Blood flow sustains diffusion gradients in alveoli

Steps:

• Gas exchange relies on concentration gradients for oxygen and CO2 diffusion across alveolar walls.
• Blood arriving at alveoli has low O2 and high CO2 compared to air.
• As gases diffuse, blood flow continuously replaces this blood with fresh, low-O2/high-CO2 blood.
• This prevents equilibrium, keeping the gradient steep for ongoing diffusion.

Why A is correct:

• Blood flow acts as a countercurrent mechanism, removing O2-rich blood and supplying deoxygenated blood, per Fick's law of diffusion where gradient drives net flux.

Why the others are wrong:

• B: Haemoglobin binds O2 for transport but does not renew blood to sustain the gradient.
• C: Single-celled walls enable rapid diffusion by shortening path length, not maintaining gradient.
• D: Squamous epithelium minimizes diffusion distance in capillaries but does not influence blood renewal.

Final answer: A