Human digestive system

The human digestive system starts with mouth and ends with anus.The  function of digestive system  starts with   intake food -Ingestion, Chewing and mixing of food with Saliva - Mastication,Swalloing of food -Deglutition, Break down of food in to simpler form - Digestion, Imbibition of digested food in to blood- Absorption, expulsion of undigested food- defecation.The part of digestive system which starts with mouth and ends with anus is also termed as Alimentary canal.

Mouth

Mouth encloses a cavity called Oral cavity  other wise called Buccal cavity. There are 32 teeth in mouth.  The human dental formula is 2/2,1/1,2/2,3/3. First four teeth  Incisors, next two canines, next four  pre-molar and last three molars.The last pair of molars are called wisdom teeth which appear at age of 20 years. The floor of buccal cavity has muscular organ called tongue. It has sensory papillae with taste buds and hence works as gustatory organ( Perception of taste) and masticating organ. There are three pairs of salivary glands surrounding the buccal cavity. A pair are located below tongue called Sublingual salivary glands, a pair in maxilla called Submaxillary salivary glands and a pair near ear called Parotid glands. These three pairs of salivary glands pour their secretions called Saliva in to buccal cavity. Saliva is slightly alkaline and have enzyme called Salivary Amylase(Ptyalin).

This enzyme digests starch to simple dextrins and glucose. It also helps the food to become soft and slimy, easy enough to swallow. The chewing of good breaks food into tiny food particles mixed with saliva, as a result a ball of food is formed called Bolus. The upward movement of tongue forces bolus in to an opening called Pharynx. Pharynx is common junction for wind pipe and food channel called esophagus.

Guarding the wind pipe(Trachea) there is a flap like structure called Epiglottis which regulates the passage of food into esophagus and prevents accidental entry of food into wind pipe.To err is human, Epiglottis sometimes allows food particles into wind pipe. Respiratory system immediately gets rid of these particles by protective mechanism called Coughing.

Pharynx opens into esophagus. Esophagus is a long tube lined with longitudinal and circular muscles.It passes through neck, thorax and diaphragm and finally opens in to stomach. The junction point of  esophagus and stomach is called Cardiac stomach. When bolus is swallowed the involuntary peristalitic movements direct the food into stomach. Peristalitic moments are wave like moments.

Stomach

Stomach is muscular bag like structure. The anterior part is called Cardiac stomach, the middle part called Fundic stomach and posterior part is called Pyloric stomach. Pyloric stomach into U shaped tube called Duodenum through a guarded valve called Pyloric spincter. Muscular stomach walls consists of longitudinal muscles and circular muscles which help in churning movements of stomach.The mucosal lining of stomach has gastric glands. Gastric glands have three major types of cells namely -mucus neck cells which secrete mucus,peptic or chief cells which secrete the proenzyme pepsinogen; and parietal or oxyntic cells which secrete HCl. The secretions of gastric glands is called Gastric juice. Hcl present in gastric juice kills bacteria and denatures proteins. Gastric juice contains enzymes namely Pepsin, Rennin and small amounts of Lipase.HCl provides the acidic pH optimal for pepsins. Rennin is a proteolytic enzyme found in infants which helps in the digestion of milk proteins.Lipase digests fats.Pepsin converts proteins into peptones. Lipase converts fats into Dyglycerides and Monoglycerides. Once churning of food is completed the partially digested food in stomach is called chyme. Chyme enters duodenum through controlled opening of Pyloric spincter.

Duodenum

Duodenum is U shaped tube like structure. Bile duct from Liver and Pancreatic duct from Pancreas opens into duodenum. Bile juice is temporarily stored in Gall bladder. It contains Bile pigments namely Bili-verdin, Bili-rubin and Bile salts mainly bicarbonates.

Pancreas

The pancreatic juice contains inactive enzymes Trypsinogen, Chymotrypsinogen, Procarboxypeptidases, Amylases, Lipases and Nucleases. Trypsinogen is activated to Trypsin by an enzyme Enterokinase secreted by the intestinal mucosa. Proteins, peptones, proteoses are converted to dipeptides by action of Trypsin, Chymotrypsin and carboxypeptidases. Carbodydrates in Chyme are hydrolysed to Disaccharides. Nucleases convert nucleic acids into nucleotides and nucleosides.

Small intestine 
It is narrow coiled tube like structure with length of 8 mts and width of 3cms. Coiled intestine is held in place by Mesentaries.The intestinal mucosal epithelium has goblet cells which secrete mucus.  The secretions of the brush border cells of the mucosa along with the secretions of the goblet cells constitute the intestinaljuice or succus entericus. This juice contains a variety of enzymes like   Maltase, Sucrase, Lactase dipeptidases, lipases, nucleosidases, nucleotidases etc.The mucus along with the bicarbonates from the pancreas protects the intestinal mucosa from acid as well as provide an alkaline medium  for enzymatic activities.

Dipeptides are converted by dipeptidases to Amino acids. Maltose is converted by Maltase to Glucose. Lactose is converted by Lactase to Glucose and Galactose. Sucrose is converted by Sucrase to Glucose and Fructose. Nucleotides are converted to Nucleosides by Nucleotidases. Nucleosides are converted to sugars and bases by Nucleosidases. Di and Monoglycerides are converted to Fatty acids and Glycerol by Lipases. The simple substances thus formed are absorbed in the jejunum and ileum regions of the smallintestine through finger like projections called Villi. The undigested and unabsorbed substances are passed on to large intestine.

Absorbtion

Absorption is the process by which the end products of digestion pass through the intestinal mucosa into the blood or lymph. It is carried out by passive, active or facilitated transport mechanisms. Small amounts of monosaccharides like glucose, amino acids and some electrolytes like chloride ions are generally absorbed by simple diffusion. The passage of these substances into the blood depends upon the concentration gradients.However, some substances like glucose and amino acids are absorbed with the help of carrier proteins. This mechanism is called the facilitated transport.

Large intestine

The large intestine is considerably thicker in diameter, thus giving it its name. The large intestine is about 5 feet (1.5 m) in length and 2.5 inches (6-7 cm) in diameter in the living body. The large intestine is connected to the ilium of the small intestine via the ileocecal sphincter.The inferior region of the large intestine forms a short dead-end segment known as the cecum that terminates in the vermiform appendix which is vestigial organ in human.The rectum is the enlarged final segment of the large intestine that terminates at the anus.A slurry of digested food, known as Chyme, enters the large intestine from the small intestine via the ileocecal sphincter. Chyme passes through the cecum where it is mixed with beneficial bacteria Bacterial fermentation converts the chyme into feces and releases vitamins including vitamins K, B1, B2, B6, B12, and biotin. Vitamin K is almost exclusively produced by the gut bacteria and is essential in the proper clotting of blood.

The absorption of water by the large intestine helps to condense and solidify feces and allows the body to retain water to be used in other metabolic processes. Ions and nutrients released by gut bacteria and dissolved in water are also absorbed in the large intestine and used by the body for metabolism. The dried, condensed fecal matter is finally stored in the rectum  until it can be eliminated from the body through the process of defecation. 

Gastro-intestinal tract is under neural and hormonal control for proper coordination of different parts. The sight,smell or the presence of food in the oral cavity can stimulate the secretion of saliva. Salivary secretion occurs during day and does not happen during night. Gastric and intestinal secretions are also, similarly, stimulated by neural signals. The muscular activities of different parts of the alimentary canal can also be moderated by Second brain also called Gut nervous system, both local and through CNS. Hormonal control of the secretion of digestive juices is carried out by local hormones produced by the gastric and intestinal mucosa.