Page 165 - Interactive Theoritical Notes of Bioharmaceutics and pharamcokinetics.docx compressed

PharmD clinical pharmacy program Level 3, Semester 2 Biopharmaceutics & Pharmacokinetics (PT608(

Glomerular filtration

• The kidneys receive approximately 25% of the cardiac output, or 1.2–1.5 L of blood

per minute via the renal artery.

• Approximately 10% of this volume (i.e. 120–150mL) is removed every minute as it

passes through the glomeruli of the kidneys.

• The types of solute present in this filtrate are normally limited by the size of the solute

molecule; however, the pores of the glomerular capillaries are sufficiently large to

permit the passage of most drug molecules.

• In addition, since the glomeruli effectively restrict the passage of blood cells and

plasma proteins, only free drug can be filtered. (i.e. drug that is not bound to

plasma proteins).

• Changes in glomerular filtration rate affect the rate of elimination of drugs which are

primarily eliminated by filtration (e.g., digoxin, kanamycin).

Active tubular secretion

• In addition to glomerular filtration, certain drugs can also be secreted into kidney

tubules (e.g., dicloxacillin).

• This secretion process is normally considered to be active, carrier-mediated and,

consequently, requires energy input, because the drug is transported against a

concentration gradient.

• The carrier system is capacity limited and may be saturated.

• Drugs with similar structures may compete for the same carrier system.

• Two active renal secretion systems have been identified, systems for:

– (1) weak acids and

– (2) weak bases.

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