A good experiment to show chemical digestion in the stomach is to create your own gastric juices. Obviously, since you will be dealing with acid wear the appropriate safety gear and act responsibly. Making gastric juice is quite simple:
- Add 600ul of concentrated HCl to 100ml of distilled water
- Dissolve 2g of pepsin in the solution from step 1
Caution: Always add concentrated acid to distilled water! Never add water to concentrated acid!
Above is an interactive diagram of the digestive system, scroll over each word to see a brief explanation of its function.
The main regions of the digestive system are the: mouth and mouth cavity, oesophagus, stomach, small intestine, large intestine, rectum and anus. The pancreases, liver and salivary glands are outgrowths of the digestive system.
Let start from the beginning
Mouth and mouth cavity
Both physical and chemical digestion occurs in the mouth. If you look at our teeth, they are specifically designed for our meat and plant diet.
As you can see by the picture to the right, we have teeth for cutting, shearing, grinding and chewing.
This is the first part of digestion, breaking down food into smaller pieces
Oesophagus
The oesophagus is essentially a long muscular tube which connects the mouth to the stomach.
Muscles within the oesophagus contract in a Mexican wave formation, pushing the food down. This is called Peristalsis and is seen in the animation to the left.
At the end of the oesophagus is a sphincter which closes, and prevents back flow from the stomach.
The oesophagus produces mucus which lubricates the food.
Stomach
The stomach is an important food storage organ. Through chemical and physical digestion it breaks down food, turning it into chyme. Chyme is a soft semi-
Physical digestion:
You will notice by the diagram to the right that the stomach is made up of many muscles. This allows the stomach to be pulled in all direction.
Small intestine
The small intestine is actually the longest organ possible. On average your small intestine is 6.9 meters long. Its called small, because of its diameter.
When you see food, you start to salivate. This is an automatic response, preparing your body to digest the incoming food. We have 3 main salivary glands
This is seen in the animation to the left.
So what actually is in saliva?
Saliva contains:
Mucus: which lubricates food and prevents damage to the oesophagus.
Amylase: a digestive enzyme which breaks down starch.
Your tongue and other contractions in your mouth push the food to the back of the pharynx.
Try swallowing without moving your tongue back. Its impossible.
When you swallow, the epiglottis moves, covering the trachea and opening the oesophagus. This ensures food goes down the oesophagus and not the trachea.
This is seen in the animation to the right.
Chemical digestion:
Hydrochloric acid (HCI), pepsinogen and gastric lipase is released from the wall of the stomach.
HCI splits pepsinogen into pepsin (enzyme which breaks down proteins). HCI also produces the ideal pH for enzyme activity.
Gastric lipase breaks down fats into fatty acids and glycerol.
You can see chemical digestion of the stomach in the video to the left.
Since the majority of nutrients are absorbed in the small intestine we have to increase its surface area to volume ratio, to make it more efficient.
The small intestine has the tiny folds called villi (seen to the left) and microvilli. This increases the surface area to volume ratio. Within these villi are cappileries, allowing nutrients absorbed to travel the body.
As with the oesophagus, food moves through the small intestine through peristalsis.
The small intestine is made up of three parts, the Duodenum (pink), Jejunum(green) and the Ileum (yellow).
Small amount of chyme are released from the stomach into the Duodenum.
Once the food enters the Duodenum, the Pancreas then releases enzymes, the liver releases bile and the intestine produces intestinal enzymes into the Duodenum.
You can see how these organs are connected in the diagram to the left.
Bile is stored in the gallbladder and is important as it neutralises the acid from the stomach and provides a favourable environment for duodenal enzymes.
As the nutrients travel through the small intestine, the enzymes break down protein into amino acids, carbohydrates into simple sugars and lipids into fatty acids and glycerol molecules.
Once they have been broken down they can travel through the membrane of the intestinal cells. This is shown in the animation to the right.
While this process dose use diffusion, the majority of nutrient movement is done through active transport.
Some infections can cause the colon to release large amounts of salt and water, prevent their re-
A great way to show the digestive system and for student to get an understanding of it, is to do a rat dissection.
If we zoom in we can see where each type of molecule will move.
Carbohydrate (green) and amino acids (blue) pass into the capillaries (pink) to be transported around the body.
Lipids (orange) pass through the cell membrane and are collected in the lacteal (yellow). The lacteal returns fat to the blood via the lymph system.
Click here to see a diagram
Large intestine
The large intestine is much simpler than the small intestine. Although it is significantly shorter it is much wider than the small intestine.
It is made up of a appendix, Ceacum, Colon and Rectum. However we will only be focusing on the Colon and rectum.
In the initial parts of the Colon a small amount of nutrient absorption occurs. Salts are actively absorbed and water moves along the osmotic gradient.
Within the Colon there is a large amount of bacteria. These bacteria are responsible for giving us vitamins such as K and B12.
The main role of the large intestine is to store and move faecal matter out of the body. The walls are thin and easily stretched too allow to a higher storage capacity.
The faeces moves through the large intestine by peristalsis. When the faeces reaches the rectum it is stored. There are two main sphincters here, an internal sphincter (involuntary control) and an external sphincter (voluntary control).
