Q.What is the minimum number of plasma membrane that oxygen has to diffuse across to pass from air in the alveolus to haemoglobin inside a R.B.C.?

Gas transport covers both external respiration — the exchange of O2 and CO2 between alveolar air and pulmonary capillary blood, driven by diffusion down partial-pressure gradients across the thin respiratory membrane (alveolar epithelium plus capillary endothelium) — and internal respiration, how the blood then carries those gases to and from the tissues.

Only about 3% of oxygen travels dissolved in plasma; the remaining 97% binds haemoglobin (Hb) in red blood cells, each Hb molecule carrying up to four O2 as oxyhaemoglobin (Hb + 4O2 ⇌ Hb(4O2)). How saturated haemoglobin becomes depends on the surrounding oxygen tension (ppO2), and plotting saturation against ppO2 gives the S-shaped oxyhaemoglobin dissociation curve — near-maximal (95-97%) at the high ppO2 of the alveoli, falling steeply as ppO2 drops at the tissues. The curve shifts rightward (haemoglobin releasing O2 more readily) when H+, CO2, temperature or 2,3-DPG rise — exactly the conditions in active tissue; the shift specifically due to changing CO2 is the Bohr effect, while the Haldane effect describes oxyhaemoglobin acting as a weak acid that helps drive bicarbonate back into CO2 and water at the lungs. Carbon monoxide is dangerous here because haemoglobin binds it about 250 times more readily than oxygen, forming stable carboxyhaemoglobin that blocks oxygen transport even though the blood appears saturated.

Carbon dioxide is carried in three ways: about 7% dissolved directly in plasma as carbonic acid; about 70% as bicarbonate, formed inside red blood cells with the help of the enzyme carbonic anhydrase and then exported to plasma (balanced by the chloride shift/Hamburger's phenomenon, with the resulting H+ buffered by haemoglobin); and about 23% bound directly to haemoglobin's globin chains as carbaminohaemoglobin. All three forms reverse at the lungs, releasing CO2 into the alveolar air where its partial pressure is low.