Objective

This topic gives you an overview of;

• Digestion in Humans.
• The mouth and Buccal Cavity.
• The foodpipe / oesophagus.
• The stomach
• Absorption in the Small Intestine
• Large Intestine

Digestion in Humans

We take in food through the mouth, digest and utilise it. The unused parts of the food are defecated.Have you ever wondered what happens to the food inside the body? The food passes through a continuous canal (Fig. 2.2) which begins at the buccal cavity and ends at the anus. 

The canal can be divided into various compartments: (1) the buccal cavity, (2) foodpipe or oesophagus, (3) stomach, (4) small intestine, (5) large intestine ending in the rectum and (6) the anus. Is it not a very long path?

These parts together form the alimentary canal (digestive tract). The food components gradually get digested as food travels through thevarious compartments.

The inner walls of the stomach and the small intestine, and the various glands associated with the canal such as salivary glands, the liver and the pancreas secrete digestive juices. The digestive juices convert complex substances of food into simpler ones. The digestive tract and the associated glands together constitute the digestive system.Now, let us know what happens to the food in different parts of the digestive tract.

 The mouth and Buccal Cavity

Food is taken into the body through the mouth. The process of taking food intothe body is called ingestion. We chew the food with the teeth and break it down mechanically into small pieces. Each tooth is rooted in a separate socket in the gums.Our teeth vary in appearance and per for m dif ferent functions. Accordingly they are given different names.

 The foodpipe / oesophagus

The swallowed food passes into the foodpipe or oesophagus. 

The foodpipe runs along the neck and the chest. Food is pushed down by movement of the wall of the foodpipe. Actually this movement takes place throughout the alimentary canal and pushes the food downwards  At times the food is not accepted by our stomach and is vomited out. 

The stomach

The stomach is a thick-walled bag. Its shape is like a flattened U and it is the widest part of the alimentary canal. It receives food from the food pipe at one end and opens into the small intestine at the other.

The inner lining of the stomach secretes mucous, hydrochloric acid and digestive juices. The mucous protects the lining of the stomach.The acid kills many bacteria that enter along with thefood and makes the medium in the stomach acidic and helps the digestive juices to act.The digestive juices break down the proteins into simpler substances.

The small Intestine

The small intestine is highly coiled and is about 7.5 metres long. It receives secretions from the liver and the pancreas. Besides, its wall also secretes juices.The liver is a reddish brown gland situated in the upper part of the abdomen on the right side. It is the largest gland in the body. It secretes bile juice that is stored in a sac called the gall bladder.

The bile plays an important role in the digestion of fats.The pancreas is a large cream coloured gland located just below the stomach.The pancreatic juice acts on carbohydrates, fats and proteinsand changes them into simpler forms.The partly digested food now reaches the lower part of the small intestine where the intestinal juice completes the digestion of all components of the food. The carbohydrates get broken into simple sugars such as glucose, fats into fatty acids and glycerol, and proteins into amino acids.

Absorption in the Small Intestine

The digested food can now pass into the blood vessels in the wall of the intestine. This process is called absorption. The inner walls of the small intestine have thousands of finger -like outgrowths. These are called villi (singular villus).The villi increase the surface area for absorption of the digested food.

Each villus has a network of thin and small blood vessels close to its surface. The surface of the villi absorbs the digested food materials. The absorbed substances are transported via the blood vessels to different organs of the body where they are used to buildcomplex substances such as the proteins required by the body. This is called assimilation. In the cells, glucose breaks down with the help of oxygen into carbon dioxide and water, and energy is released. The food that remains undigested and unabsorbed enters into the large intestine.

Large Intestine

The large intestine is wider and shorter than small intestine. It is about 1.5 metre in length. Its function is to absorb water and some salts from the undigested food material. The remaining waste passes into the rectum and remains there as semi-solid faeces. The faecal matter is removed through the anus from time-to-time. This is called egestion.

 Summary:

• The human digestive system consists of the alimentary canal and secretory glands.
• It consists of the (i) buccal cavity, (ii) oesophagus, (iii) stomach, (iv) small intestine, (v) large intestine ending in rectum and (vi) anus.
• The main digestive glands which secrete digestive juices are (i) the salivary glands, (ii) the liver and (iii) the pancreas.
• The stomach wall and the wall of the small intestine also secrete digestive juices.