It is true that an adequate digestion is essential for absorption, and as a matter of fact, the whole digestive machinery has evolved with the only intention of making the foodstuff suitable for absorption. But if we look to the phenomenon from the opposite standpoint, it will be seen that absorption also is no less important for digestive process. The two processes-digestion and absorption -are closely, interdependent. One cannot go without the other.
The following considerations will clarify:
i. As digestion proceeds the number of discrete particles increase and unless they are removed by absorption, the osmotic pressure of the medium will rise a considerable amount of water will be drawn out from the surrounding tissues, thus upsetting the very purpose of digestion altogether. But since, absorption does take place, no such osmotic disturbance occurs at all.
ii. Absorption helps digestion also in another way. It is known that the action of the digestive enzymes is reversible. So that unless the end products are quickly removed they will accumulate and after a time reversible action will be set up by which the digested particles will again be synthesized. In this way the rate of digestion will come to a standstill with the establishment of a dynamic equilibrium. Absorption removes the end products, prevents the reversible process and thereby helps to make digestion complete.
One feature of histological interest is also to be noted in this connection. The cells of the absorbing epithelium are found to be longitudinally striated in their free border. This type of brush border epithelium is also found in other places whose chief function is absorption, for instance the renal proximal tubules. Examination under electron microscope reveals that the brush border is actually minute villi like projections, named microvilli. This increases the effective absorbing surface of the villi to a great extent.
Stomach absorbs very little food because, under normal conditions, the end products of gastric digestion are not absorbable. For this reason gastric mucosa has not been adapted for absorption and therefore contains no villi. But those simple substances which do not require digestion at all may pass through the gastric epithelium. Hence water, alcohol, saline, glucose and certain easily diffusible drugs are absorbed by stomach to some extent.
Small intestine is the chief place for absorption (Fig. 9.63). Its upper part is more concerned with secretion, whereas, the lower part for absorption. The end products of digestion as well as salts, vitamins and water-are all absorbed through small intestine. The enormous length of the small intestine affords a large surface area for maximum possible digestion and absorption. The villi of the small intestine are the absorbing units. They are more numerous in the lower parts than in the upper.
There are about 5,000,000 villi in the human intestine with a total absorbing area of nearly 10 sq. metres. The end products of protein and carbohydrate digestion are absorbed by the vascular capillaries in the villi and are then drained into the portal system. But the fats mainly pass through the lacteals of the villi and are then carried by the lymphatics. The nature of the factors which determine different routes of absorption for the different foodstuffs is not known.
Large intestine mainly absorbs water. Saline, glucose alcohol, and certain drugs may to some extent be also absorbed from this part.
Although large amount of water is absorbed from the small intestine, yet, it cannot be complete, because a watery medium must be maintained in this part to facilitate digestion and absorption of foodstuffs. But when the chyme reaches the large intestine, it seldom carries any undigested and unabsorbed foodstuff.
Consequently, a watery medium is no more necessary. For this reason all the water is absorbed producing what is known as the ‘formed’ stool. This water absorption reduces bacterial putrefaction by making the medium dry and is also very useful in conserving the water content of the body.