The nutrition in human beings takes place through human digestive system. It consists of alimentary canal and its associated glands. Various organs of Human digestive system in sequence are: Mouth, Oesophagus (or Food Pipe), Stomach, Small Intestine and Large Intestine. The glands which are associated with this system are: Salivary glands, Liver and Pancreas. The human alimentary canal runs from mouth to anus and is about 9 metres long tube. The ducts of various glands open in to the alimentary canal and pour the secretions of the digestive juices in to the alimentary canal.
Now we will understand the various steps involved in the nutrition of human digestive system of human beings:
1. Ingestion: Food is ingested in human beings through the mouth and it is put in to the mouth with the help of hands.
2. Digestion: In the mouth itself the digestion of food begins in the mouth itself. Process of digestion is as follows: The mouth cavity or buccal cavity contains teeth, tongue and salivary glands. The teeth cut the food in to small pieces, chew and grind it. So, teeth help in the physical digestion. The salivary glands present in our mouth produce saliva and with the help of tongue saliva is mixed with food. As, we know that saliva is a watery liquid so it wets the food in our mouth and helps to swallow it easily. Many a times we have observed that when we see or eat a delicious food our mouth ‘waters’. This is due to the saliva produce by the salivary glands. The salivary glands help in chemical digestion by secreting enzymes. Human saliva contains an enzyme known as salivary amylase which digests the starch present in food in to sugar. Thus, the digestion of starch or carbohydrate begins in the mouth itself. But food remains for very short time in mouth so, digestion of food remains incomplete in mouth.
Now, the slightly digested food goes to stomach through food pipe i.e oesophagus. This happens as follows: The walls of food pipe have muscles which can contract and expand alternately. When slightly digested food enters the food pipe, the walls started movements of contraction and expansion and this movement is known as peristaltic movement. And this peristaltic movement pushes food inside the stomach.
The Human Digestive System
The stomach is J- shaped organ present on the left side of the abdomen. The food is churned in the stomach for about three hours. During this time, the food breaks down in to still smaller pieces and forms a semi-solid paste. The glands present in the stomach walls secrete gastric juice and it contains three substances: hydrochloric acid, the enzyme pepsin and mucus. Due to the presence of hydrochloric acid food is acidic in nature and the enzyme pepsin begins the digestion of proteins present in food to form smaller molecules. Thus, the protein digestion begins in the stomach.
Function of Hydrochloric acid is:
(i) It makes pepsin enzyme active.
(ii) It kills any bacteria which may enter in stomach with food.
Mucus helps to protect the stomach wall from its own secretions of hydrochloric acid otherwise it may cause ulcers in the stomach. The partially digested food goes from the stomach to small intestine. The exit of food from stomach is regulated by ‘sphincter muscle’ which release it in small amounts in to the small intestine.
The small intestine is the largest part of the alimentary canal. It is about 6.5 metres long in an adult man. It is known as small intestine because it is very narrow. The small intestine is arranged in the form of a coil in our belly. The small intestine in human beings is the site of complete digestion of food like carbohydrates, proteins and fats. This happen as follows:
(i) The small intestine receives the secretions of two glands: Liver and pancreas. Liver secretes bile. Bile is a greenish yellow liquid made in the liver which is normally stored in the gall bladder. Bile is alkaline and contains salts which help to emulsify or break the fats or lipids present in the food. It makes the acidic food alkaline which comes from stomach so that pancreas can act on it and it also break the fats present in the food in to small globules making it easy for the enzymes to act and digest them. Pancreas is a large leaf like gland which lies parallel to and beneath the stomach. Pancreas secretes pancreatic juice which contains digestive enzymes like pancreatic amylase, trypsin and lipase. Amylase breaks down the starch, trypsin digests the proteins and lipase breaks down the emulsified fats.
(ii) The walls of small intestine contain glands which secrete intestinal juice. The intestinal juice contains a number of enzymes which complete the digestion of complex carbohydrates in to glucose, proteins in to amino acids and fats in to fatty acids and glycerol. Glucose, amino acids, fatty acids and glycerol are small, water soluble molecules. In this way, the process of digestion converts the large and small insoluble food molecules in to small, water soluble molecules. The chemical digestion of food is brought about by biological catalysts called enzymes.
3. Absorption: After digestion the molecules of food becomes small and passes through small intestine and goes in to our blood. So, we can say that small intestine is the main region for the absorption of digested food. The inner surface of small intestine has millions of tiny, finger like projections called villi which gives large surface area for absorption and the absorbed food goes in to our blood.
4. Assimilation: The blood carries digested and dissolved food to all the parts of the body where it becomes assimilated as part of the cell. This assimilated food is used by the body cells for obtaining energy as well as for growth and repair of the body. The undigested food stored in the liver in the form of carbohydrate called ‘glycogen’ and can be used by the body during requirement.
5. Egestion: A part of the food which we eat cannot be digested by our body. This undigested food cannot be absorbed in the small intestine. So, undigested food passes from small intestine to large intestine. The walls of large intestine absorb most of the water from this food and become solid. Last part of the large intestine called ‘rectum’ stores this undigested food for some time and finally, egested from our body through anus as faeces or ‘stool’. This process is known as egestion or defecation.
The formation of small cavities or holes in the teeth is due to the action of acid-forming bacteria and improper dental care is called dental caries. If the teeth are not cleaned regularly, they become covered with a sticky, yellowish layer of food particles and bacteria cells called ‘Dental Plaque’.
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