Physiology of Digestion - Mouth, Stomach, Small intestine

Physiology of Digestion

Digestion takes place in three parts of the alimentary canal. They are:

Mouth - with the help of saliva from three pairs of salivary glands.

Stomach - with the help of gastric juice from the stomach wall and

Small intestine - with the help of pancreatic juice from the pancreas, bile juice from the liver and the intestinal juice from the small intestine.

Most of the digestive juices contain chemicals known as enzymes which do the work of breaking down foods chemically.

Digestion in the Mouth

The food that is ingested is broken down mechanically by the help of teeth present. This is known as mastication or chewing. This chewing action is aided by saliva that moistens and lubricates the food, before it can be made into a bolus ready for swallowing. The saliva helps in chemical digestion of the food. Saliva contains the enzyme amylase or ptyalin. Salivary amylase acts on starch which is a complex polysaccharide, breaking it to maltose, a disaccharide.

Starch  -> Salivary amylase -> Maltose

Digestion in the Stomach

The food material after being broken down by mechanical grinding and having been converted into a bolus with the saliva reaches the stomach, which pours a large quantity of gastric juice everyday. The mechanism of production of gastric juice is chemical or hormonal in nature. When the digested food is in contact with the gastric mucosa, the duodenum secretes gastrin a hormone that belongs to the group of gastrointestinal hormones. This causes the discharge of gastric juice.

The gastric juice contain mainly Hydrochloric acid secreted by parietal cells in the gastric glands and enzymes - 1) Pepsin 2) Renin.

Functions of Hydrochloric acid

1.     Kills bacteria present in the food.

2.     Acidifies the food and stops the action of salivary amylase.

3.     Converts inactive form of pepsinogen into active form of pepsin.

Functions of Enzymes

1. Pepsin - Converts protein into peptones, proteases and polypeptides.

2. Renin - Converts the undigestible protein of milk into easily digestible one.

The smooth muscle layers of the stomach enable the gastric contents to be liquefied to chyme. When the chyme is sufficiently acidified and liquefied, the pyloric antrum forces small jets of gastric contents through the pyloric sphincter into the duodenum.

Digestion in the Small Intestine

The small intestine is the organ where completion of chemical digestion of carbohydrates, proteins and fats occur. The small intestine secretes the intestinal juice called succus entericus. This consists of water, mucus, and enzyme enterokinase. Digestion in the small intestine is aided by secretions from the accessory glands such as liver and pancreas.