme called salivary amylase, which initiates the process of carbohydrate digestion.

Mechanical Digestion : In the oral cavity, the process of mechanical digestion serves several functions as follows:
 * 1) First, the action of the tongue and teeth break the food into small portions so that it may be sent to the stomach via the esophagus.
 * 2) Second, the process of mechanical digestion increases the surface area of food, allowing the secretions of the salivary glands to mix freely with the food.

Enzymatic Digestion : The amount of enzymatic digestion within the oral cavity is small as compared to the activity of the lower GI tract. However, there is some initial digestion of both carbohydrates and lipids in the oral cavity : As the bolus forms in the rear of the oral cavity the swallowing reflex begins and pushes the food into the esophagus.
 * 1) The salivary amylase in the saliva begins to break down the starches in the food until deactivated by the acidic pH of the stomach.
 * 2) The glands on the surface of the tongue secrete an enzyme called lingual lipase that begins the breakdown of lipids.

__**LAYERS OF THE DIGESTIVE SYSTEM**__ Before following the bolus on its breif journey through the esophagus, it is necesary to undertand the tissue structure of digestive tract.From the esophagus to the anus, the walls of the digestive tract have the same general structure, with minor variations in each organ to enable specific functions. Within the wall of the digestive tract are four major tissue layers namely:
 * 1) **Serosa**: It is the outermost layer and is comprised of connective tissue.This layer is important in that it forms aconnection between the digestive tract and the mesentery that suspends the organs of the digestive tract within the abdominal cavity.It secretes a water -based mixture to prevent friction between the organs of the system.
 * 2) **Muscularis Externa**: It is double layered and is made of smooth muscle. The two muscle layers are the inner circular muscle and the outer longitudinal muscle.The muscles contract in different directions, with the circular layer controlling the diameter of the digestive tract and the longitudinal layer controlling the length.
 * 3) **Submucosa** : It is single layered and made up of connective tissue.This layer contains the major blood and lymphatic vessels and nerves.
 * 4) ** Mucosa : ** It is the innermost layer that lines the interior of the digestive tract and is thus in direct contact with the nutrients passing through the system.The epithelial cells of this layer serve several functions, depending upon the region of the gastrointestinal tract.In some cases, these cells secrete a mucus layer that serves to lubricate the passage and protect the ceells. Other cells may secrete digestive juices, while still others may release hormones to regulate the activity of the region.

Follow this link to see the enlarged diagram of the inner layers of stomach and detailed desciption. __**ESOPHAGUS**__ Once the food leaves the oral cavity it enters into a muscular tube called the esophagus. The esophagus is not a major digestive organ becuse the only enzymes that are active here are the salivary amylase and lingual lipase from th oral cavity.It serves a s a conduit from the oral cavity to the stomach.To ensure one way movement of food, the esophagus is regulated by two sphincters, or valves namely the pharyngoesophageal sphincter at the upper end and the gastroesophageal sphincter at the lower end.The gastroesophageal sphincter also inhibits the reflux, or backup of gastric juice from the stomach into the esophagus.Without this valve the highly acidic gastric juices would damage the delicate mucosa layer of the esophagus. ** STOMACH ** It is a muscular sac that functions as a holding site for food before it enters into the small intestine as well as the location where the food is mixed and partially digested by mechanical and enzymatic processes. **Structure**:It is an elastic J-shaped organ whose bounadries are defined at the upper end by the //**gastroesophageal sphincter**// and at the lower end by the //**pyloric sphincter**//. the stomach contains the smae tissue layers as are found in the esophagus, small intestine, and colon, with some important variations in the secretions and the structure of the mucosa layer. The uppermost part located above the gastroesophageal sphincter is called the //**fundus,**// below is the main region of the stomach called the //**body**// and the lower portion of the stomach which connects to the small intestine is called the //**antrum**//. **Secretions**: The stomach produces about 2.12 quarts(2 litres) of //**gastric juice**// per day.The mucosa layer of the stomach contains folds called //**gastric pits**//.At the top of the gastric pit are the //**mucous neck cells**// that secrete mucous coating that protects not only the cells within the gastric pit but also the mucosa layer of the stomach.The mucus is alkaline and thus serves to neutralize any stomach acid before it comes in contact with the stomach mucosa.The mucus also serves to lubricate the interior of the stomach for mechanical digestion.Located within the gastric pits are the //**chief cells**// and the //**parietal cells**//.The chief cells secrete an inactive enzyme called the //**pepsinogen**// and the parietal cells are responsible for manufacturing //**hydrochloric acid**//.The parietal cells also secrete an //**intrinsic** **factor**// that aids in the absorption of vitamin B12 in the small intestine. **Enzymatic Digestion**: The prime nutrient target of enzymatic action in the stomach is proteins.The pepsinigen secreted by the chief cells makes its way through the protective mucous coat into the lumen of the stomach.The hydrochloric acid in the lumen activates pepsinogen by cleaving off a small fragment from one end of the molecule.This active form of the enzyme is called pepsin. Once activated, pepsin breaks down some proteins into smaller peptide fragments for further digestion in the small intestine.

