Physiology and Pharmacology: 4

Question 1

6 Methods of drug absorption?

Answer 1

Oral (most desirable for consumers)
Sublingual (absorption from oral cavity)
Rectal
Epithelial surfaces
Inhalation
Injection (most direct, fastest)

Question 2

Factors affecting drug absorption? What do they affect?

Answer 2

Site/method of administration (distance travelled)
Molecular weight (affects diffusion) 
Lipid solubility (ability to cross membranes easily)

pH and ionisation
Carrier mediated transport (active of facilitated)

Question 3

Describe Blood brain barrier experiment by Paul Erlich

- what did the results confirm?

Answer 3

Injected dye into bloodstream
-> stained every organ except brain

Injected into brain
-> dye stayed in brain

Endothelial cells of blood vessels supplying the bran form tight junctions drugs cannot pass between cells so must cross membrane (excludes lipophilic or very small drugs) (junctions can also become leaky due to inflammation)

Affects blood access from extracellular fluid to CSF
-> important consideration for drugs to work on brain/CNS

Question 4

Phases of drug metabolism

Answer 4

Phase 1: oxidation, reduction of hydrolysis
- P450 monooxygenase system (cytochrome P450 enzymes)

Phase 2: conjugation
- addition of substituent groups

Question 5

Methods of drug excretion?

Answer 5

Renal excretion

• Glomerular filtration
• Active tubular secretion
• slow reabsorption (into glomerular filtrate)

GI Excretion
- bilary excretion from the liver

Lung Excretion

Question 6

Which model can be used to predict time-course of drug action

• list terms, equation
• comment on graph

Answer 6

One compartment model
Ka = rate of drug absorption
Vd = volume of drug distribution
Cp = conc of drug in plasma
Kel = rate of elimination (excretion and metabolism)
Q = total amount of drug administered 
C0 = concentration of zero time

Decay of drug is exponential -> can plot as log graph to form straight line

Vd=Q/C0

Extrapolation of log graph leads to estimation of C0

Question 7

What model should he used when drug disappearance does not follow a single exponential

Answer 7

2 compartment model

- invokes a second peripheral compartment which drugs can enter/leave e.g. fat/brain