Digestion (cont'd)
Monacia Williams, Contributor

Sheriffa Munroe shares in an interactive session at the recently held Ministry of Education National Sexuality and Reproductive Health Expo. - Contributed

Last week, we began our study of digestion. In that lesson, we took note of the processes occurring in the buccal cavity, the oesophagus and the stomach. Today, we will continue our study as we direct our attention to what happens in the other regions. We begin at the next section, which is the small intestine.

Small intestine

This is a long, coiled, narrow tube, about five metres in length. It is divided into three regions:

• The duodenum, which forms a 'U'-shape region next to the stomach.
• The jejunum, which lies between the duodenum and the ileum.
• The ileum, which is connected to the colon and which forms the last region.

The Duodenum

• It receives secretions from the pancreas, which is a cream-coloured gland just beneath the stomach. It is connected to the duodenum by the pancreatic duct. The pancreas produces pancreatic juice which contains:

• Amylase
• Trypsin
• Lipase

• Sodium hydrogen carbonate, which is alkaline and hence will neutralise the acid of the chyme. Why is this necessary?

• The duodenum also receives bile. Bile is produced by the liver and stored in the gall bladder. The duodenum is connected to the gall bladder by the bile duct. The bile:

• Is a yellowish-green watery liquid
• Does not contain enzymes
• Contains bile salts, which break up fats into small pieces making it easier for the lipase enzymes in the pancreatic juice to digest them. This process is known as emulsification.

Have you been able to figure out why there is a need to neutralise the chyme in the duodenum? Do you remember the pH of the stomach? That's right, it was 1.2 - 2.0! Do you remember what happened to the digestion of the starch? Digestion of starch stopped because amylase could not work at that pH. Did you pay attention to the enzymes in the pancreatic juice? Amylase was one of them. This should have suggested two things to you: one - that the pH must change to facilitate these enzymes and two - that the digestion of starch was going to continue. It follows then that Trypsin and the lipase enzymes do not work in an acid (pH 1.5 - 2.0) medium, but in an alkaline one (pH 7).

Jejunum and ileum

Here, other enzymes are secreted to complete the digestion of the foods consumed. These are:

• Maltase - which completes the digestion of starch, breaking down maltose into glucose.
• Sucrase - which breaks down sucrose into glucose and fructose.
• Lactose - which breaks down the lactose from milk into glucose and galactose.
• Proteases - which breaks down polypeptides to amino acids.
• Lipases - which breaks down fats to fatty acids and glycerol.

If you look at the enzymes and the foods they were acting on, you would realise that the digestion of foods end in the small intestine. What comes next? These digested foods must be transferred to the blood stream for them to be transported to their site of use. This process is known as absorption.

Absorption

The ileum of the small intestine is the site of absorption. Absorption is the passage of digested foods, salts, vitamins and water through the walls of the gut into the blood stream. The ileum is highly suited for the purpose of absorption because it possesses the following adaptations:

• It is very long.
• It has a large surface area. This is because it is covered by finger-like projections which are known as villi, which serves to increase the surface area.
• It is richly supplied with blood vessels. It has a network of capillaries serving each villus. These capillaries connect to form the hepatic portal vein.
• It has lacteals to receive the products of fat digestion. These lacteals are part of the lymphatic system.

The end products of digestion are taken to the liver, via the hepatic portal vein. The liver is the largest organ in the body, weighing over 1 kilogramme. It functions as a chemical factory, a food store, and a central heating system. Doesn't this sound exciting?

See you next week when we look in more detail at the functions of the liver.