Flashcards in Clearance Deck (17):
What is the concept of clearance?
measure of efficiency of elimination of drug from body
What is the definition of clearance based on plasma?
The apparent volume of plasma from which all of the drug molecules have been irreversibly removed per unit of time
Is clearance dependant on concentration?
No, clearance is constant for each drug regardless of drug concentration.
What is the extraction ratio?
The efficiency of drug removal during one pass across an organ / the fraction of drug molecules eliminated during one pass across an organ. (0-1)
What is clearance proportional to?
the extraction ratio. A low extraction ratio means a low clearance constant
What does an extraction ratio of 0 indicate?
all of drug has avoided first pass metabolism and has passed into the circulation.
What happens to clearance if you increase concentration?
What happens to the rate of elimination if you increase concentration?
Increases (up to a point until pathway is saturated) . First order elimination is driven by concentration
Explain the concept of steady state concentration
At steady state, the rate that drug is coming into the body is equal to the rate of drug being eliminated.
Typically, how many half-lives does it take to reach steady state?
What is e-k.t
The fraction of drug remaining in the body. Therefroe 1-e-k.t is the fraction of drug that has been eliminated
How will an increase in clearance effect half life?
k = cl/v so an increase in clearance means an increase in the rate of elimination and therefore a decrease in half-life
How will an increase in volume of distribution effect half-life?
An increse in V means a decrease in the rate of elimination and so an increase in half-life.
Why does an increase in volume of distribution cause an increase in the half-life?
A larger volume of distribution means that more drug has been distributed into tissues and so less is circulating and passing through the eliminating organs such as the liver.
What is the elimination half-life?
the time it takes for the body to eliminate 50% of the drug present at the beginning of the interval
The time it takes to achieve Css depends solely on?
the elimination half-life of the drug