Human Digestive System

Food is not directly utilized by our body its original form. Thus, they are subjected to a process called digestion and the process which helps to complete process of digestion 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 metre 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 silt bounded by two 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 the buccal (mouth) cavity is called palate. The upper and lower jaws of the buccal cavity consists of two separate sequences of teeth.

Teeth

These are hard structures present in the mouth and both the jaws (i.e., upper and lower jaw). Each tooth is embedded in a socket of jaw bone. A tooth consists of 3 major parts, namely crown (upper part), neck (middle part) and root (lower part).

In human beings, the number of teeth present is 32, 16 teeth present in each jaw. The half teeth of the jaw is towards left while the rest half is towards right. Arrangement of these teeth include two incisors, one canine, two premolars and three molars on each side (half).

Each of the tooth is specalised to perform a particular function. The main function of teeth is to perform physical digestion.

(a) Incisors are outer mostly forward, flatten and extremely sharp, which help in biting or cutting the food.

(b) Canines are sharply pointed which crack and split or tear the food.

(c) Premolars and molars, crush, grind and chew food smoothly.

Terms related to Teeth

Enamel    The outermost, shining layer in the crown region is called enamel. It is the hardest substance in the human body.

Cement Layer    The outermost layer in the neck and the root region is called cement layer

Pulp Cavity    A cavity in the centre of the tooth containing pulp (mass of cells, blood vessels, lymph vessels and nerves) is called pulp cavity.

Dentine    The layer made up of hard ivory like substance, which surrounds the pulp cavity is called dentine.

4.Tongue

It is a 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.

Functions

The tongue helps in tasting and swallowing of food. It also helps in the process of speech and masticating the food by mixing saliva in it.

Everyday Science

Women have more taste buds than men.

The blue whale has the largest tongue of all animals, i.e., about 2.7 metric tonne.

Dogs and Cats often use their tongue to clean their fur and body removing oil and parasites.

85% of the population can curl their tongue into a tube.

5.Pharynx

It is 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 trachea. During swallowing of food, trachea is covered by epiglottis (a cartilagenous flap or lid) to prevent the entry of food into trachea (wind pipe).

6.Oesophagus

It is also called as food pipe. 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 the oesophagus helps in the forward movement of food. A muscular gastro-oesophageal sphincter regulates the opening of oesophagus into the stomach.

Function  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 J-shaped organ which churns, breaks up food and mix the pieces with gastric juice (include enzyme like renin, pepsin and HCl).

The inner lining of the stomach secretes various components like mucous, Hydrochloric acid and digestive juices. The mucous lining of stomach protects it from the acidic environment and allow it to work firmly.

Stomach has three major parts as given below

(a) Cardiac   stomach, the upper portion into which the oesophagus opens.

(b) Fundic    stomach, the middle portion.

(c) Pyrolic   stomach, the lower portion which opens into the first part of small intestine, (i.e., opening of stomach into duodenum) is guarded by pyrolic sphinter.

Functions

(i) It acts as a short term reservoir of food.

(ii) Food become liquified in the stomach before being released into the small intestine.

8. Intestine

Small Intestine

It is the longest part of the alimentary canal. It is about 6 metre long.

It is divisible into 3 main parts as

(i) Duodenum  It is U-shaped, about 25 cm long and it is the widest part of the small intestine.

(ii) Jejunum  It has a diameter of about 4 cm. It is the middle part of the small intestine  and is about 2.5 metre long.

(iii) Ileum It is the last part with diameter around 3.5cm. Its wall is thinner than that of jejunum. It is the longest part of small intestine. Ileum opens into the colon of large intestine.

Functions   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.

Villi   The inner mucosa layer of small intestine has villi (about 1mm in height), covered with columnar epithelial cells.

Microvilli    Numerous microscopic projections of microvilli are produced by the cell lining of villi. These microvilli gives it a brush border appearance and enormously increase the surface area of the intestine.

Large Intestine

Although it is shorter, but its diameter is larger than that of the small intestine thus, it is known as large intestine. It is about 1.5 metres long and is divisible into three main part as

(i) 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.

(ii) Colon  The caecum part leads into the colon. The colon has three main parts as ascending, transverse and descending part.

(iii) Rectum  The descending portion of colon leads into the rectum which is the last part of our the intestine. Rectum is of about 20 cm in length and opens into anus.

Functions  The absorption of water and nutrients and the elimination of semisolid wastes takes place mainly in large intestine.

8.Anus

The semi-solid waste is excreted from anus.

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