Human digestive system - Description, Parts and Functions - CBSE Tuts (2024)

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What is digestion ? and How digestion process takes place in humans?

When we eat food, it gets broken down into smaller and soluble substances during digestion. The digested food is absorbed and utilised by our body. The undigested and unabsorbed portion of food is removed from the body in the form of waste material called faeces. We will now discuss the human digestive system in detail.

The human digestive system consists of the alimentary canal and its associated glands. The various organs of the human digestive system in sequence are : Mouth (Buccal cavity), Oesophagus (or Food pipe), Stomach, Small intestine, Large intestine, Rectum and Anus. The glands which are associated with human digestive system and form a part of human digestive system are : Salivary glands, Liver and Pancreas. Salivary glands are located in our mouth (or buccal cavity).

Liver is a reddish-brown gland situated in the upper part of abdomen on the right side. Liver is the largest gland in the body. Pancreas is a large, cream coloured gland located just below the stomach. The ducts (or pipes) of various glands open into the alimentary canal and pour the secretions of their digestive juices into the alimentary canal. The human digestive system is shown in Figure. We will now describe how food gets digested when it passes through the various parts of alimentary canal.

1. In the Mouth (or Buccal Cavity)

Food is taken into the body (or ingested) through the mouth. The digestion of food starts as soon as we put the food in our mouth. The mouth (or buccal cavity) contains teeth, tongue and salivary glands. The teeth cut the food into small pieces, chew and grind it. The salivary glands secrete a watery liquid called saliva. The tongue helps in mixing saliva with food. Saliva is a digestive juice which helps to digest the,starch present in the food partially. The slightly digested food is swallowed by the tongue and goes down into oesophagus (or food pipe) (see Figure).

2. In the Oesophagus (or Food Pipe)

The oesophagus is a tube (or pipe) which connects the mouth (or buccal cavity) to stomach. Oesophagus is commonly known as food pipe. It runs along the neck and chest. Oesophagus carries the slightly digested food from the mouth to the stomach (see Figure). The food coming from mouth moves down through oesophagus by peristalsis.

This happens as follows : When the food enters oesophagus at the top end, the muscles in the walls of oesophagus start alternate contractions and relaxations producing a wave-like movement which pushes the food downwards towards the stomach. (During vomiting, however, the food moves in the opposite direction, from stomach to mouth, by the process of anti-peristalsis in the food pipe).

3. In the Stomach

The stomach is a thick walled bag present on the left side of the abdomen (see Figure 2). It is the widest part of the alimentary canal. Oesophagus (or food pipe) brings the slightly digested food from the mouth into stomach. The food is further digested in the stomach. The food is churned in the stomach for about three hours. During this time, the food breaks down into still smaller pieces and makes a semi-solid paste. The inner lining of stomach secretes mucus, hydrochloric acid and digestive juices. Mucus protects the lining of stomach (from its own secretions of hydrochloric acid).

Hydrochloric acid kills any bacteria which may enter the stomach with food. Hydrochloric acid also makes the medium in the stomach acidic (which is necessary for the proper action of digestive juices on proteins in the stomach). The digestive juices secreted by the stomach lining break down the proteins present in our food into simpler substances (This happens in the acidic medium). The partially digested food then goes from the stomach into the small intestine.

4. In the Small Intestine

The small intestine is a very long tube. It is about 7.5 metres long. Though small intestine is very long, it is called small intestine because it is a quite narrow tube. The small intestine is arranged in the form of a coil in our belly (see Figure). The small intestine in human beings is the site of complete digestion of food. The small intestine is also the main region for the absorption of digested food. This happens as follows.

(a) Complete Digestion of Food in Small Intestine. The partially digested food from stomach comes into small intestine. The small intestine receives secretions of digestive juices from the liver, pancreas and its own walls. All these digestive juices carry out the complete digestion of food as follows : Liver secretes a liquid called bile (which is stored temporarily in the sac called gall bladder). The bile plays an important part in the digestion of fats. Actually, bile converts fats into tiny droplets so that their further breakdown becomes easy.

Pancreas secretes pancreatic juice. Pancreatic juice breaks down fats completely into fatty acids and glycerol. Pancreatic juice also breaks down starch carbohydrate and proteins into simpler forms. The walls of small intestine secrete a digestive juice called ‘intestinal juice’. Intestinal juice breaks down the starch carbohydrate completely into the simplest sugar called glucose, and the proteins into amino acids. In this way, our food breaks down completely into very small, water soluble substances like glucose, fatty acids, glycerol and amino acids. This is called digested food.

