Pharmacokinetics - Distribution

Question 1

Define exponential, and explain why logarithms of concentrations are plotted

Answer 1

Exponential describes a process where the rate of change of the calculated value, is proportional to the value itself.

Logarithms of concentrations are plotted because concentration-response curves are often hyperbolic & difficult to compare as they look very similar.
Using the logarithm compresses the curve, and different drugs can be compared more easily

Question 2

What is the volume of distribution?

Answer 2

Defined as the dose administered, divided by the concentration at time zero - this is extrapolated backwards as it is impossible to measure the concentration at time zero. The gradient of this logarithmic graph is the elimination constant.

The apparent volume that would give the observed plasma concentration, if the body was all one homogenous fluid compartment.

Drugs initially distribute into tissues with high blood flow (Brain, thyroid, kidney, lung)

Then to tissues with lower blood flow
Muscle (Sometimes considered as a fourth ‘medium’ compartment)
Fat