7.2 - The Digestive System

The organs of the digestive system

• The digestive system breaks food down into small molecules the body can absorb and use, and passes out what's left.
• Its central part is the alimentary canal (the gut) - a long muscular tube running from mouth to anus.
• The alimentary canal plus the organs that empty into it make up the whole digestive system.
• Food travels along the canal in one direction, and each region has its own job.

The alimentary canal, from mouth to anus

1. Mouth - Takes food in and starts breaking it down. The teeth break food into smaller pieces, and the salivary glands release saliva, which contains a digestive enzyme that starts to digest starch.
2. Oesophagus - The muscular tube carrying food from the mouth down to the stomach.
3. Stomach - A muscular bag that churns food with its muscular walls into a paste. It releases gastric juice containing hydrochloric acid, which kills many bacteria taken in with food and makes the contents strongly acidic - the condition the stomach's own enzyme needs.
4. Small intestine - A long, narrow tube whose first part is the duodenum and last part is the ileum. Digestion is completed here, and the small, soluble products of digestion (plus most of the water) are absorbed through the wall into the blood.
5. Large intestine (the colon) - Most of the remaining water is absorbed back into the body, leaving a semi-solid waste.
6. The waste, called faeces, is stored in the rectum (the last part of the large intestine) before being passed out through the anus.

Three organs sit alongside the canal and release their secretions into it:

1. Liver - Makes bile, a fluid that helps with the digestion of fats.
2. Gall bladder - Stores the bile made by the liver and releases it, through the bile duct, into the small intestine.
3. Pancreas - Makes digestive enzymes and releases them into the small intestine to help complete digestion.

The five processes of the digestive system

Food doesn't pass through the body unchanged - from being taken in to what's left being passed out, it goes through five processes.

The five processes in order

1. Ingestion - The taking of substances, such as food and drink, into the body through the mouth
2. Digestion - The breakdown of food, where large pieces and large molecules are broken down into small, soluble molecules
3. Absorption - The movement of these small nutrient molecules out of the intestines and into the blood, happening mainly in the small intestine
4. Assimilation - The absorbed nutrients are taken up by the body's cells and used, becoming part of the cells or being used to release energy
5. Egestion - The removal from the body, as faeces, of food that was not digested or absorbed

Examples of assimilation include glucose being used in respiration to release energy or stored as glycogen, and amino acids being used to build the body's own proteins.

Egestion is not the same as excretion

• Excretion - The removal of waste products made by the body's own chemical reactions, such as carbon dioxide and urea, and of substances in excess of the body's needs
• Egestion - The removal of undigested food as faeces

Undigested food removed as faeces was never taken into the body's cells and never took part in its reactions, so it is not a waste product of the body's chemistry. Losing it is egestion, not excretion.

Physical and chemical digestion

Digestion happens in two ways that work together: physical digestion and chemical digestion.

Physical digestion

• Physical digestion is the breaking of food into smaller pieces without changing the molecules themselves.
• It's carried out by:
  • The teeth biting and grinding food
  • The stomach churning food with its muscular walls
• Each piece is still made of the same molecules as before - no chemical change has happened.
• Breaking food up greatly increases its surface area:
  • A large lump has only its outside exposed
  • The same food in many small pieces exposes far more surface
• This gives the enzymes of chemical digestion a larger area to work on, so they can act faster.

Chemical digestion

• Chemical digestion is the breaking of large molecules into small ones - here the molecules really are changed.
• The starch, proteins, and fats in food are large molecules, and most are insoluble, so they're far too big to pass through the wall of the alimentary canal into the blood.
• Chemical digestion breaks these large, insoluble molecules into small, soluble ones, which can then dissolve and pass easily through the gut wall to be absorbed.
• This breakdown is carried out by digestive enzymes.

The villus and its adaptations

• The small intestine is adapted to absorb the products of digestion quickly and efficiently.
• It is very long, giving plenty of time to digest and absorb food before what's left reaches the end.
• Its inner wall has an enormous surface area - it is folded and covered in millions of tiny finger-like projections called villi.
• Each villus is a microscopic finger-like extension sticking out into the inside of the gut.
• The surface cells of each villus are themselves covered in even smaller projections called microvilli.
• Together, the length, folds, villi, and microvilli give a very large surface area, so a great deal of food can be absorbed at once.
• Most of this absorption takes place in the ileum, the last and longest part of the small intestine, whose inner wall is covered in villi.

A short diffusion distance

• A villus has a single, thin layer of surface cells - the wall is only one cell thick, and the cells have thin, permeable membranes.
• So the products of digestion have only a very short distance to travel from the gut into the blood, making absorption fast.

A rich blood supply

• Inside each villus is a network of blood capillaries lying very close to the thin surface.
• The capillaries absorb the soluble products - such as glucose and amino acids - and carry them away in the blood.
• This constant removal keeps the concentration inside the villus lower than in the gut, maintaining a steep concentration gradient that drives more products to diffuse in.
• Some absorption also uses active transport, which is why the surface cells contain many mitochondria to provide the energy needed.

The lacteal and fat absorption

• Inside each villus, alongside the network of blood capillaries, there is a single small vessel called a lacteal.
• The lacteal is part of the lymphatic system, not the blood system.
• Its job is to absorb the products of fat digestion - fatty acids and glycerol - which are taken up here rather than directly into the capillaries.

How absorbed fats reach the blood

1. Once absorbed into the lacteal, the fatty acids and glycerol are carried away in a fluid called lymph.
2. The lacteals from the many villi join up into larger lymph vessels.
3. Eventually the lymph, along with the fats it carries, empties into the blood, returning the products of fat digestion to the bloodstream to be transported around the body.

Two transport routes compared

1. Fatty acids and glycerol - Absorbed into the lacteal and carried away as lymph
2. Glucose and amino acids - Take the more direct route through the blood capillaries