Normal Digestive Functioning

To have a better understanding of what IBS or functional gut problems are, it useful to know what the digestive system looks like and understand how it works. Below is an illustration and description of the process of digestion.


The digestive system
The aim of the digestive system is essentially to absorb useful nutrients and excrete the useless waste. Digestion begins before you put food in your mouth. Just the sight or smell is enough to activate the secretion of gastric juices in order to prepare the gut for the breakdown and assimilation of food.
The sight and smell of food causes a significant elevation in dopamine, a neurotransmitter in the brain that is associated with feelings of pleasure and reward.

The mouth
Once food enters the mouth, saliva is secreted via the parotid, sub-mandibular and sublingual salivary glands. The enzyme Amylase that helps breaks down carbohydrates and lipase that helps break down fats is secreted in order to help to prepare the food to be further broken down in the stomach. Once the food has been masticated (chewed) enough to form a bolus (shape of chewed food), the tongue pushes the food to the back of the throat that activates the swallow reflex and the food is swallowed.
The food then travels down to the Oesophagus to the stomach with the help of muscular peristalsis (organized muscle contractions). The food then reaches the Oesophageal Sphincter that relaxes and opens up to allow food to enter the stomach.

In an adult the stomach it is approximately 30cm long, and 15cm wide. It has a capacity of approximatly1-litre. Once food enters the stomach, the oesophageal sphincter closes to prevent food moving back up the oesophagus, which can cause reflux. The stomach is composed of five layers. The innermost is the mucosa that is involved in releasing protein-digestive enzymes (proteases) like pepsin, and hydrochloric acid, which can kill or inhibit bacteria and provide the correct PH for the proteases to work effectively. It is this layer that is also responsible for absorbing fluids like water. The next layer is the submucosa that is surrounded by the muscularis and is responsible for moving and mixing the stomach contents. The outer layers that follow are the subserosa and the seros. When the food has been partially broken down (Chyme), it moves down the stomach to the Pyloric sphincter. The Pyloric sphincter then opens and the food enters the duodenum (beginning of the small intestine).

Small Intestine
The small intestine is approximately 6 meters long and 2-3 centimetres in wide. It comprises of three sections: the duodenum, Jejunum, and ileum. In a healthy human, it takes approximately 8-12 hours for the food to reach the large intestine.

The duodenum is approximately 25 centimetres in length.
As the food enters the duodenum, the pancreas releases bicarbonate and digestive enzymes (lipase, amylase & Trypsin) that help break down proteins, fats and carbohydrates. The liver and gall bladder release bile to help break down fats.

The Jejunum is approximately 1.4 metres in length and where the majority of nutrients are absorbed.

The ileum is approximately 3.5 metres in length. The ileum’s function is to mainly absorb the products of digestion that were left over from the jejunum. The surface of the ileum wall is made up of finger-like projections called villi. The villi is lined with epithelial cells that contain even larger numbers of micro villi. The purpose of these villi and microvilli is to provide a large surface area for the absorption of nutrients from the ileum. Although absorption is taking place in the ileum, it is still responsible for the final stages of carbohydrate and protein digestion.

Large intestine
The large intestine is approximately 1.5 m long and compromises of the cecum, ascending & descending transverse colon, sigmoid and rectum. Its main function is to absorb water and electrolytes from the remaining indigestible material and to absorb vitamins like Vitamin K, vitamin B12, thiamine and riboflavin, that is produced by the colonic bacteria. The large intestine’s other main function is to facilitate the removal of waste, by compacting the fecal material and transporting it to the rectum, where it is stored until defecation through the anus occurs. The entire journey though the colon takes approximately 16 hours.

The sigmoid colon, also known as the pelvic colon, is the largest section of the large intestine. It is approximately 40 centimeters long and forms a loop near the rectum and anus.

The rectum is approximately 12 centimeters long where output is governed by the internal and external rectal sphincters. It major role is to temporarily store faeces. The build up of faeces causes the rectum wall to expand and stretch, activating stretch receptors. The stretch receptors send signals via the nervous system to stimulate defecation. Delay of defecation can result in the faeces moving back into the colon where more water is absorbed. Constipation or hardened faeces can occur if defecation is excessively prolonged.
When ready to defecate, the fecal matter moves down into the rectal canal via peristalsis, causing the rectum to shorten and activating the internal and external sphincters that pull the anus up over the faeces, propelling it out.

What does the Appendix do?
The Appendix is approximately 10 centimeters long, with its base located 2 centimeters below the ileocecal valve. Its exact role is not entirely known. According to Charles Darwin, the appendix was used in order to help us digest leaves during evolution when we were primates. Another possible function of the appendix is it may harbour and protect bacteria in the colon.