Digestion of food in human beings: Digestion of food begins in the mouth. The mouth cavity contains teeth, tongue and salivary glands. The teeth cut the food into small pieces, chew and grind it. This is called physical digestion. Salivary glands produce saliva which mixes with the food. This involves chemical digestion of food. The saliva contains an enzyme called salivary amylase which digests the starch and converts it into maltose sugar. Mouth opens into a small funnel shaped area called pharynx which leads to a long tube called oesophagus. It carries the food down into the stomach.
The wall of oesophagus is muscular. When the slightly digested food enters the food pipe, the walls of the oesophagus starts contraction and expansion movements called peristaltic movements which push the food into the stomach. Digestion does not take place in the oesophagus. The glands present on the walls of the stomach secrete gastric juice that contains hydrochloric acid, the enzyme pepsin and mucus. A small amount of gastric lipase is also present that breaks down the fats present in the food. Gastric juice is acidic due to the presence of HCl which is necessary for the pepsin to become active and converts the proteins into peptones.
The mucus protects the stomach walls from HCl. From the stomach, the partially digested food goes into the small intestine through sphincter muscle. Small intestine is divided into two parts: Duodenum and Ileum.
Duodenum receives the secretions of two glands, liver and pancreas through a common duct. Liver secretes bile which is alkaline and contains salts to emulsify the fats (or lipids). The bile secreted by the liver is stored in the gall bladder. Pancreas secretes pancreatic juice which contains trypsin, lipase and pancreatic amylase. Trypsin digests the proteins, lipase emulsifies the fats and pancreatic amylase breaks down the starch. Thus, small intestine is the site of complete digestion of carbohydrates, proteins and fats. The walls of ileum secrete succus entericus which completes the digestion process.