This viva is relevant to the objectives of Section C(v) of the 2017 CICM Primary Syllabus, which encourages the exam candidate to "to explain the Law of Mass Action.
The equation, if the candidate wishes to write one, is:
A + B ⇌ a + b
Consider two "masses", A and B.
It can be used to describe the interaction between the drug [D] and the receptor it binds [R]:
kon[D][R] = koff [DR]
Here, the rate of association can be described as the product kon[D][R]. The rate of dissociation of the drug from the receptor can be described as koff [DR], where koff is the rate constant for dissociation.
The mass action equation for drug/receptor interaction is therefore:
[DR] = [D][RT] / [D] + Kd
where RT is the total number of receptors, [DR] is the concentration of the complex of drug and receptor
Another wayy of wording this question is, "can you draw a graph which demonstrates the relationship between drug concentration and receptor occupancy?" They should drraw something like this:
This relationship makes several assumptions:
Moreover, there are problems when the drug and receptor exist in a complex system of interrelated and mixed mass action reactions.