(b) Absorption of Digested Food in Small Intestine. The digested food can now pass into the blood vessels in the walls of the small intestine. This process is called absorption. The small intestine is especially adapted for absorbing the digested food. This happens as follows : The inner surface of the small intestine has millions of tiny, finger-like outgrowths called villi (read as vee-la-ee ; singular of villi is villus). The presence of villi gives the inner walls of the small intestine a very large surface area (which helps in the rapid absorption of food). Thus, the role of villi in the small intestine is to increase the surface area for the rapid absorption of digested food.

Each villus has a network of thin and small blood vessels (called blood capillaries) close to its surface. The surface of villi absorbs the digested food materials into blood flowing through them. Blood carries the absorbed food materials to the cells in all the parts of the body. In the cells, food is used for energy, growth and repair. This is called assimilation. Glucose breaks down in the cells with the help of oxygen to form carbon dioxide and water, and releases energy. Fatty acids and glycerol build components of cells and form fats to be stored in the body as food reserves. Amino acids are used to make proteins required for the growth and repair of the body.

A part of the food which we eat cannot be digested by our body. This undigested food cannot be absorbed in the small intestine. The food that remains undigested and unabsorbed passes from the small intestine into large intestine.

5. In the Large Intestine

The large intestine is about 1.5 metres long. It is called large intestine because it is a quite wide tube. The undigested and unabsorbed food from the small intestine enters into large intestine. The large intestine absorbs most of the water from the undigested food material. Due to the removal of water, the undigested food becomes semi-solid. This undigested food (or waste material) is stored in the last part of the large intestine called rectum for some time. When we go to toilet, the undigested, semi-solid waste is passed out from our body through anus in the form of faeces. This is called egestion.

Activity
To Study the Effect of Saliva on Starch Present in Food
Our mouth contains salivary glands which secrete saliva. During digestion, saliva breaks down complex carbohydrate ‘starch’ present in food into a ‘sugar’. We can observe the effect of saliva on starch containing food by performing a simple activity as described below. Since our common food ‘rice’ contains starch, so we will use boiled rice as a source of starch in this activity.

Take one teaspoonful of crushed boiled rice in a beaker and add about 20 rhL water to it. Stir with a glass rod. In this way, we will get a mixture of crushed boiled rice with water which contains starch carbohydrate. Divide this mixture into two parts.

(i) Take one part of crushed boiled rice and water mixture in a test-tube and add 2 or 3 drops of dilute iodine solution to it. A blue-black colour is produced [see Figure (a)]. This shows that boiled rice contains starch.

(ii) Take the second part of crushed boiled rice and water mixture in another test-tube and add saliva from mouth into it. Keep this test-tube in warm water for about 15 to 20 minutes. Then add 2 or 3 drops of dilute iodine solution. No blue-black colour is produced in this case [see Figure (b)]. This shows that there is no starch in the boiled rice which has been treated with saliva.

The above observation can be explained on the basis of action of saliva on starch as follows : When the mixture of boiled rice and water is treated with saliva, then saliva breaks down the starch present in boiled rice to form a sugar. Since no starch is left in the test-tube after treatment of boiled rice with saliva, therefore, no blue-black colour is produced on adding dilute iodine solution. Thus, saliva breaks down starch into sugar (The sugar which is formed by the breakdown of starch with saliva is called maltose).

Why Do We Get Hiccups

The windpipe (called trachea) carries air from nostrils to our lungs. And food pipe (called oesophagus) carries food from mouth to stomach. The windpipe runs adjacent to the food pipe in the body. Inside the throat, however, air and food share a common passage. Then how the food which we eat is prevented from entering our windpipe ? This happens as follows : Normally, when we swallow food, then a flat piece of cartilage (called epiglottis) drops over the mouth of windpipe forming a bridge over which food passes and goes into food pipe (without falling into windpipe). Sometimes, however, when we eat too fast in a hurry, or talk too much or laugh while eating, then a little of windpipe remains open due to which food particles may enter the windpipe. When food particles enter the windpipe we may get hiccups (or a choking sensation). Hiccups produce a characteristic gulping sound’ repeatedly. Hiccup is called ‘hichki in Hindi. The blockage in windpipe caused by food particles is usually cleared by coughing.