Gas exchange

Question 1

What are the ideal gas equation and Boyle’s law? (2)

Answer 1

PV=nRT

P proportional to 1/V

Question 2

Define the partial pressure of a gas. (2)

Answer 2

The pressure that one gas exerts within a mixture of gases as a fraction of the total gas pressure.

Question 3

Describe the atmospheric pressure at sea level and explain why is is lower at higher altitude. (3)

Answer 3

1atm = 101kPa

Less air above pressing down means less pressure.

Question 4

Calculate the partial pressures of O2, N2 and CO2 in the atmosphere if N2 is 78%, O2 is 21% and CO2 is 0.03%. (3)

Answer 4

p(N2) = 101 x 78% = 78.7kPa
p(O2) = 101 x 21% = 21.2kPa
p(CO2) = 101 x 0.03% = 0.03kPa

Question 5

Describe what happens to the air pressure of gases if the air is saturated with water. (3)

Answer 5

The water in the air exerts “vapour pressure” in the air of constant 6.28kPa.
This means the gas pressure in the air is now 101 - 6.28 = 94.7.

Question 6

Describe how the partial pressure of gas in air and the amount of dissolved gas differ. (3)

Answer 6

Amount dissolved = partial pressure x soluability constant

Soluability constants are often really small so amount dissolved at that temp is much smaller than you’d think.

Question 7

Describe how the blood contains lots of oxygen even though the dissolved amount of oxygen is really small. (3)

Answer 7

Because content of gas in the blood is small amount dissolved plus all of the oxygen bound to haemoglobin. Until Hb is saturated there is a gradient to dissolve more oxygen in the blood because it is all removed.

Question 8

Describe the partial pressures of gas in the alveoli. (3)

Answer 8

p(O2) is lower than atmospheric
p(CO2) is higher than atmospheric
Because inspired air mixes with residual volume, which has had all the oxygen removed already and a load of carbon dioxide added.

Question 9

Describe the factors affecting diffusion. (5)

Answer 9

Partial pressure gradient 
Surface area
Thickness of surface
Diffusion co-efficient 
           Soluability (higher = faster)
           Molecular weight (lower = faster)

Question 10

Explain why carbon dioxide diffuses 20x faster than O2. (3)

Answer 10

Molecular weight is higher in CO2
CO2 is more soluable
Difference in pp is much greater in CO2.

Question 11

Describe what oxygen must cross to get from alveolar air to RBC. (3)

Answer 11

5 cell membranes
3 layers of cytoplasm
2 layers of plasma