A first-order process is a process that has a rate proportional to the amount of the reactant involved in this process.

For example, if the dg elimination rate is proportional to the amount of the drug in the body, this means that this drug is eliminated by a first-order process. The proportionality constant is the first-order elimination rate constant. i.e.

Elimination rate = elimination rate constant x drug amount

The rate of drug elimination decreases with time because the amount of the drug remaining decreases with time.

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 vloume/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 dependent pharmacokinetic parameters.).

The volume of distribution is the apparent volume in which the drug can be distributed 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.

The half life is the time required for the amount of the drug or the plasma concentration of the drug to decrease by 50% . The half life has units of time.

The half life and the first order elimination rate constant are dependent on the total body clearance and the volume of distribution.