Bioavallability is a measure of the fraction of the administered dose that reaches the systemic circulation.

Absolute bioavailability is the fraction of the total dose that reaches the systemic circulation. It can have values between zero when the drug is not absorbed at all to 1 when all the administered dose reaches the systemic circulation. The absolute bioavailability can be expressed in terms of percentage (0% - 100%).

The relative bioavailability is the bioavailability of a drug product relative to the bioavailability of another drug product (reference standard preparation). The relative bioavailability can have any value above zero. It can be more than one when the product under investigation has bioavailability higher than that of the reference standard.

The first-order absorption rate constant is the rate constant that determines the rate of drug absorption from the site of administration. Larger absorption rate constant results in faster rate of drug absorption.

The absorption rate constant determined after drug administration is an operative rate constant that accounts for all the necessary steps required for drug absorption into the systemic circulation including disintegration, dissolution and absorption.

The first-order absorption rate constant has units of time^-1.

The total body clearance is the volume of the plasma or blood which is completely cleared from the drug per unit time. It has units of volume/time.

The CL_T for a drug is constant within a patient (dose and concentration independent) when the elimination processes follow first-order kinetics.

The total body clearance is a measure of the efficiency of all eliminating organs in eliminating the drug and it is the sum of all organ clearances (i.e. CL_T is the sum of the renal clearance, hepatic clearance and all other organ clearances).

The elimination rate constant and the half life (the dependent pharmacokinetic parameters) are dependent on (is determined from) the total body clearance and the volume of distribution (the independent pharmacokinetic parameters).

The volume of distribution is the apparent volume in which the drug can distribute in. It is the factor that relates the amount of drug in the body to the concentration of the drug in the sampling site. It is not an actual volume, however it is a hypothetical volume determined by the drug distribution behavior.

The volume of distribution ranges from 3-5 liters to more than 25 L/kg. Drugs with higher affinity for tissues, have higher volume of distribution.