The Human Digestive System

The human digestive system is the system of organs that take part in breaking down of food to release energy and absorb nutrients.

It consists of two parts: alimentary canal and digestive glands.
Alimentary canal(or gut) is the path which food takes from the mouth to the anus.
Digestive glands are associated with it and secrete digestive juices and enzymes to help in digestion.
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Journey of Food

Mouth

When we take in food, it enters our mouth. Our mouth is the anterior opening of the digestive system. Digestion of food begins here. The space in the mouth is known as buccal cavity. The Teeth, tongue and salivary glands are present here.

Teeth

An adult person has 32 permanent teeth and 16 in each jaw. There are four types of teeth in humans: incisors, canines, premolars and molars. We have 8 incisors, 4 canines, 8 premolars and 12 molars in our mouth. Incisors are used for cutting food, canines for tearing food whereas premolars and molars are used for grinding and crushing food. The upper layer of teeth consists of enamel, the hardest substance in the body. The process of chewing food with help of teeth and jaws is known as mastication.

Tongue

The tongue is a muscular organ in the mouth that has taste buds on its surface. It helps in mixing the food with saliva and swallowing the food.

Salivary glands

There are three pairs of salivary glands which secrete saliva into the mouth. This saliva softens the food and helps for chewing. Moreover, this contains an enzyme called salivary amylase that helps to convert the starch into sugar in the mouth.

While swallowing our food is pushed into the pharynx from which food starts it's journey through oesophagus.

Oesophagus

Oesophagus is a long tube like structure that allows a passage from the mouth to the stomach. Here, no digestion takes place. The food is pushed downwards by the contraction of muscles above and relaxing of muscles below. This series of wave like movements pushing the food down the oesophagus is know as peristalsis.

Stomach

The Stomach is a J shaped organ of the digestive system that lies on the left side of abdomen. This is the part where food gets partially digested. The Stomach is a muscular organ that is flexible and can expand to hold more food. Stomach's walls produce gastric juice and mucus. The mucus protects the stomach from action of gastric juice. Gastric juice contains enzymes like Pepsin and Hydrochloric acid(HCl). Pepsin changes protein into peptons and Hydrochloric acid kills the bacteria and makes the medium of stomach acidic. The juice mixes with the food and begins to digest it by churning it slowly. After this, food moves to the small intestine.

Liver and Gallbladder

Liver is the largest organ of our body located just above the stomach. It secretes Bile juice that is stored inside a small pear shaped organ called gallbladder. This juice does not contain any enzymes and is helpful in breaking down of fats into tiny droplets that can be digested easily.

Pancreas

The Pancreas is a leaf shaped organ just below the stomach that has a very important role to play. It secretes Pancreatic juice which contains Pancreatic amylase, Trypsin, and Lipase. Pancreatic amylase converts carbohydrates into glucose, Trypsin breaks down proteins into peptides and Lipase breaks down fat into fatty acids and glycerol. Lipase is activated by Bile juice.

Small intestine

This is the longest part of digestive system with a length of 6-7 metres. This is divided into three parts: duodenum, jejunum and ileum. The liver, the pancreas and gallbladder pour their juices into the small intestine. Besides, small intestine also secretes intestinal juice from its intestinal glands.
The intestinal juice contains enzymes like Peptidase that breaks peptides into amino acids, Sucrase that changes sucrose into glucose and fructose, Lactase that changes lactose in glucose and galactose and Maltase that changes maltose into glucose.
The small intestine has tiny finger like projections called villi. These have blood capillaries in them which speed up absorption of food into the blood. Here, digestion is completed.

Large Intestine

It is comparatively wider than small intestine and absorbs the leftover water and salts. Escherichia coli and other bacteria present in the large intestine produce moderate quantities of Vitamin B and Vitamin K. The appendix, cecum, colon and rectum are part of large intestine. Appendix is fixed to the cecum that is the first part of large intestine. Then comes the colon and then rectum. The rectum stores the undigested food material or faeces for some time.

Anus

It is the opening at the end of the digestive system through which faeces are expelled.