The Digestive System of Human


Man has gastro intestinal tract which runs from mouth to the anus. It begins with mouth and buccal cavity followed by pharynx, oesophagus, stomach, small intestine bearing duodenum, jejunum and ileum. Large intestine consist of caecum bearing appendix, colon and rectum terminating at the anus oral cavity has three pairs of salivary glands. Saliva contains 95% water, some mucus, and amylase and lysozyme enzymes. Mucus moistens and lubricates the food, semi solid food is passed into oesophagus in the mass of bolus. 

Oesophagus is narrow muscular tuber about 25 cm long and conveys food from pharynx to stomach. Stomach is muscular bag lying below diaphragm on left side of abdominal cavity. It performs three functions: storage of food, mechanical digestion by peristalsis and chemical digestion of food by enzymes, which is reduced to creamy paste called chyme. Stomach has cardiac region, middle region the fundus and posterior pyloric region. It opens into duodenum through pyloric sphincter or pylorus. Gastric juice contains pepsin, HCl which kills bacteria, pepsin converts proteins into polypeptides.

Next to stomach is small intestine about 6 metres long. It is in coiled loops and is divided into duodenum, jejunum and ileum. Duodenum is 30 cm long and runs parallel to stomach. It receives bile duct from liver and pancreatic duct from pancreas. Chyme meets with juices of liver and pancreas. Bile salts neutralize the acid of gastric juice and make chime alkaline. Bile pigments bilirubin red, and biliverdin (green) are excretory products formed by breakdown of haemoglobin of worm out RBC in liver. Pancreatic juice contains four enzymes trypsin (protease). Chymotrypsin, amylase and lipase. Trypsin acts on polypeptides and proteins and convert them into polypeptides. 

Chymotrypsin converts casein (milk protein) into short chain amino acids. Amylase converts starch and glycogen into maltose and lipase converts emulsified fats into fatty acids and glycerol which are soluble products of fat. Thus digestion of fat is completed in the duodenum. Duodenum passes into jejunum which is about 2.4 meter long. Digestion of food is completed within the jejunum by enzymes like maltase, sucrase, lactase and peptidase. End products are monosaccharides and amino acids are liberated into the lumen of small intestine. Jejunum passes into ileum which is about 36 meter long. It receives much diluted food chyle containing digested food in true solution form. It has finger like villi which can contract and relax. This increases the absorptive surface area. 

Monosaccharides and amino acids are absorbed into the blood capillaries by diffusion or active transport, while the fatty acids and glycerol enter the epithelial cells of villi. Here they are reconverted into simple fats which then enter the lacteals and pass into blood stream.

The blood capillaries converge to form hepatic portal vein which delivers absorbed food to liver where it is stored and is distributed to all the cells of the body.

Small intestine opens into large intestine bearing short caecum, a long colon and a terminal rectum. Caecum gives off a blind tube about 18 cm long from its lower portion known as vermiform appendix which is vestigial organ. Colon is longest part; vitamins and water are absorbed here. Rectum is last portion of large intestine, undigested food in passed from rectum and out by anus in the form of faeces. Liver and pancreas are glands present behind diaphragm. Liver has two main lobes a right and a left. Left lobe is further divided into two lobes. Liver is redish brown in colour. Both produce juices to digest food.

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