What do the the skin, lungs, and digestive system do when drugs enter the system?

Drugs undergo four stages within the body: absorption, distribution, metabolism, and excretion. Drugs can be absorbed through the skin.
Or, some drugs can be inhaled as gases. Gases penetrate the lining of the esophagus and lungs very quickly, which allows the drug to enter the bloodstream. 
 Once a drug has entered the bloodstream,the blood throughout the body carries the substance with it. 
The digestive system is involved in the process of metabolism of the drugs. and transform drugs into substances that can be expelled by the body. Enzymes within the liver break down the drugs.

Processes above and be sure to mention the movement of electrons and energy as the carbon atom moves from one reaction to another.