L7, 8 & 9 - Digestive System

objectives

• Define digestion as the break-down of large, insoluble food molecules into small, water-soluble molecules using mechanical and chemical processes
• Identify the main regions of the alimentary canal and associated organs including mouth, salivary glands, oesophagus, stomach, small intestine: duodenum and ileum, pancreas, liver, gall bladder, large intestine: colon and rectum, anus
• Define absorption as movement of digested food molecules through the wall of the intestine into the blood
• Describe the significance of villi in increasing the internal surface area of the small intestine
• Identify the small intestine as the region for the absorption of digested food
• Describe the role of the liver in the metabolism of glucose (glucose → glycogen)
• Describe the role of the digestive system in the ingestion, digestion, absorption, assimilation and egestion of food.

What is Digestion?

The body carries out digestion of food to convert large insoluble food molecules into smaller soluble ones.

Small food molecules can pass through the walls of the small intestine and then dissolve into the blood stream. Large food molecules cannot do this.

vitamins and minerals, exist in foods as small molecules able to be absorbed.

Fibre is important for keeping the digestive tract working smoothly. Since we do not digest it, the fibre in food passes into the intestine and absorbs water. The undigested fibre creates "bulk" so the muscles in the intestine can push waste out of the body. Eating enough fibre helps prevent constipation.

water can be directly absorbed without needing to be digested

Mechanical Digestion

Digestion begins in the mouth where food is first ingested and then broken down by the teeth. This is called mechanical breakdown. The small parts of food are mixed with saliva and swallowed. This food then passes through the Oesophagus.

The salivary glands are found in the back of the mouth and secrete saliva - a mixture of mostly water plus digestive enzymes and other chemicals that aid digestion.

The Enzyme Amylase - which breaks down carbohydrates - is found in the saliva.

The Oesophagus

The Oesophagus is the muscular tube that attaches the mouth to the stomach.

It can contract or expand to allow food through. This expansion and contraction is called Peristalsis.

The Stomach

After food is swallowed it enters the stomach, which is basically a muscular bag filled with hydrochloric acid.

A process known as chemical digestion (which starts with the saliva in the mouth) is continued in the stomach.

Three things happen to food in the stomach:

• the chemical breakdown of food begins as the Hydrochloric acid in the stomach breaks down food as does the Enzyme PEPSIN which breaks down proteins
• microbes are destroyed.
• Mechanical Digestion continues as the muscular wall of the stomach churns and grinds food.

Liver, Pancreas and Gall Bladder

Small Intestine

From the stomach, food enters the small intestine where digestion is completed and the small digested food molecules are absorbed into the bloodstream.

Large Intestine and Colon

Rectum and Anus

summary video