Digestive System or Alimentary Canal of Man

The alimentary canal is a long tube stretching from the mouth to the anus. The digestive system or alimentary canal of man includes: the mouth, oesophagus, stomach, duodenum, small intestine or ileum, caecum, appendix, large intestine or colon, rectum and anus. 

[Photo credit: Bruce Blaus]

The description and importance of the parts are as follows:

Mouth: The alimentary canal of man starts from the mouth. The mouth contains the teeth, salivary gland and tongue.

The teeth

• They are used to cut and grind food into tiny particles. 
• They expose large area of food for the action of enzymes.

The tongue

• The tongue rolls the food into bolus. 
• It aids movement of food in the mouth. 
• It allows the mixing of food with saliva or ptyalin. 
• It aids swallowing of food into the oesophagus.

The salivary gland

• The salivary gland secretes saliva which contains enzyme called ptyaline. The ptyaline breaks down starch into maltose which is later swallowed into the gullet in form of bolus.
• It allows easy chewing or movement of food in the mouth before swallowing.
• It also serves as solvent for food.

The saliva is slightly alkaline

Oesophagus: The oesophagus connects the mouth to the stomach. The food swallowed is passed down through the oesophagus by a peristalitic movement into the stomach.

Stomach: In the stomach, the food is temporarily stored for few hours and it is released at regular intervals by the opening of the pylotic sphincter. In the stomach, the gastric gland secretes gastric juice which contains two enzymes - renin and pepsin.

The renin acts on milk, while the pepsin breaks down proteins to peptones. The gastric gland also secretes hydrochloric acid which creates an acidic medium for the two enzymes to act and also helps to kill some bacteria in the stomach. 

The food is churned by by muscular contraction of the stomach wall (churning movement) which enables the mixing of food with digestive juice. The churning movement then converts the food into a semi-liquid state called chyme.

Duodenum: The duodenum contains pancreas which secretes pancreatic juice that contains three enzymes: 

• Amylase: This converts starch to maltose.
• Lipase: This converts fats and oil to fatty acid and glycerol.
• Trypsin: It converts proteins and peptones to polypeptides.

The pancreatic juice is alkaline and provides that medium for enzymes. At the end of digestion in the duodenum, the food now in liquid form called chyle passes to the small intestine.

Small intestine: This is found between the duodenum and the large intestine. Two major events take place in the small intestine: Digestion and absorption of the digested food.

(i) Digestion: The digestion of food also takes place in the small intestine. The intestinal wall secretes intestinal juice which contains the following enzymes - lipase, erepsin, maltase, sucrase and lactase. 

The lipase converts fats and oil to fatty acid and glycerol; erepsin converts polypeptides to amino acids; maltase converts maltose to two untis of glucose; sucrase converts sucrose to glucose and frucose; while lactase converts lactose to glucose and galactose.

The final digestion ends in the small intestine. The end products in the digestion of protein are amino acids; that of fats and oil are fatty acids and glycerol; and that of starch is glucose.

(ii) Absorption of the digested food: The end products of food are absorbed in the small intestine by tiny finger-like structures called villi (singular: villus). The folding of the small intestine combined with the presence of numerous villi creates a large surface area for the absorption of digested food.

The inner surface layer or epithelium of each villus is thin. This allows the absorption of the end products by either diffusion or active transport through it. The glucose and amino acids are easily absorbed by blood capillaries which eventually empty their content into the blood vessels near the heart.

The blood then carries the fats and other food materials to various parts of the body where they are needed.

Caecum and Appendix: In man, the function of caecum and appendix are not well known but the caecum usually contains some bacteria which aid minor digestion of cellulose. Some vitamins K and B-complex are partially synthesized in this region. 

Large Intestine: The undigested food passes into the colon or large intestine. Here, water is absorbed. This absorption of water concentrates the waste products and turn them into faeces. The faeces is passed into the rectum and finally out of the body through the anus.

Categories: Biology, Digestion, Digestive System