The digestive system, teeth and physical digestion

Last time

Before we start: last time, in Diet, nutrients and food tests, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

A nine-metre tube

From your mouth to your anus is a tube about nine metres long. As food travels along it, it is broken down, absorbed, and what is left is removed. Today: the organs of the digestive system, your teeth, and physical digestion, including bile.

Five processes

Ingestion is taking food and drink into the body through the mouth. Digestion is breaking down large, insoluble molecules into small, soluble ones. Absorption is the movement of these small molecules from the gut into the blood. Assimilation is using them in cells. Egestion is removing undigested food as faeces through the anus.

The digestive system

Here is the digestive system. Mouth, oesophagus, stomach, small intestine, which is the duodenum and ileum, then the large intestine, which is the colon and rectum, and the anus. The liver, gall bladder and pancreas add juices.

What each organ does

The mouth: teeth chew the food and salivary glands add saliva containing amylase. The oesophagus carries food to the stomach by peristalsis, waves of muscle contraction. The stomach churns food, and adds hydrochloric acid and protease.

What each organ does

The duodenum receives bile and pancreatic juice. Much of the chemical digestion happens here. The ileum absorbs digested food and water into the blood. The colon absorbs more water. The rectum stores faeces until egestion through the anus.

Physical digestion

Physical digestion breaks food into smaller pieces without changing the molecules. This gives a larger surface area for enzymes to work on, so chemical digestion is faster. Chewing by teeth and churning by the stomach wall are both physical digestion.

Human teeth

Adults have four types of teeth. Incisors and canines bite and cut. Premolars and molars, at the back, crush and grind.

Inside a tooth

The crown is covered with enamel, the hardest substance in the body. Under it is dentine, and in the middle the pulp cavity with nerves and blood vessels. Bacteria turn sugar into acid, which dissolves enamel and causes tooth decay. Brushing and fluoride help prevent it.

Bile

Bile is made in the liver, stored in the gall bladder, and released into the duodenum. Bile is alkaline. It neutralises the acid from the stomach, giving the right p H for enzymes in the small intestine. Bile emulsifies fats: it breaks large fat drops into many small droplets, giving a larger surface area for lipase. Bile contains no enzymes. Emulsifying is physical digestion.