Normal GI function
Dr. Widjaja Luman
Consultant Gastroenterologist and General Physician
B Sc (St. Andrews), MB ChB (Man), MRCP (UK), M.D. (Edin),
CCST (UK), FRCP (Edin)
(Ahli penyakit pencernaan dan hepar)
NORMAL FUNCTIONS OF OUR GASTROINTESTINAL TRACT
Mouth : Apart from providing the pleasure of eating, the function of the mouth is to help in the chewing of food so that food can be broken into smaller pieces for swallowing and digestion. The greater the surface area of the food means greater amount of food would be available for digestion by enzymes secreted by our bowel. Our saliva also contains some enzymes which help in the digestion of food before it gets to the small bowel.
Oesophagus/ gullet : the gullet is a muscular tube of approximately 25 cm in length. It helps to deliver food into our stomachs. The gullet has one sphincter at the upper end which controls the movement of food bolus from our mouths into the oesophagus. There is another sphincter at the lower end which controls the entry of food into the stomach. When the lower sphincter relaxes, food will be allowed into stomach. This lower sphincter works as one way valve allowing entry of food into the stomach. However, this sphincter can sometimes become weakened and will become a two way valve instead. It fails to contract at the right time thus permitting the reflux of acid contents from the stomach into the gullet. This sphincter becomes more relaxed by alcohol, nicotine, fatty food and obesity. The acid so refluxed into the gullet can cause inflammation of the wall as it is not resistant against corrosive effect of acid. This condition is called ‘oesophagitis’ and produces pain at the upper abdomen and heartburn.
Stomach : the stomach can be considered as a muscular sac which performs the following functions:
- Reservoir for food
- Mixing of foods ( as a blender )
- Gatekeeper for food delivery into our small bowel for digestion. This is to prevent the small bowel being overwhelmed by food.
- The lining of the stomach secretes acid which helps in the digestion of protein.
Common disorders of the stomach are gastritis, stomach (gastric) ulcer and cancer.
Small intestine : this is a muscular tube of around 5 metre in length. The internal surface area of the small bowel is increased by numerous folds in order to increase the surface area for absoprtion of nutrients. Absorption of nutrients (digestion) from food is the primary function of the small bowel. Protein, fat and carbohydrate molecules are broken down into smaller fragments by enzymes secreted by small bowel wall, pancreas and gallbladders. The smaller fragments of nutrients are absorbed across the small bowel wall and into the blood stream and lymphatic system. This process is called digestion. As much as 9 litre of water can be absorbed across the small bowel per day. Vitamins and minerals such as iron are also absorbed by the small bowel.
The absoprtion of nutrients across the small bowel can become impaired by bacterial toxin, inflammation and poor blood supply.
Colon ( large bowel ) : the large bowel starts from the caecum which joins the end of small bowel and ends at the rectum. The rectum is about 12 cm long and joins the anus. Anal canal has muscular sphincter which controls defaecation. The principle function of the large bowel is for absorption of water. Up to 3 litres of water can be absorbed by the colon daily. Some food particles, particularly roughage and fibre, are not digested by the small bowel. The large bowel is colonised by large amount of bacteria and when roughage and fibre get to the large bowel, the enzymes from this resident bacteria in the large intestine will break down these food particles. Some of the nutrients will be absorbed across the large bowel into the blood stream. However, the energy derived from this route is not essential for our daily requirement. Entry of faeces into the rectum causes relaxation of the muscular sphincter and at the appropriate social situations, the rectum can be emptied by contraction of abdominal muscular wall and relaxation of anal sphincter.
Common disorders affecting the large bowel are bacterial infection, inflammatory bowel disease ( see other articles in this website) and cancer.
Liver : The liver is the largest organ in the human body and performs some of the most complex functions. It receives 25% of blood supply at any one time and weighs about 1.5 kg. Blood rich in nutrients from the small and large intestine will flow into the liver so that these nutrients can be processed by the liver into more complex products. Its functions are :
- It synthesizes all the circulating proteins, antibody and fats in the body.
- It produces sugar from fat when the body is low in sugar and vice versa when the sugar level is high.
- It breaks down and inactivates many drugs and alcohol. The broken down products can then excreted into the gallbladder and bile ducts and subsequently into the bowel for excretion.
- It acts as a sieve to ‘trap’ bacteria and inactivates many toxins that have gained access into the body through the bowel wall.
Impairment of liver functions can result in decreased poor body immunity to fight against infection, confusion, weight loss.