兽医药理学药物代谢动力学y

1、Pharmacokinetics(how the body handles drugs) (a,d,m,e)Drugs cross membranes by passive diffusion or active transportThe vast majority of drugs gain access to their site of action by passive diffusionmost drugs are weak acids or bases:when H increases get more HA and BH+only the non-ionized form can

2、cross cell membranesHA H+ + A-BH+ B + H+ absorption of drugsoral absorption is more complex:tablet-dissolves in gut water-lipid soluble form crosses gut barrier-must escape liver (first pass metabolism)amount absorbed is bioavailabilitydistribution of drugsonce in the body, drugs distribute to tissu

3、esVolume of distribution (Vd) is a calculation of the apparent volume in which a drug is dissolvedVd = Dose (mg)/Cp0 (mg/L)= Litres sometimes the Vd is anatomically “correct” sometimes its notdrugs with low Vd are often bound to plasma proteinsdrugs with high Vd are often bound to tissue components

4、(proteins or fat)metabolism The purpose of drug metabolism is to render drugs water soluble, so they can be excreted by the kidney The liver is the main organ of metabolism and often inactivates drugs on their way from the GI tract to the body.This is called the first-pass effect filtration: free dr

5、ug only, not protein bound reabsorption:passive, lipid soluble form only (pH) secretion:active, acids and bases, saturableif the drug is water soluble, the kidney can excrete it.excretionRate processesfirst-order constant fraction or percent of drug eliminated per unit of timeMost drug disappear fro

6、m plasm by this processThe half life is constantRate processeszero-order constant amount of drug eliminated per unit of timeThe half life is not constant, but depends on the concentrationTime to steady state or elimination is independent of dosage, but depends only on the half lifeRequires 5 elimination half-lives to reach 97% of steady stat