The gastrointestinal (GI) system includes the gastrointestinal tract (mouth, pharynx, oesophagus, stomach, small intestine, large intestine) and accessory organs (salivary gland, liver, gallbladder, pancreas) that secrete substances into the tract via connecting ducts.
GI system breaks down particles of ingested food into molecular forms by enzymes (digestion) that are then transferred to the internal environment (absorption).
Functions of GI organs
The GI tract begins at the mouth, where digestion begins with chewing. Saliva containing mucus and the enzyme amylase is secreted from 3 pairs of salivary glands, located in the head. Mucus moistens the food and amylase partially digests polysaccharides (starches). Food then reaches the stomach through the pharynx and ooesophagus.
The stomach is the sac that stores and digests food macromolecules into a solution called chyme. Glands lining the stomach secrete hydrochloric acid that dissolves food particles and protein-digesting enzymes, called pepsin.
Final stages of digestion and most of the nutrient absorption occurs in next portion of the tract: the small intestine. The small intestine is divided into 3 segments - duodenum, jejunum, and ileum.
The pancreas is a gland located behind the stomach. From its exocrine portion it secretes (1) digestive enzymes and (2) a fluid rich in HCO3- ions to neutralize the acid from stomach. The liver secretes bile. Bile contains HCO3- ions and bile salts to solubilise fats. Bile reaches the gall bladder through hepatic ducts and is stored in the gall bladder between meals. During a meal, bile is secreted from the gland by smooth muscle contraction and reaches the duodenum portion of the small intestine by the common bile duct.
Monosaccharide, amino acids and mineral salts are absorbed by transporter-mediated processes while fatty acid water diffuse passively.
Undigested material is passed to large intestine, where it is temporarily stored and concentrated by reabsorption of salts and water. Finally, contractions of rectum, the last part of large intestine, expel the faeces through the anus.
