Diffusion of medicines in the body

A tablet's journey is diffusion

Medicine molecules also diffuse, from a region of higher concentration to one of lower concentration. The rate of diffusion controls how quickly and how well a drug is absorbed, distributed and eliminated.

The same rule as for gases: particles spread from high to low concentration. In the body, that spreading is how a medicine reaches where it is needed.

Key terms

Absorption

A drug passing from the stomach and intestine into the blood.

Distribution

A drug diffusing from the blood into tissues and organs.

Onset of action

How soon a medicine starts to work; faster diffusion makes it sooner.

Lipid-soluble drug

A drug that crosses cell membranes easily and acts in a shorter time.

Short questions with model answers

1. Q1. Put a tablet's journey in order, using your book's words.

   • Swallowed.
   • Absorbed.
   • Distributed.

   Stomach and intestine → blood → tissues and organs, with diffusion at each step; then the drug is eliminated.

2. Q2. Drug A diffuses quickly into its target organ; drug B diffuses slowly. Same dose. Which is likely to give a stronger response?

   • Concentration at the target.

   Drug A: a higher concentration at the target means a more effective response.

Common mistakes

• ✗ A medicine moves from low concentration to high concentration.

  ✓ Diffusion is always from higher to lower concentration.

• ✗ Diffusion only matters while the tablet is in the stomach.

  ✓ It also moves the drug from the blood into tissues and organs.

• ✗ Faster diffusion leaves less drug at the target organ.

  ✓ Your book: faster diffusion generally gives a higher concentration there.

MCQs

1. 1. Medicine molecules diffuse from a region of:

   1. (a) Lower to higher concentration
   2. (b) Higher to lower concentration
   3. (c) Only the stomach to the mouth
   4. (d) Nowhere; they stay put
   Show answer

   (b) p. 12: from a region of higher concentration to one of lower concentration.

2. 2. A faster rate of diffusion between the stomach and intestine means:

   1. (a) The drug is never absorbed
   2. (b) Slower onset of action
   3. (c) The drug leaves the body unused
   4. (d) Earlier absorption and a faster onset of action
   Show answer

   (d) p. 13: a faster diffusion rate means early absorption and hence a faster onset of action.

3. 3. Lipid-soluble drugs act in a shorter time because they:

   1. (a) Diffuse more easily through cell membranes
   2. (b) Are always injected
   3. (c) Never enter the blood
   4. (d) Have larger doses
   Show answer

   (a) p. 13: lipid-soluble drugs diffuse more easily through cell membranes.

4. 4. Generally, a faster rate of diffusion leads to:

   1. (a) A lower concentration at the target organ
   2. (b) No change in the response
   3. (c) A higher concentration at the target organ and a more effective response
   4. (d) The drug staying in the stomach
   Show answer

   (c) p. 13, as printed.

5. 5. Rates of diffusion control how rapidly drugs are:

   1. (a) Manufactured
   2. (b) Absorbed, distributed and eliminated
   3. (c) Priced
   4. (d) Coloured
   Show answer

   (b) p. 13: absorbed, distributed and eliminated from the body.

Quick revision

• Medicines diffuse from higher to lower concentration, like gases.
• Diffusion rates control absorption, distribution and elimination, and so the onset of action.
• Lipid-soluble drugs cross cell membranes easily; faster diffusion gives a higher concentration at the target.