The digestive system
The table shows the main structures and associated organs of the human (or gut), and their functions.
| Structure | Function |
| Mouth | Where food enters the alimentary canal and digestion begins by amylase enzyme in the saliva starting the digestion of starch. |
| Oesophagus | Muscular tube which moves ingested food to the stomach. |
| Stomach | Muscular organ where digestion continues. Protease is secreted to begin the digestion of proteins. |
| Pancreas | Produces the digestive enzymes carbohydrase, protease and lipase. These enzymes are secreted from here into the small intestine. |
| Liver | Produces bile and secretes it to the gall bladder for storage. |
| Gall bladder | Stores bile before releasing it into the first section of the small intestine. |
| Bile duct | Takes bile from the gall bladder to the first section of the small intestine. |
| Small intestine | Where food is mixed with digestive enzymes and bile and digested food is absorbed into the blood. |
| Large intestine | Where water is reabsorbed. |
| Anus | Where faeces leave the alimentary canal. |
| Structure | Mouth |
|---|---|
| Function | Where food enters the alimentary canal and digestion begins by amylase enzyme in the saliva starting the digestion of starch. |
| Structure | Oesophagus |
|---|---|
| Function | Muscular tube which moves ingested food to the stomach. |
| Structure | Stomach |
|---|---|
| Function | Muscular organ where digestion continues. Protease is secreted to begin the digestion of proteins. |
| Structure | Pancreas |
|---|---|
| Function | Produces the digestive enzymes carbohydrase, protease and lipase. These enzymes are secreted from here into the small intestine. |
| Structure | Liver |
|---|---|
| Function | Produces bile and secretes it to the gall bladder for storage. |
| Structure | Gall bladder |
|---|---|
| Function | Stores bile before releasing it into the first section of the small intestine. |
| Structure | Bile duct |
|---|---|
| Function | Takes bile from the gall bladder to the first section of the small intestine. |
| Structure | Small intestine |
|---|---|
| Function | Where food is mixed with digestive enzymes and bile and digested food is absorbed into the blood. |
| Structure | Large intestine |
|---|---|
| Function | Where water is reabsorbed. |
| Structure | Anus |
|---|---|
| Function | Where faeces leave the alimentary canal. |
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 . 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.
In the stomach
The stomach produces hydrochloric acid. It kills many harmful that might have been swallowed along with the food. The enzymes in the stomach work best in 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 conditions - but the food is acidic after being in the stomach. is a substance produced by the liver and stored in the gall bladder. It passes down the bile duct into the small intestine.
Bile is secreted into the small intestine where it has two effects:
- it the acid - providing the alkaline conditions needed in the small intestine
- it 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 .
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 from plant cell walls
The absorbs most of the remaining water. This leaves semi-solid waste material called faeces. happens when these faeces pass out of the body through the anus.
