Human alimentary canal

 Human alimentary canal

The stomach related arrangement of human comprises of a long cylinder that reaches out from mouth to anus.this tube is called nutritious waterway. its mane segments are oral depression, pharynx, oesophagus,stomach, small digestion tracts and internal organs. moreover there are numerous organs related with wholesome canal.these are as three sets of salivary organs a pancreas and a liver.
Oral cavity- selection, grinding, partial digestion 

Oral cavity

1 *  Oral cavity is the space behind mouth and has many important function in the whole process of digestion. food selection is one of them.when food enters oral cavity,it is tasted and felt. if the taste of mutton suggests that it is old, we reject it. if teeth or tongue detect some hard object, such as dirt, we also reject that bite. the sense of smell and vision also help Oral cavity in the selection of food.

2 * The second function of Oral cavity is the grinding of food by teeth. it is known as chewing of mastication. this is useful because oesophagus can pass only small pieces. Enzymes also cannot act on large pieces of food. they require small pieces with large surface areas to attack.

3 * the third and fourth functions of oral cavity are lubrication and chemical digestion of food. the chewing process stimulates the three pairs of salivary glands ( under tongue, behind jaws, and in front of ears ) to release a juice called saliva in oral cavity. Saliva through oesophagus. Saliva also contains an enzyme salivary amylase, which helps in the semi-digestion of starch.

During the processes of chewing, lubrication and semi-digestion, the pieces of food are rolled up by the tongue into small, slippery, spherical mass called bolus. we swallow bolus and  push it in oesophagus through pharynx.

pharynx and oesophagus- swallowing and peristalsis

During swallowing, bolus is pushed to the back of mouth by tongue. when tongue pushes bolus, the soft palate also moves upward and to rear. In this way, the opening of nasal cavity is closed. When swallowed, the  bolus passes pharynx to enter oesophagus. Pharynx has adaptations to prevent the entry  of bolus particles in trachea ( wind pipe to lunge ) . During swallowing larynx ( the top of trachea) moves upward and force the epiglottis ( a flap of cartilage ) into horizontal position. thus glottis i.e. the opening of trachea is closed. The beginning of swallowing action is voluntary, but once food reaches the back of mouth swallowing becomes automatic.

after being swallowed , food enters the tube called oesophagus, which connects pharynx to stomach.Neither pharynx nor oesophagus contributes to digestion and the previous digestive actions of saliva continue. Peristalsis moves food from oral cavity to rectum. peristalsis is defined as the waves of contraction and relaxation in the smoooth muscles of alimentary canal walls.

stomach- Digestion, churning and melting

stomach

Stomach is a dilated part of alimentary canal. it is J-shaped, located in the left of abdomen, just beneath diaphragm. Stomach has two main portions. Cardiac portion of stomach is present immediately after oesophagus and pyloric portion is located beneath cardiac portion. stomach has two sphincters ( opening which are guarded by muscles ). Cardiac sphincter is between stomach and oesophagus while pyloric sphincter is between stomach and small intestine. Bolus enters stomach from oesophagus though cardiac sphincter.

When food enters stomach, the gastric glands found in the stomach wall are stimulated to secrete gastric juice. Gastric juice is composed chiefly of mucous, hydrochloric acid, and a protein digesting enzyme pepsinogen. Hydrochloric acid convert the inactive enzyme pepsinogen into its active form i.e.pepsin. HCL also kills microorganisms present in food. pepsin partially digests the protein portion of food ( bulk of mutton) into polypeptides and shorter peptide chains.

In stomach food is further broken apart trough a process of churning. the walls of stomach contract and relax and these movements help in through mixing of gastric juice and food. The churning action also produces heat which helps to melt the lipid content of food.

The starch in our bite of bread and the protein in mutton have been partially digested and the food has been converted to a soup like mixture called chyme. after it, the pyloric sphincter allows a little mass of chyme to enter duodenum.

Small Intestine- complete digestion and absorption

Duodenum comprises of the first 10 inches ( 25cm) of small intestine and it is the part of alimentary canal where most of the digestive process occurs. Here, food is further mixed with 3 different secretions:

1* Bile from liver helps in the digestion of lipids through emulsification i.e. by keeping the lipid droplets separate from one another.

2* pancreatic juice from pancreas contains enzymes trypsin, pancreatic amylase and lipase which digest proteins, carbohydrates and lipids respectively.

3* Intestinal juice from intestinal walls contains many enzymes for the complete digestion of all kinds of food.

Next to the duodenum is 2.4 meters long jejunum. it is  concerned with the rest of the digestion of proteins, carbohydrates and lipids of our bite.

Last 3.5 meters long part of small intestine is ileum. It is concerned with the absorption of digested food. There are circular folds in the inner wall of ileum. These folds have numerous finger like projections called villi (singular:villus). Villi increase the surface area of the inner walls and it helps a lot in the absorption of digested food. Each villus is richly supplied with blood capillaries and a vessel of lymphatic system, called lacteal. The walls of villus are only single cell thick. The digested molecules i.e. simple sugars and amino acid are absorbed from intestine into the blood capillaries present in villi. Blood carries them away from small intestine via the hepatic portal vein and goes to liver for filtering.Here, toxin are removed and extra food is stored. From liver, the required food molecules go towards heart via the hepatic vein. Fatty acids and glycerol, present in small intestine, are absorbed into the lacteal of villus. Lacteal carries them to the main lymphatic duct, from where they enter in bloodstream.

Large intestine- Absorption of water and defecation

After the digested products of our bite have been absorbed in blood, the remaining mass enters large intestine. it has 3 parts; caecum ( or pouch that forms the T-junction with small intestine ), colon and rectum. From colon, water is absorbed water into blood. As water is absorbed, the solid remains of food are called faeces. Faeces mainly consist of the undigested material. Large number of bacteria, sloughed off cells of alimentary canal, bile pigments and water are also part of faeces.

Faeces are temporarily stored in rectum, which opens out through anus. Under normal conditions when the rectum is filled up with faeces, it gives rise to a reflect and anus is opened for defecation. this reflex is consciously inhibited in adults but in infants it is controlled involuntarily. During growth, child learns to bring this reflex under voluntary control.

Role of liver 

liver 

Liver is the largest gland of body. It is multi lobed and dark reddish in appearance. It lies beneath the diaphragm on the right side of abdomen. In an adult human , it weighs about 1.5 kg and is the size of a football. A pear shaped greenish yellow sac i.e. gallbladder lies along the right lobe of liver on ventral side.

Liver secretes bile, which is stored in gallbladder contracts,bile is released into  duodenum through common bile duct. Bile has no enzymes but contains bile salt for the emulsifications of lipids. it helps the lipid digesting enzyme to attack on lipids. Besides digestion, liver carries out a number of other functions, some of which are summarized here;

* Removes amino groups from amino acids ( de-amination )

* converts ammonia to a less toxic from i.e urea

* Destroys the old red blood cells.

* Manufactures blood clotting proteins called fibrinogen.

* Convert glucose into glycogen and, when required, breaks glycogen into glucose.

*Convert carbohydrates and proteins into lipids and produces cholesterol.

*Produces heat to maintain body temperature.

* Stores fat-soluble vitamins (a, d, e, and k ) and mineral ions, such as iron