Revision - The Intestines

• The small intestine is involved in digestion and absorption of nutrients while the large intestine is involved in the re-absorption of water and other materials and the formation of semi-solid faeces.

• The first part of the small intestine is the duodenum where digestion of lipids, carbohydrates and protein occur. Food enters the stomach as acidic chyme and bile from the gall bladder is added to the chyme. Bile is an alkaline liquid that neutralises the acid in the chyme, making it alkaline for the enzymes from the pancreas to have optimum conditions.

• Bile also contains salts, which help break large particles of fat into small particles, forming an emulsion. The emulsion has a large surface area for enzymes to attach which speeds up digestion. 

• Enzymes released from the pancreas break down food into small molecules that can be absorbed into the blood stream. The muscle movement (peristalsis) of the duodenum walls help mix the food and enzymes enabling rapid digestion as well.

• The end part of the small intestine has villi that stick out, from the inside surface of the gut, into the digested food. Villi create an increase in surface area over which nutrients from food can be absorbed.

• The small intestine is narrow in diameter but the large intestine is much wider. The material in the large intestine is indigestible fibre, water and minerals. The water and minerals are reabsorbed into the walls while the fibre is formed into faeces.