Digestion Of Food

Digestion in the Buccal Cavity: Food ingested in the buccal cavity is chewed or masticated with the teeth. The teeth are well suited for this function, being exceedingly hard. The incisors are used for cutting the food while premolar and molar are for crushing food. The canines are poorly developed in humans but well developed in carnivores, which use it to hold unto its prey tear flesh. While the food is being chewed, it mixes saliva secreted by the salivary glands. The flow of saliva is initiated by the sight, smell, taste or thought of food. Saliva contains mucus and the enzyme ptyalin or salivary amylase, which hydrolyses cooked starch to maltose. Saliva is neutral or slightly alkaline, which is the optimum pH for the action of ptyalin. The mucus softens the food for easy swallowing. The tongue rolls the food into a bolus, which is swallowed into the stomach. The bolus moves by peristalsis through the oesophagus. The opening of the larynx, the glottis, is guarded by the valve-like epiglottis, which prevents food from entering the trachea and directs it into the oesophagus.
Digestive System of Humans

Gastric glands in the thick stomach wall secrete gastric juice. which contains the enzymes: pepsin and rennin and hydrochloric acid. The hydrochloric acid is secreted by the special oxyntic cells in the gastric gland. Pepsin is secreted as pepsinogen, which is activated by hydrochloric acid to pepsin. Rennin is secreted as inactive prorennin and it is also activated to rennin by the hydrochloric acid. Pepsin hydrolyses proteins into short chain polypeptides. Rennin coagulates the soluble milk protein, caseinogen, forming insoluble or solid casein, which can then be hydrolysed by pepsin.

The presence of food in the stomach stimulates the stomach walls which cause the secretion of the hormone gastrin. Gastrin circulates through the blood and causes the gastric glands to secrete larger quantities of gastric juice. Secretion of gastric juice, like saliva, is also stimulated by the sight, smell, taste or expectation of food.

The wall of the stomach contracts rhythonically to mix gastric juice with the food to form a semi-solid chyme.

Through the peristaltic contraction of the stomach, the chyme moves into the duodenum, the first loop of the small intestine. The pyloric sphincter or pylorus, a ring of muscles situated at the junction between the stomach and the ,duodenum, controls the passage of the chyme into the duodenum. This is achieved by the alternate contraction and relaxation of the pylorus.

The duodenum is the main seat of digestion in the alimentary canal. The liver, pancreas and wall of the small intestine, that is the duodenum and ileum, produce digestive juices which help the process of digestion. The liver produces bile, from the break down of wornout or dead red blood cells, which is stored in the gall bladder. The bile, which contains bile salts, Sodium taurocholate and glycocholate, and bile pigments, billirubin and billiverdin, passes through the bile duct into the duodenum. The bile salts emulsify fats by lowering their surface tension and causing them to break into smaller droplets. Bile also contains Sodium hydrogen trioxocarbonate (IV), or Sodium bicarbonate, which neutralizes the Hydrochloric acid from the stomach and creates a suitable pH for enzymes to function in the duodenum.

Pancreatic juice secreted by the pancreas passes through the pancreatic duct, which opens into the bile duct, into the duodenum. Pancreatic amylase, contained in pancreatic juice, hydrolyses starch into maltose. Trypsin is secreted as inactive trypsinogen, is activated by the enzyme, enterokinase; secreted by the wall of the small intestine. Trypsin, an endopeptidase, hydrolyses proteins into polypeptides. Pancreatic lipase hydrolyses fats and oils into fatty, acids and glycerol. which are the end products of lipid digestion and can thus be absorbed.

Secretory cells in the wall of the small intestine produce. intestinal juice, which contains mucus and a variety of enzymes that complete the process of digestion. These enzymes include: maltase, which digests or ~ hydrolyse maltose into glucose and fructose; lactase, digests lactose into glucose and galactose; erepsin or amino peptidase, which digests polypeptides into amino acids; and enterokinase which activates trypsin.

The flow of pancreatic juice is controlled by nervous reflex triggered or initiated by the smell, sight and taste of food. It is also controlled by the hormones, sucretin and Cholecystokinin. Nervous reflex and the hormone secretin also control the secretion of bile while the hormone, cholecystokinin controls the release of bile from the gall bladder.

The secretion of intestinal juice by the intestinal glands is triggered mainly by the presence of food in the intestine or contact of
food with the lining of the intestine.

As the food mixes with the various digestive juices secreted into the duodenum, it is converted into a watery emulsion called chyle. The chyle moves along the small intestine towards the large intestine, caecum and colon by peristalsis.