The human digestive system consists of a long muscular tube and several accessory organs such as the salivary glandspancreas and gall bladder. It is responsible for food ingestion and digestion, absorption of digestion products and the elimination of undigested materials.
Ingestion – the taking in of food
Food is taken into the mouth where it is physically broken down by the teeth into smaller pieces.
The presence of food in the mouth triggers a nervous reflex that causes the salivary glands to deliver a watery fluid called saliva to the mouth.
Saliva moistens and lubricates the food, with the aid of a slippery substance called mucin, making swallowing easier as well as dissolving some of the food and allowing it to be tasted.
The presence of a digestive enzyme, known as amylase, in saliva allows chemical digestion of starches to begin.
Digestion – breaking the large into the small
The swallowing reflex allows food from the mouth to be moved into the oesophagus. Here, waves of muscular contractions known as peristalsis move food down this thin-walled tube to a muscular bag known as the stomach.
Both physical and chemical digestion occurs within the stomach. The continual churning movements of the muscular walls of the stomach mix food with a digestive fluid, known as gastric juice.
Cells lining the stomach produce this highly acidic fluid, and the enzymes present commence the chemical breakdown of the protein component of the food. Eventually, the food is reduced to a creamy paste known as chyme.
A structure at the bottom of the stomach known as the pyloric sphincter controls the entry of chyme into the first part of the small intestine, called the duodenum. Ducts from the gall bladder and pancreas feed fluids rich in bile salts and digestive enzymes into the duodenum. In addition, some of the cells lining the small intestine produce a fluid known as ‘succus entericus’ made up mostly of water, mucus and sodium bicarbonate.
Collectively, these fluids help to lubricate the partially digested food as it moves down the gut, neutralise the acidic chyme, emulsify fats and oils and enzymatically digest the protein, carbohydrate and fatty acids present. This breaking down of large molecules into small molecules is essential as it enables the absorption of these smaller molecules into the bloodstream.
Absorption – uptake of the soluble products of digestion
The small intestine is 5–6m in length, and most of the chemical digestion occurs within the first metre. Once digested into smaller molecules, absorption can take place.
Millions of tiny finger-like structures called villi project inwards from the lining of the small intestine. These structures greatly increase the surface area of contact that the products of digestion have with the small intestine, allowing for their rapid absorption into the bloodstream. Once absorbed, they are then transported to the liver by means of the hepatic portal vein.
Egestion – the removal of undigested food materials
On reaching the end of the small intestine, all the digested food products, along with the minerals and vitamins that are useful to the body, should have been removed from the watery contents. What remains consists of the indigestible components of food such as cellulose from the consumption of plant-based foods. These materials are then passed on to the large intestine.
The 4 main functions of the large intestine are:
- recovery of water and electrolytes (sodium, chloride) from indigestible food matter
- formation and storage of faeces
- fermentation of some of the indigestible food matter by bacteria
- maintaining a bacterial population.
It has been estimated that there are over 500 species of bacteria present in the large intestine, and these friendly (commensal) bacteria perform a variety of functions. For example, undigested carbohydrates (fibre) are metabolised to short-chain fatty acids, and small amounts of vitamins, especially vitamin K and the vitamin B group, are produced for absorption into the blood.
As undigested material accumulates in the rectum, it stimulates a response that leads to the evacuation of the waste through the anus.