# Time and Elimination Rate

-Drugs may be eliminated from the systemic circulation by different pathways such as through * Urine

* Bile

* Intestine

* Saliva

* Alveolar air

* Sweat

* Milk

Kidney –

Nephron

Glomurelar filtration-

Refer to book

Order of rxn-it refers to the way in w/c the conc of a drug or reactant influences the rate of chemical rxn

1st order reaction

-causes constant percentage of the drug to be lost per unit time -An elimination rate constant can be describe

-Ke( elimination rate constant) the unit is t ½

-One the log plot, the slope is equal to- ke/2.303

-The factor 2.303 converts from natural log to base 10 log units -Half-life-is the period of time required for the conc of a drug to decrease by 1/2 -This is a constant and is related to ke drugs that hav 1st order kinetics -T1/2 = 0.693/ke

1st order elimination rate -rate constant X Plasma conc X vol. of distribution. -K(1/2 min) X P(mg/ml) X Va(ml)=mg/ml

Elimination rate – rate of removal of a drug in units of weight per unit time

| First order| Zero order|

| 500mg(20%/hr)| 1000mg(20%/hr)| 500mg(100mg/hr)| 1000(100mg/hr)| 1| 400| 800| 400| 900|

2| 320| 740| 300| 800|

3| 256| 640| 200| 700|

4| 204.8| 512| 100| 600|

5| | | | 500|

6| | | | |

7| | | | |

8| | | | |

9| | | | |

10| | | | |

| | | | |

1st oreder elimintation - half life is constant

* Lower risk of toxicity

* Very predictable

Zero order elimination- half life is not constant

* As the amount of the drug inc, the time for half life inc. Clearance of a drug is different from the elimination rate

Clearance=rate of removal of drug(mgmin)plasma conc of drug(mgml)

Clearance is the volume of fluid cleared of a drug per unit time

Type equation here.

For 1st order reaction

Vd,t1/2,Ke areinterelated

Clearance = elimination rate/plasma conc...

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