The gastrointestinal system comprises of the digestive tract and accessory organs. The digestive tract is a hollow muscular tube starting at the mouth, continuing through the pharynx, oesophagus, stomach, intestines, rectum and ends at the anus. The accessory organs include the salivary gland, liver, pancreas and gall bladder, which secrete enzymes that help break down the food into nutrients. The food is propelled through the digestive system by peristaltic movements.
The main function of the gastrointestinal tract is to break down the food consisting of proteins, carbohydrates and fat into nutrients which are absorbed into the body to provide energy. The wastes are excreted from the body as faeces.
The oral cavity
The oral cavity or mouth is responsible for the intake of food. It consists of the following:
- The tongue which manipulates the food to get to the teeth. It is also able to sense taste, temperature and touch of the food using sensors called papillae.
- The palate helps push food to the teeth.
- Teeth mechanically chop the food.
Chemical breakdown is done by the saliva.
The salivary glands
There are three pairs of salivary glands – parotid, sublingual and submandibular, which empty their secretion through ducts into the mouth. Salivation occurs due to the taste, smell or appearance of food. Saliva helps to moisten the mouth and act as a lubricant. Saliva produces an enzyme called amylase which helps the first stage of the digestion process.
The oesophagus is a muscular tube that carries the food from the mouth to the stomach.
The stomach is a J-shaped bag. Its functions include:
- Storage of food for about 4 hours
- Mechanical breakdown of food by churning and mixing
- Chemical breakdown through acids and enzymes
- The acid destroys germs.
The stomach produces a hormone called grehlin which stimulates appetite.
The small intestine is made up of the following:
- Duodenum: Into which pancreatic, gall bladder and liver secretions enter and digest the food
- Jejunum: Is where the majority of digestion and absorption of nutrients occurs
- Ileum: Carries remnants of food to the caecum
The large intestine
The large intestine consists of the following:
- Ascending colon
- Transverse colon
- Descending colon
- Sigmoid colon
The large intestine mainly absorbs water and passes on the undigested material to be excreted.
The rectum holds the faecal matter before it is excreted through the anus. Thick bands of muscle called the sphincter hold the faeces in the rectum, till it is excreted. The glands secrete lubricant to enable faeces to pass out smoothly. The main functions of the large intestine are:
- Accumulation of unabsorbed material to form faeces
- Digestion by bacteria
- Resorption of water, salts, sugar and vitamins
The liver is made up of 4 lobes and has several important functions including:
- It acts as a mechanical filter for blood from the intestines.
- It detoxifies the body by breaking down several metabolites like bilirubin and oestrogen.
- It produces albumin and blood clotting factors.
- It produces bile and helps metabolism of nutrients.
- Bile produced by the liver enters the intestines and helps in digestion of lipids.
The gall bladder stores and thickens the bile secreted by the liver and empties it into the biliary system when required.
The pancreas connects with the duodenum. It produces insulin, glucagon and other substances which if deficient causes diabetes. It also produces enzymes to digest different components of food.