Learn — Cell, tissues & general physiology

1 note · 11 questions on the topic

Diffusion

Diffusion is the net movement of molecules from a region of higher concentration to one of lower concentration by their own random thermal motion, so it needs no energy and it stops when the gradient is gone. It is how oxygen crosses into the blood, how carbon dioxide leaves it and how most drugs reach their cells, which means every gas-exchange problem a nurse manages is at bottom a diffusion problem.

  • Simple diffusion carries lipid-soluble and small uncharged molecules straight through the lipid bilayer — oxygen, carbon dioxide, nitrogen, urea, alcohol and the steroid hormones; water crosses both directly and through aquaporin channels.
  • Facilitated diffusion uses a carrier or channel protein, is still downhill and still needs no ATP, but unlike simple diffusion it is specific, saturable (it has a transport maximum) and competitively inhibitable — glucose entering muscle and fat through GLUT4 under insulin is the standard example.
  • Fick's law: the rate of diffusion is directly proportional to the surface area, the concentration or partial pressure gradient and the diffusion coefficient of the gas, and inversely proportional to the thickness of the membrane.
  • The respiratory membrane is about 0.2 to 0.6 micrometre thick over a surface area of roughly 70 square metres — thin and vast on purpose; pulmonary oedema and fibrosis add thickness and emphysema destroys area, and both show as hypoxaemia.
  • The gradients that drive it at the alveolus: alveolar oxygen about 100 mmHg against mixed venous 40 mmHg, and alveolar carbon dioxide 40 mmHg against venous 45 mmHg — only 5 mmHg for carbon dioxide, which is enough because carbon dioxide is about 20 times more diffusible than oxygen.
  • Because of that difference in solubility, a thickened membrane causes hypoxaemia long before it causes carbon dioxide retention — a pure diffusion defect is an oxygen problem first, and a normal carbon dioxide never excuses a low saturation.
  • Blood spends about 0.75 second in the pulmonary capillary at rest but equilibrates in about 0.25 second, so there is two-thirds of reserve in hand; exercise shortens the transit time and a thickened membrane lengthens the equilibration, and hypoxaemia appears when the two meet.
  • Diffusion time rises with the square of the distance, so it is efficient only over micrometres — which is why no cell sits far from a capillary, why an abscess or a thick exudate blocks oxygen delivery, and why circulation, not diffusion, moves substances over any real distance.

Free today: the 10-question daily quiz · the NORCET 11 recall paper · what ₹999 unlocks — once, no auto-renewal.