Drug absorption

                             ABSORPTION

For the drug to act on the system, its adequate concentration (conc.) is required at the target tissue. The two processes by which the conc. of the drug at any moment and in any region can be determined: translocation of drug molecule, chemical transformation.

Translocation
The drug is translocated in the body by bulk flow and diffusion. If the drugs  chemically differ, still the transfer by bulk flow can occur by the same mechanism but if the drugs are moving by diffusion, it means that their movement is markedly different. The transfer of drug is depends on its solubility in lipid or water.

For movement of the drug from the GIT to the system the sink condition is playing a vital role. Sink condition means, the drug is always in circulation due to blood circulation. So, the conc. of drug is not reaching at equilibrium.

Thus, the drug can be diffused due to no equilibrium state.
The smaller molecule can be move faster than larger ones

 

The movement of drug through cell membrane
The drug can be moved by intercellular or intracellular from GIT to systemic circulation.

Cell Membrane:

It is made up of lipid bilayer and some proteins in it. The membrane allows the passage of lipid soluble substances but acts as a barrier to the entry or exit of charged or polar substances. Protein carriers presents on the membrane, allows the polar substances selectively. Some proteins act as receptors.

It means that cell membrane is amphipathic, it means it have both polar and non polar parts. But out of these layers, outer layer is hydrophilic and inner layer is hydrophobic.

The epithelial of the GIT is tightly packed. So molecules must transverse atleast two cell membranes. Gaps between these cell membranes are very small and presents due to proteins that acts as filters.

So water is transferred rapidly; smaller molecules can also be passed but larger molecules cannot passed easily.

In glands, the porous epithelial are present by which hormones or other molecules can be enter or leave the circulation. It is controlled by VEGF( vascular endothelial growth factor) released from glands.