HUMAN DIGESTIVE SYSTEM

Biomacromolecules which we consume in our food are not directly utilised by our body in its original form. Thus, they are subjected to a process called digestion and the system that helps in complete process of digestion by mechanical and biochemical methods is called digestive system.

The human digestive system consists of various parts(organs and glands) that are concerned with the uptake, digestion and elimination of indigestible remains of food from the body.

ALIMENTARY CANAL

The alimentary canal in human beings is a long tube(about 8 to 10 metres in length) which begins with an anterior opening i.e,. mouth and ends posteriorly through the anus. The various parts associated with the alimentary canal are as follows:

1- MOUTH

It is a transverse slit bounded by soft, movable lips which are covered with skin on the outer side and lined with mucous membrane on the inner side.

2- VESTIBULE

Mouth mainly leads to vestibule which is a narrow space enclosed between the lips and cheeks externally and the gums and teeth internally. Its lining contains mucous glands.

3- BUCCAL CAVITY

It is bounded by lips and cheeks. It contains teeth, tongue and salivary glands. Mouth(buccal) passes through both the jaws. The uppermost portion of buccal cavity is called palate. The upper and lower jaws of the buccal cavity consists of two separate sequences of teeth.

4- TONGUE

 

It is highly muscular organ containing voluntary muscles attached to the floor of buccal cavity with the help of a connective tissue(frenulum linguae). There are taste buds present on the tongue to realise the nature of the food like sweet, bitter, salt and sour. The tongue helps in tasting and swallowing the food. It also helps in the process of speech  and masticating the food by mixing it with saliva.

5- PHARYNX

It is a small(12 cm long) vertical canal beyond the soft palate of the oral cavity. It acts as a common passage for both air and food i.e,. it communicates with both oesophagus and treachea. During swallowing of food, trachea is covered by epiglottis to prevent the entry of food into treachea(wind pipe).

6- OESOPHAGUS

It is a thin, long highly muscular and purely conducting (23 to 27 cm long) tube. It opens in stomach. Its opening is called gullet and carries food to it. The secretion of fluid from the mucous glands present in the wall of oesophagus helps in the forward movement of food. A muscular gastro-oesophageal sphincter regulates the opening of oesophagus into the stomach. It transfers food from the pharynx to the stomach.

7- STOMACH

It is the widest organ and the most dilated organ of the alimentary canal. The stomach is a J-shaped organ which churns, breaks up food and mix the pieces with gastric juice (include enzymes like Renin, Pepsin and HCl). The mucous lining of the stomach protects it from the acidic environment and allows it to work firmly. It acts as a short term reservoir of food. Food liquifies in the stomach before being released into the small intestine.

Stomach has three major parts as given below:

1. Cardiac stomach, the upper portion into which the oesophagus opens.
2. Fundic stomach, the middle portion.
3. Pyrolic stomach, the lower portion which opens into the first part of small intestine, i.e,. duodenum.

8- SMALL INTESTINE

 

It is the longest part of the alimentary canal. It is about 6 metres long. It is divisible into three main parts as:

• DUODENUM- It is U-shaped, about 25 cm long and is the widest part of the small intestine.

• JEJUNUM- It has a diameter of about 4 cm. It is the middle part of the small intestine and is about 2.5 metres long.

• ILEUM- It is the last part with diameter around 3.5 cm. Its wall is thinner than that of jejunum. It is the longest part of small intestine. It opens into the colon of large intestine.

Small intestine completes digestion of components like proteins, carbohydrates, nucleic acid and fats. Thus acts as a major site in digestion of food. It absorbs nutrients into the blood and lymph and also helps in absorption of fats.

9- LARGE INTESTINE

Although it is shorter, but its diameter is larger than that of the small intestine. The absorption of water and nutrients and elimination of solid wastes takes place mainly in large intestine. It is about 1.5 meters long and is divisible into three main parts:

 

• CAECUM- It is a small pouch like structure of about 6 cm. It also has an outgrowth known as vermiform appendix, which is slightly coiled tube of about 8 cm long.

• COLON- The caecum part leads into the colon. The colon has three main parts as ascending, transverse and descending part.

• RECTUM- The descending portion of colon leads into the rectum which is the last part of the intestine. It is about 20 cm in length and opens into the anus.

 

POINTS TO REMENBER

1- Human stomach cannot digest cellulose containing food such as raw grasses, leaves etc.

2- The stomach of an adult can hold 1.5 litres of material.

3- The lining of human stomach is replaced every 36 hrs.

4- The stomach can breakdown goat milk faster than cow milk.

5- The inner mucosa layer of small intestine has villi(about 1mm in height).

 

6- Numerous microscopic projections of microvilli are produced by the cell lining of villi.