Alveolar – arterial O2difference due to shunts between arterial and venous blood
5 – 15 mm Hg normal
PB= 253 mm Hg
PO2 = 53
Fraction of inspired air as oxygen
[O2 dissolved O2x P O2
KO2 – solubility = 0.3 mL O2/100 mL blood
250 mL O d2livered/min
Blood flow would have to be 18 times greater without hemoglobin
O 2apacity of Hb 1.35 mL O /2 Hb
20 mL O /200 mL blood
Bulk flow is the movement of air in response to a driving total pressure (pressure gradient)
whereas diffusion is the movement of a particular gas according to its partial pressure gradient.
Diffusion occurs until there is no longer a pressure gradient.
Henry’s Law describes the fact that a gas in contact with a liquid (plasma) will dissolve in
proportion to the partial pressure of that gas. Hence at the respiratory membrane, the higher
the partial pressure of oxygen in the alveoli, the greater the amount of oxygen which could
potentially dissolve in the plasma. Carbon dioxide is exposed to the systemic capillary blood
and Henry’s Law applies.
Graham’s Law states that the rate of diffusion of a gas is directly proportional to its solubility
coefficient and inversely proportional to the square root of its molecular weight.
Fick’s Law – the volume of the gas which flows by diffusion is proportional to the surface area
between the blood and the gas, the pressure gradient of the gas, and the diffusion constant for
The diffusion constant (D) is the solubility coefficient divided by the square root of the molecular
weight of the gas.
A hemoglobin molecule is composed of four protein globin chains, each surrounding a central