The fraction of the dose that reaches the systemic circulation (bioavailable drug) is the fraction of the dose that escapes hydrolysis and metabolism in the gut lumen, metabolism in the gut wall, and metabolism during the first pass through the liver. Elimination of the drug before it reaches the systemic circulation is called presystemic elimination or first-pass effect.

Bioavailability 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 take any value above zero. It can be more than one when the product under investigation has bioavailability higher than that of the reference standard.

Drug molecules can be actively secreted in the proximal renal tubules. There are different transport systems for the different groups of compounds with similar function groups in the renal tubules. These transport systems actively secrete the drug molecules from the plasma to the lumen of the nephron.

These transport systems are not specific, however they can transport compounds with similar characteristics. So, drugs can compete with each other for the transport system, and drugs with higher affinity for the transport system can inhibit the secretion of other drugs with lower affinity. (e.g. probenecid inhibits the active secretion of penicillins in the renal tubules).

Since most of the water filtered in the glomeruli is reabsorbed in the renal tubules, the drug concentration in the nephron lumen is always higher than the plasma drug concentration. So, the drug secretion in the renal tubule is against the concentration gradient.

Enzyme induction results in increasing the metabolic capacity of the enzyme system. This can happen by a wide variety of agents known as enzyme inducers. Enzyme inducers usually increase enzyme synthesis leading to an increase in the total amount of the enzyme and an increase in the metabolic capacity of the metabolic pathway.

The effect of enzyme induction on drug metabolism depends on the drug and the inducer. Some inducers are specific for a certain sub-family of CYP-450 which will affect all the drugs metabolized by that particular CYP-450 sub-family.

Drugs that are metabolized by high capacity metabolic pathways (high extraction ratio drugs, or drugs that have high intrinsic metabolic clearance) will not be affected significantly by enzyme induction. On the other hand, enzyme induction significantly affects the rate of metabolism of drugs that have low intrinsic metabolic clearance (low extraction ratio drugs).