Diffusion and Gas Transport

Question 1

Fick’s law of diffusion

Answer 1

V_g = 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

Question 2

What does a molecule of O₂ have to go through in order to get to the pulmonary capillaries?

Answer 2

gas space within alveolus

“alveolar-capillary membrane”

plasma

erythrocyte membrane and intraerythrocyte fluid

Question 3

What is the most important factor that limites the transfer of O₂ from alveolar gas to blood?

Answer 3

the reaction rate of O₂ and hemoglobin

Question 4

diffusion limited gas transfer

Answer 4

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

Question 5

perfusion limitation

Answer 5

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

ex. NO₂

Question 6

Is oxygen diffusion or perfusion limited?

Answer 6

diffusion limited until about 0.25 sec, then turns into perfusion limited

Question 7

What additional resistances to diffusion are there after a gas travels inside red blood cells?

Answer 7

diffusion through a blood-gas barrier (alveolar membrane)

chemical reactions (binding of O₂ to Hb)

Question 8

diffusion capacity (D_L)

Answer 8

the rate of gas transfer divided by the pressure gradient across the alveolar/capillary membrane

Question 9

What is the partial pressure of CO₂ in the alveoli?

Answer 9

45 mmHg

Question 10

What is the partial pressure of CO₂ in the venous return?

Answer 10

40 mmHg

Question 11

What is the minute ventilation for purely dissolved oxygen?

Answer 11

250 mL/min

Question 12

What chemical reactions of CO₂ can limit CO₂ exchange? What are the implications of this?

Answer 12

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

Question 13

How much hemoglobin (gm%) is in a normal individual?

Answer 13

~15 gm%

*binds 1.34 mL O2

O₂ capacity in the blood is 20.1 vol%

Question 14

P₅₀

Answer 14

the pressure of oxygen at which hemoglobin is half saturated

this value does not change with varying hemoglobin concentration

Question 15

factors that increase P₅₀

Answer 15

increased temperature

Bohr effect - decreased pH or increased PCO₂

increased 2,3-diphosphoglycerate

Question 16

relationship between saturation and capacity

Answer 16

% saturation = content/capacity

Question 17

What effect does pH have on the hemoglobin saturation curve?

Answer 17

as the pH goes down, the curve shifts to the right

Question 18

What effect does CO2 pressure have on the hemoglobin saturation curve?

Answer 18

as the partial pressure of CO2 increases, the saturation curve shifts right

Question 19

Bohr effect

Answer 19

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

Question 20

Haldane effect

Answer 20

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