The digestive system

The table shows the main structures and associated organs of the human alimentary canal (or gut), and their functions.

StructureFunction
MouthWhere food enters the alimentary canal and digestion begins by amylase enzyme in the saliva starting the digestion of starch.
OesophagusMuscular tube which moves ingested food to the stomach.
StomachMuscular organ where digestion continues. Protease is secreted to begin the digestion of proteins.
PancreasProduces the digestive enzymes carbohydrase, protease and lipase. These enzymes are secreted from here into the small intestine.
LiverProduces bile and secretes it to the gall bladder for storage.
Gall bladderStores bile before releasing it into the first section of the small intestine.
Bile ductTakes bile from the gall bladder to the first section of the small intestine.
Small intestineWhere food is mixed with digestive enzymes and bile and digested food is absorbed into the blood.
Large intestineWhere water is reabsorbed.
AnusWhere faeces leave the alimentary canal.
Diagram of the human digestive system showing the Mouth, Salivary glands, Oesophagus, Liver, Gall bladder, Stomach, Bile duct, Pancreas, Small intestine, Large intestine, Appendix, Rectum, and Anus.

Food is broken down into smaller pieces in the mouth by chewing. This is an example of mechanical digestion. The teeth cut and crush food, and the pieces are mixed with saliva to form a ball of food called a bolus. This is then swallowed and is carried down the oesophagus by peristalsis.

Peristalsis

Food is moved through the digestive system by a process called peristalsis. The muscles in the oesophagus contract above the bolus to push food down the oesophagus.

The muscles work together to produce wave-like contractions. These have a squeezing action that pushes the bolus through the gut.

Peristaltic wave moving food down the oesophagus to the stomach.Peristalsis continues to move food through the small intestine and the large intestine

In the stomach

The stomach produces hydrochloric acid. It kills many harmful micro-organisms that might have been swallowed along with the food. The enzymes in the stomach work best in acidic conditions – at a low pH. Protein digestion starts here.

Bile

After it has been in the stomach, food travels to the small intestine. The enzymes in the small intestine work best in alkaline conditions - but the food is acidic after being in the stomach. Bile is a substance produced by the liver and stored in the gall bladder. It passes down the bile duct into the small intestine.

Bile production in liver. Bile stored in gall bladder. Bile duct. Small intestine (duodenum). Hepatic duct. Digestive enzymes produced in pancreas. Pancreatic duct.Bile and enzyme production in the liver and pancreas

Bile is secreted into the small intestine where it has two effects:

  • it neutralises the acid - providing the alkaline conditions needed in the small intestine
  • it emulsifies fat - turning large droplets of fat into lots of smaller droplets, thus providing a larger surface area over which the lipase enzymes can work

Absorption

Absorption is the movement of digested food molecules through the wall of the intestine into the blood.

The small intestine is the region where digested food is absorbed. The small intestine has a large internal surface area for absorption to happen quickly and efficiently. This large surface area is due to the presence of many finger-like projections called villi.

A villus in the small intestine. The lining surrounds the blood capillary and the lacteal.

The good blood supply around the villi quickly takes away absorbed nutrients, this maintains a steep concentration gradient so that more diffusion of digested nutrients from the small intestine into the blood can occur.

Egestion

At the end of the small intestine, the remaining material in the gut consists of:

  • water
  • bacteria (living and dead)
  • cells from the lining of the gut
  • indigestible substances - such as cellulose from plant cell walls

The large intestine absorbs most of the remaining water. This leaves semi-solid waste material called faeces. Egestion happens when these faeces pass out of the body through the anus.