Large Intestine of Human Body | Digestive System | Biology

In this article we will discuss about the movements and functions of large intestine of human body.

Movements of Large Intestine:

The colon receives mixed residues of food which have escaped digestion and absorption in the stomach and small intestine, i.e., undigested or indigestible food residues, whatever remains of digestive juices including large amount of H₂O and the fluid that has been swallowed or secreted and has escaped absorption in the small intestine in liquid form.

The colon extracts mainly H₂O and other substances to some extent, viz., glucose, amino acids, NaCl and drugs etc., from this mixture and converts it to a solid form of faecal masses to be evacuated through the anal canal by means of its different movements which may be divided into two classes:

(1) Stationary, and

(2) Translatory.

(1) Stationary Movements:

These are localised movements having no absolute forward movement of masses and are responsible for the agitation of the colonic contents and extraction of H₂O and other substances from liquid mixture of food residues received in the colon which appear to be performed by the four types of movements successively.

i. Segmenting contraction, as seen in small intestine.

ii. Haustral contraction (bulging of wall of colon between the teniae).

iii. Kneading movements (fairly large segment contract while adjacent segment relaxes and followed by contraction and relaxation in reverse phase).

iv. Finally by means of peristalsis and antiperistalsis.

These movements occur principally in the ascending and transverse colon for the maximum absorption of H₂O in this area. Each type of these movements not necessarily occurs in every species of animal.

(2) Translatory Movements:

The second class of movements includes:

i. Peristalsis.

ii. Mass peristalsis which propels the colonic contents anal wards.

i. Peristaltic Movement:

In colon is the same as that of small intestine except in frequency. The power of contraction is greater in the descending colon due to dry and hard character of the material to be moved. In this type of movement, integ­rity of myenteric plexuses is not required.

ii. Mass Peristalsis:

It is also of propulsive nature but is not true peristalsis since it involves simultaneous contraction of large segments of the colon. This occurs generally when the stomach is filled with food (unconditioned) or as reflex process called gastrocolic reflex which may be also due to conditioned stimulus. This gastrocolic reflex may be preceded by gastro-ileac reflex. This movement serves to empty of the contents of the proximal colon into the more distal portion and finally into the rectum giving rise to feeling or desire to defaecate.

Functions of Large Intestine:

Large intestine serves the following functions:

1. Absorption:

a. Water absorption and formation of stool is one of the chief functions of large intestine. Daily about 350 gm of fluid chyme is passed into the large gut and about 135 gm of moist faeces is produced on average. About 60 – 80 % of water is absorbed here.

b. Saline- Normal saline is freely absorbed.

c. Glucose- Isolated large intestine absorbs glucose at the rate of 6 gm per hour. 5% glucose solution is suitable for administration per rectum in the human subject.

d. Certain drugs, e.g., some anaesthetics are absorbed.

e. Amino acids are also absorbed.

Absorption in the proximal colon is better than in the distal.

2. Excretion:

Heavy metals like bismuth mercury, arsenic, etc., are excreted through the large gut. The diffusible substances present the bolus may be excreted if the concentration of these substances in the colon is lower than the blood. Due to the basis of which, this part of intestine may be used as artificial kidney for removal of body waste product whose kidneys is in trouble provided the concentration of these substances in the intestine is kept lowered by withdrawing them from it constantly. When they are injected subcutaneously they appear in the faeces.

3. Secretion:

The goblet cells of the large intestine secrete mucus which acts as a lubricant. Mechanical irritation stimulates some watery secretion. The secretion of the large intestine has a distinct alkaline reaction (pH 8.4), but normal stool has an acid reaction due to acids produced by bacterial action.

4. Synthetic Functions:

Bacterial flora of the large intestine synthesises vitamin K, folic acid and some other members of vitamin B complex. Large amounts of vitamin B₁₂ are also synthesised but they are not absorbed.

5. Bacterial Digestion:

Large intestine is the seat of growth of various types of micro-organism or bacteria. Whether they are useful or not, is not definitely known. But the following facts indicate that they may be useful to some extent.

These bacteria are very rich in cytochorme. Some observers believe that it is from the dead body of these bacteria that the human body derives a large part of its cytochrome requirement. Some people suggest that the normal flora of the large intestine prevent the growth of other pathogenic bacteria and thus serve a very useful purpose.

In the herbivora, celluloses are digested by the cytases present in the vegetable cells or by the action of bacteria. In human being, cellulose, hemicellulose, etc., form the dietary residue. It has been suggested that some people suffering from chronic constipation can break down cellulose more than normal ones, thus reducing the bulk.

The un-absorbed foodstuffs pass into the large intestine where they are attacked by the bacteria with the formation of products as follows:

a. Carbohydrates are converted to CO₂ organic acids, etc. Cellulose is converted into carbonic acid and methane.

b. Fats are converted to lower fatty acids and glycerol. Choline is converted to a toxic product called neurine.

c. Proteins are converted into amino acids, ammonia, etc.

i. By Decarboxylation Amines are Produced:

Putrescine and cadaverine together constitute ptomaine which is said to be responsible for the so- called ptomaine poisoning.

ii. By Reductive Deamination:

6. Movements:

One great function of the large intestine is its capacity to move. Its mass peristalsis is essential for defaecation.