Absorption is the process by which the end products of digestion pass through the intestinal epithelium and enter the blood stream. Attempts have been made to explain the mechanism of absorption on the basis of known physico-chemical principles.
The following factors are believed to be involved in the process:
i. Diffusion:
This depends upon the relative concentration of the substances on either side of a permeable membrane. The concentration of the end products of digestion is much higher in the intestine than in blood in health. This difference initiates diffusion currents from the lumen of the intestine into the blood stream and thus helps absorption.
ii. Hydrostatic Pressure:
Hamburger found that amount of absorption is directly proportional to the hydrostatic pressure in the intestine. The intestinal content at the height of digestion exerts considerable pressure which is all the more enhanced by the tone and movement of the intestine. This higher pressure causes filtration of water and dissolved substances across intestinal epithelium.
iii. Osmotic Pressure:
The colloidal osmotic pressure in the capillary loop of the villus is fairly high and is nearly two to three times higher than the blood pressure existing in it. This high osmotic pressure helps in drawing the fluid from the lumen into blood. [It should be noted that the same factors help in absorbing water and metabolites from the tissues into the venous side of the capillaries.]
iv. Adsorption:
This process is also involved in the absorption of certain substances. It is a very subtle process of union by which a particular substance while being absorbed from the intestine can carry another substance along with it.
v. Hydrotropy:
This is a process by means of which substances insoluble in water are converted into soluble forms. In this way they are brought into a suitable state for absorption. This action is effected by bile salts upon fatty acids and similar products.
vi. Electrical Effects:
Heber has suggested that a process of cataphoresis (the transfer of a solute across a membrane from anode to cathode) may be set up during absorption and may be of considerable importance in this respect. The cations will move towards the side with a negative potential charge and the anions towards the positive potential charge. In addition to this electrical phenomenon the fluid will exert effect on the solute particles. This is known as solvent drag phenomenon.
vii. Passive and Active Transport:
In the passive transport mechanism, special physical force is not required. As for instance if any particular food substance remains in higher concentration in the lumen of the intestine the energy for movement is derived from the higher concentration of the food substance.
In the active transport mechanism, there is a carrier mechanism which helps in movement of the food substance against the electrical forces, ATP supplies energy to this carrier system.
viii. Chemical Action:
In many cases it is seen that brisk chemical action goes on inside the cells of the epithelium during absorption.
Although it is certain that some of the above processes are essentially involved in the phenomenon of absorption, yet, there are certain aspects of the process which cannot be explained on these principles only. For instance, the rapidity of absorption, the selective absorption of certain substances, the absorption of glucose and NaCI from hypotonic solutions and many other similar phenomenon cannot be explained in this way. This shows that in addition to this, some other forces are also at work.
It has been shown that the epithelial covering of the villus is not a passive membrane, submitting itself to physical forces only (like Bowman’s capsule of the nephron), but intense chemical activity goes on inside the epithelium during active absorption. This is proved by the fact that absorption entails a good deal of oxygen consumption and expenditure of energy on the part of the absorbing epithelium.
Moreover, when the temperature of a loop of intestine is reduced to 0°C., preferential absorption of glucose ceases altogether. All these facts show that besides physical forces, some form of chemical activity is certainly and essentially involved in the process of absorption.