Fick's law of diffusion
Vg = A/t x D x deltaP
D is proportional to solubility/sqrt(molecular weight)
A = surface area available for diffusion
D = diffusion coefficient
t = thickness of diffusion barrier
deltaP = pressure gradient driving diffusion
What does a molecule of O2 have to go through in order to get to the pulmonary capillaries?
gas space within alveolus
erythrocyte membrane and intraerythrocyte fluid
What is the most important factor that limites the transfer of O2 from alveolar gas to blood?
the reaction rate of O2 and hemoglobin
diffusion limited gas transfer
the diffusion pathway provides a barrier that limits gas transfer - perfusion has no impact on the transfer rate
ex. the effective solubiligy of blood for CO is very high because it binds very tightly to hemoglobin
as CO diffuses into the blood and its concentration rises, the partial pressure of CO int he blood only increases slowly
equilibration of gas partial pressures in the alveoli and capillary is achieved before the end of the capillary
rate of diffusion exceeds capacitance of blood
transport of gas from alveoli to tissue could be increased if cardiac output is increased
Is oxygen diffusion or perfusion limited?
diffusion limited until about 0.25 sec, then turns into perfusion limited
What additional resistances to diffusion are there after a gas travels inside red blood cells?
diffusion through a blood-gas barrier (alveolar membrane)
chemical reactions (binding of O2 to Hb)
diffusion capacity (DL)
the rate of gas transfer divided by the pressure gradient across the alveolar/capillary membrane
What is the partial pressure of CO2 in the alveoli?
What is the partial pressure of CO2 in the venous return?
What is the minute ventilation for purely dissolved oxygen?
What chemical reactions of CO2 can limit CO2 exchange? What are the implications of this?
formation of bicarbonate
the formation of carbamino compounds with blood proteins, mostly hemoglobin
the result is that the equilibration rate for CO2 is not much faster that that for O2
How much hemoglobin (gm%) is in a normal individual?
*binds 1.34 mL O2
O2 capacity in the blood is 20.1 vol%
the pressure of oxygen at which hemoglobin is half saturated
this value does not change with varying hemoglobin concentration
factors that increase P50
Bohr effect - decreased pH or increased PCO2
relationship between saturation and capacity
% saturation = content/capacity
What effect does pH have on the hemoglobin saturation curve?
as the pH goes down, the curve shifts to the right
What effect does CO2 pressure have on the hemoglobin saturation curve?
as the partial pressure of CO2 increases, the saturation curve shifts right
deoxyhemoglobin is a weaker acid than oxyhemoglobin, and it more readily accepts H+
higher levels of H+ in the tissue lowers the affinity of Hb for O2 in the tissue
oxyhemoglobin is a stronger acid than deoxyhemoglobin
oxy-Hb therefore has a lower affinity for H+
addition of O2 in the lungs causes H+ to be released from Hb