The rate of absorption is proportional to the drug concentration gradient across that barrier and the surface area available for absorption. This isn't the other end, but you will have active transport in the excretion. When we come to the kidney, you do use active transport to actually secrete or get rid of some drugs from the blood into the nephron,. But that's using ATP. We'll talk about when we get to under the tongue. The real two main drugs that are given administered through the lungs volatile acids.