= Lower Gatrointestinal Tract(GI) = It consists of :
 * Small Intestine
 * Large Intestine

__**SMALL INTESTINE**__ It is the main organ of digestion and absorption in the huamn body. It connects to the stomach at the pyloric sphincter and empties into the colon through the ileocecal valve, also called the //**ileocecal sphincter**//. **Structure**: The small intestine is divided into three segments namely //**duodenum**//(the first part starting at the pyloric sphincter), //**jejunum**//(the middle portion), and //**ileum**//(last part ending at ileocecal valve).The //**submucosa layer**// of the small intestine contains a group of specialized cells called the //**Brunner's glands**//.These glands release mucus into the lumen of the small intestine and help to neutralize any hydrochloric acid from the stomach remaining in the food. The //**mucosa layer**// of the small intestine consists of numerous finger-like projections called //**villi**// in order to increase the surface area to facilitate the absorption of nutrients. Within each villi, are capillaries and portions of the lymphatic section called the lacteals. As nutrients are absorbed into the villi, they pass either into the capillaries or lacteals and are then transported away from the small intestine. Along each of the villi are located four types of specialized cells namely:
 * 1) **Paneth cells**: Found at the base of each villi in pits called the intestinal glands.These cells release lysozyme that protects the small intestine from bacteria.
 * 2) **Enteroendocrine cells**: Also found in the intestinal glands. As the name implies, these cells are actually part of the endocrine system and are responsible for releasing hormones such as gastric inhibitory peptide(GIP), secretin, and cholesystokinin(CCK).
 * 3) **Goblet cells**: Located above the intestinal glands and responsible for secreting the protective mucus coating of the small intestine.
 * 4) **Absorptive or Epithelial cells**: Located along the length of the villi, but primarily above the region of intestinal glands.These cells are unique in that the plasma membrane on the lumen side of the cell contains a large number small projections called //**microvilli**//.Each of these projections is about 1 micrometer long and a typical absorptive cell may have as many as 6,000 microvilli on its surface.Under a microscope, the microvilli appear as a thin, fuzzy barrier and are thus called "//**brush border".**// This region is not only involved in nutrient absorption but also in nutrient digestion by a group of enzymes called the //**brush border enzymes**//.

**Enzymatic Digestion**:The food in the small intestine is mixed with the juices from three sources namely bile from the liver, pancreatic enzymes such as proteases, amylases, and lipases from the pancreas, and the digestive enzymes from the lining of the small intestine. //**Carbohydrate digestion**// began in the oral cavity with the activity of the salivary amylase enzyme. While this enzyme acts on the incoming food only for a brief period before becoming inactivated by the hydrochloric acid of the stomach, it does initiate the breakdown of starches and other polysachharides into disaccharides and monosaccharides.The majority of polysaccharide digestion is conducted in the small intestine by pancreatic amylase from pancreas.//**Protein digestion**// was initiated in the stomach by pepsin. Thus, when the proteins reach the small intestine, they are primarily in the form of small chains called peptides.The pancreas releases an enzyme called //**trypsinogen**// ,which is inactive, in the lumen of the duodenum via the pancreatic duct. It is activated by the brush border enzyme of the villi called //**enterokinase**//.The active form called the //**trypsin**// digests proteins into smaller peptides that can be absorbed by the villi. //**Lipid digestion**// begins in the oral cavity by the lingual lipase enzyme. In the stomach, this enzyme gets deactivated and the digestion of the lipis is continued by the gastric lipase enzyme.The major digestion of lipids takes place in the small intestine. Bile from liver emulsifies, or breaks down, droplets of fats into smaller particles. This is not enzymatic digestion, since the lipid molecules are not the target, but rather the interaction between the fat molecules.The enzyme that carries out lipid digestion is pancreatic lipase from pancreas. This lipase breaks down triglycerides into small fatty acid chains and monoglycerides.

__**LARGE INTESTINE**__ It is often mistakenly considered the location in the body where waste material is generated. In reality, this organ is more of a recyling center and temporary storage location than a waste disposal site.It begins at the ileocecal valve and is comprised of three distinct regions namely cecum, colon, and rectum. The rectum opens into anus.While the appendix has historically been considered a part of the large intestine, and often thought of as a vestigeal organ, it is actually made of the sma etype of tissue as the lymph nodes and thus is now considered a part of the lymphatic and immune systems. **Digestion and Absorption**:The small intestine is highly effective as an organ of digestion and absorption, thus the material reaching the large intestine usually contains undigested materials such as cellulose, some salts, water, and bilirubin from liver.While no digestive enzymes are secreted by the large intestine, the mucosal cells along the length of the organ secrete an alkaline mucus which serves to lubricate the internal lining of the large intestine, and protect it from any acids produced by fermenting bacteria in this region.There is some minor breakdown of bilirubin from liver which gives the characteristic color to the feces.The colon primarily absorbs water and salts and may also take in ther nutrients such as glucose and vitamins that may be present.The colon contains a natural flora of bacteria that are in a symbiotic relationship with their human host.The human host provides a stable environment, with ample water and nutrients. In return bacteria such as //**Escherichia coli**// and Clostridium species break down undigested fiber to glucose. Some of this glucose may be absorbed by the colon.In addition, some B vitamins and vitamin K are also produced that are responsible for clotting response in the circulatory system. More importantly, the presence of large colonies of beneficial bacteria inhibits the growth of pathogenic bacteria that may have entered thriugh the ileocecal valve. Click on the link to view the complete working of the digestive system.

References: The Digestive System by Michael Windelspecht Biology by Matthew Distefano