Distribution

Question 1

What is meant by distribution?

Answer 1

The process by which drugs move from the bloodstream into different tissues/organs in the body to act at receptor sites.

Question 2

What factors can affect distribution?

Answer 2

Rate of blood flow, plasma protein binding, drug solubility.

Only unbound drugs can move into tissues/organs.
Lipid-soluble drugs can distribute into fat-risch tissue however water-soluble prefers to stay in watery areas like blood.

Question 3

What does lipophillic mean?

Answer 3

Combine with or dissolve in lipids (fats).

Question 4

What does hydrophillic mean?

Answer 4

Combine with or dissolve in water.

Question 5

What is meant by bioavailiability?

Answer 5

The extent the drug reaches systemic circulation and is able to act on its receptor site. E.g., IV route bypasses first pass-metabolism. 100% bioavailiability.

Question 6

What factors can affect bioavailiability?

Answer 6

Route, GI gut motility, stomach pH, other drugs inhibit/induce metabolism, plasma-protein binding, solubility of drug.

Question 7

What is meant by half-life?

Answer 7

Time for the plasma concentration of the drug to fall to half of it’s maxiumum concentration. Determines frequency of administration. T 1/2.

Question 8

What does half-life depend upon?

Answer 8

Volume of distribution and elimination.

Question 9

What is volume of distribution?

Answer 9

How widely a drug spreads throughout the body after entering the bloodstream. How much will stay in the blood vs. how much will move into tissues.

Question 10

Low Vd =

Answer 10

Confined to blood = water soluble = high plasma concentration. More availiable to kidneys/liver so may be eliminated quicker.

Question 11

High Vd =

Answer 11

Moves into tissues = lipid soluble = low plasma concentration. Less availiable to reach kidneys/liver via bloodstream so elimination will take longer.

Question 12

How does knowing the Vd help us with prescribing?

Answer 12

Knowing the Vd can help you calculate the dose needed to achieve a critical plasma concentration.

Question 13

What can affect the Vd?

Answer 13

Solubility of the drug.
Water distribution (oedema/pregnancy).
Age.
Hydration.
Protein-binding of drug.

Water soluble = low Vd as stays in blood/extracellular fluid.
Lipid soluble = high Vd as can cross cell membrane