Digestive system of human beings consists of alimentary canal or gut and some accessory organs.
Alimentary Canal include
- Oral (Buccal) cavity
- Small intestine
- Large intestine
- Rectum and anus
Accessory organs include
- Salivary Glands
- Gall bladder
- Alimentary canal is coiled, muscular, continuous tube running through the ventral body cavity extending from the mouth to the anus. It is about 30 feet long.
- It consists of many specialised sections. these sections from top to bottom are mouth, oral cavity, pharynx, oesophagus, stomach, small intestine, large intestine, rectum and anus.
- Daily secretion of saliva in the mouth is 1 to 1.5 litre. Saliva is acidic in nature. Saliva contain ptyalin enzyme which acts on cooked starch changing them into sugar maltose.
- There are three pairs of salivary glands viz. parotids, submaxilliary (sub mandibular) and sublingual.
Buccal (Oral) Cavity
- The mouth leads into oral cavity that has two lateral walls (cheeks), an interior roof (hard palate), a posterior roof (soft palate) and a pair of jaw (upper maxilla and lower mandible).
- The buccal cavity is composed of palate, tongue and teeth.
- Pharynx is small sized (12-14 cm) conical part where food and air passages cross each other.
- Pharynx is divided into three parts viz. nasopharynx, oropharnyx and laryngopharynx.
- Oesophagus is a long (22-25 cm) narrow, muscular and tubular structure.
- It runs downward through the neck behind the trachea, passes through the diaphragm and opens in the stomach in the abdomen.
- it conducts the food to stomach by peristalsis.
- The stomach is a large muscular C-shaped sac and located on left side in the upper position of the abnormal cavity.
- Stomach secrets gastric juices with pH 1 to 3.5.
- Food mixed with gastric juice in the stomach is called chyme.
- Pepsin enzyme digests protein into peptides.
- Rennin is found in calf gastric juice. It is a milk regulating proteinase.
- HCl is also secreted in stomach. the acid (HCl) is important because pepsin can ac only in an acidic medium.
- Hydrochloric acid inactivates the salivary amylase and prevents further breakdown of carbohydrates (disachharides). Also, it kills micro-organisms and lowers the pH of the stomach between 1.5 and 2.5 (very acidic).
- After exposure to hydrochloric acid pepsinogen turns into pepsin, which is the principle proteinase (protein enzyme) of the stomach and digests proteins into peptides.
- Partially digested broth of food then leaves the stomach through its pyloric end and enters the duodenum as chyme.
- The duodenum also recieves digestive enzymes and bicarbonates from pancreas and bile from the liver via gall bladder, pancreatic acinar cells (exocrine part of pancreas) produce pancreatic juice that contains protein – digesting enzymes, namelytrypsin, chromotrypsin & carboxypeptidase and carbohydrases (carbohydrates digesting enzymes) like pancreatic amylase and lipase. All these enzymes enter the duodenum via the pancreatic duct.
- Bile, the main exocrine secretion of liver, is initially concentrated and stored in the gall bladder. On stimulation, bile is squeezed into the duodenum through the cystic and common bile duct. Bile is a watery greenish fluid mixture containing bile pigments, bile salts, cholestral and phospholipids. The bile salts play a very important role in the digestion of fats.
- Bile pigment are the products of dead erythrocytes.
- In mammals, stomach is divided into
- Anterior (cardiac) stomach
- Posterior (pyrolic) stomach
- Middle (fundic) stomach
- Small Intestine is divided into three parts:
- Duodenum: It is the widest and the shortest (25 cm) part of small intestine.
- Jejunum: It is the middle part and is about a feet long.
- Lleum: It is the longest (12 feet) part of small intestine.
- Small intestine is about 20 feet long in humans.
- Duodenum forms the typical U-shaped loop containing the pancreas.
- In man there is common opening of bile duct and pancreatic duct opening in duodenum.
- The duodenum leads into jejunum.
- Lleum is the last part of small intestine.
- Internal lining of duodenum, jejunum and lleum is raised into innumerable minute finger-like processes called villi. intestinal villi are mainly concerned with absorption.
- The absorption of digested food mainly occurs in small intestine.
- Maximum absorption takes place in lleum.
- the food in the small intestine is called chyle.
- Large intestine is 5 feet long and 2-5 inches in diameter.
- Large intestine is called large because of its large diameter, although it is much shorter in length than small intestine.
- It has three parts namely caecum, colon and rectum.
- Caecum is a small pouch like structure which ends into a tubular structure (13 cm) long called vermiform appendix.
- Caecum and vermiform appendix are vestigial in functions as these are not involved in cellulose digestion.
- Colon is thicker than small intestine and thinner than caecum.
- colon is concerned with conservation of water, sodium or other minerals and formation of faeces.
- The rectum is the last part of the alimentary canal and is about 7-8 inches long.
- The terminal 1 inch of rectum is called anal canal and its exterior opening is called the anus.
- Rectum secrets no enzymes. It stores digestive residue and absorbs water from residue.
- It plays only a minor role in the absorption of nutrients.
- Human have three pairs of salivary glands:
- Parotid glands in the cheeks.
- Submandibular (submaxillary) glands.
- Sublingual glands.
- Both submaxillary and sublingual glands opens into the floor of the mouth.
- Saliva is a mixture of water and electrolytes (Na+, K+, Cl–, HCO3-), derived from the blood plasma, mucus and resous fluids, salivary amylase or ptyalin (enzyme) and lysozyme (antibacterial agent), all are secreted by the salivary glands.
- Hormones that aid and regulate the digestive system:
- Gastrin is in the stomach and stimulates the gastric glands to secrete pepsinogen and hydrochloric acid. Secretion of gastrin is stimulated by food arriving in stomach. The secretion is inhibited by lo pH.
- Secretin is in the duodenum and signals the secretion of sodium bicarbonates in the pancreas and it stimulates the bile secretion in the liver. This harmone responds to the activity of the chyme.
- Choecystokinin (CCK) is in the duodenum and stimulates the release of digestive enzymes in the pancreas and stimulates the emptying of bile in the gall bladder. this hormone is secreted in response to fat in chyme.
- Gastric Inhibitory Peptide (GIP) is in the duodenum and decreases the stomach churning in turn slowing the emptying in the stomach.