Absorption

Parmacokinetics

Pharmacokinetics includes absorption distribution metabolism and excretion of drugs (ADME).

Absorption

Absorption is movement of the drug from its site of administration into the circulation. Not only dose of the administered dose that get absorbed but also the rate of absorption is important.

Absorption

Biological Membrane

Biological membrane is bilayer of phospholipid and cholesterol. Cell membrane thickness is about 100Å. Extrinsic and intrinsic proteins molecules are adsorbed on the lipid bilaryer.

  

Transport is movement of drug molecules from site of administration to blood circulation so that it may reach to its site of action. 

Oral drug involves absorption step but parenteral route escape absorption step because parenteral drug directly administrated into the bloodstream.  

Those drugs, which are administered by buccal cavity called oral route. Many dosages forms are available in oral route including tablet, capsule, syrup, suspension, emulsion etc. When drug is reach to blood than its produce pharmacological action.  

Factor affecting absorption

Various factor are involves in affecting absorption.

Vascularity of absorbing surface

Blood circulation removes the drug from the site of absorption and maintains the concentration gradient across the absorbing surface.

Physical state of drugs

Physical states of drugs are also playing vital role in absorption. Liquid drugs are better absorbed than solid medications. Crystalline form drugs are more readily absorbed colloids form. Aqueous solutions are more quick absorbed than oily solutions. 

  

pH of Drugs

Basic drugs are not absorbed until they reach the small intestine. 

Example – Ephedrine

Acidic drugs are rapidly absorbed from the stomach. 

Example – Aspirin, barbiturates.    

Concentration

Passive diffusion depends on concentration. Drug given as concentrated solution is absorbed faster than from dilute solution.

Route of administration

This affect drug absorption because each route has its own specialty.

 

Aqueous solubility 

Drugs given in solid dosage form must dissolve in the aqueous biphasic before they are absorbed.   

Drug absorption mechanism or Drug Transporting System 

So many of mechanisms are involves in transporting drug from site to bloodstream. These mechanisms are known as drug transport system.  

classification of drug transporting system

Passive diffusion

Diffusion process is also known as simple diffusion. Cell membrane is not actively involved in this process.

In diffusion process, high concentration to low concentration through the semi-permeable membrane. 

Diffusion is an automatic process. In this process energy is not required.

The cell membrane is a lipid bilayer and hence the lipid soluble drugs diffuse quickly than the water soluble drugs. Diffusion is bidirectional process, where the rate of transfer of drug molecules is proportional to the concentration gradient across cell membrane. 

Small non-polar molecules (such as gases) and small uncharged polar molecules such as water urea etc follow the diffusion process for transporting inside the cell and outside the cell. 

Pinocytosis

This process is also known as cell engulfing phenomena. Small molecules are engulf by cell process known as pinocytosis.

 

Large molecules are engulf by cell process known as phagocytosis. 

Carrier Transport

All cell membrane express a host of trans-membrane proteins which serve as transporter or carrier for physiologically important ions, metabolites, nutrients and transmitters etc across the cell membrane.

Eg- ion channel, transporter, antiport, symport etc.

  

Facilitated diffusion

The transpoter is belonging to the super family of solute carrier (SLC) tansporters. SLC transporter operates passively without needing energy and translocates the substrate in the direction of its concentration(i.e. higher to low concentration).

Example – the entry of glucose into fat cells and muscles by the GLUT4 transporter.

Active transport

Transports of substance against its electrochemical gradient(i.e. low to high concentration). Hydrolysis of ATP is required to power for active transport (energy is required).

Active transport is two types –

Primary active transport

Single solute transport, low to high concentration though the transporter. 

 

Secondary active transport

Two solute transport, low to high concentration through the same transporter. 

Secondary active transport is two types –

Symport – When two solutes are transport through the same transporter in the same direction. 

Antiport – When two solutes are transport through the same transporter in the opposite direction.