Diffusion

Question 1

What is diffusion?

Answer 1

•Process by which O2 (or other gases) move from higher to lower pressure; O2 goes from alveoli to blood; CO2 goes from blood to alveoli. Proportional to surface area and inversely proportional to barrier thickness

Question 2

What is normal alveolar surface area?

Answer 2

–Surface area is 50 to 100 m2

Question 3

What is normal alveolar thickness?

Answer 3

–Thickness is incredibly small (less than 1 µm)

Question 4

What is the driving force of oxygen diffusion?

Answer 4

pressure. This means that you need hemoglobin to transport oxygen because you cannot get enough oxygen to dissolve in plasma.

Question 5

What is normal Alveolar PO2?

Answer 5

–usually 100 mmHg

Question 6

What factors effect diffusion? [2]

Answer 6

Molecular weight: Bigger things don’t diffuse as well
Solubility: More soluble things diffuse better
ex. CO2 diffuses better than O2 (20 times better)

Question 7

What gasses are diffusion limited? [2] How?

Answer 7

O2, CO2 in pulmonary diseases. Need increased barrier to diffusion such as thickening of the blood-gas barrier.

Question 8

What is the diffusion of gas through a tissue sheet proportional to? [5]

Answer 8

Area
1/thickness
1/(molecular weight)0.5
Solubility
Pressure gradient

Question 9

What determines the diffusivity of a gas?

Answer 9

the solubility of the gas/square root of molecular weight so the large a molecule the slower the rate of diffusion

Question 10

What prevents CO from diffusing across blood/gas barrier?

Answer 10

diffusion properties of the barrier = diffusion limited

Question 11

What limits diffusion of N2O (nitrous oxide) across blood/gas barrier?

Answer 11

blood flow = perfusion limited

Question 12

How to tell is a gas is diffusion or perfusion limited?

Answer 12

look at difference between gas pressure and alveolar pressure at end of capillary.

If there is no difference, the gas is perfusion limited (Nitrous oxide)

If there is any difference then the gas is diffusion limited (CO)

Question 13

Is oxygen normally diffusion or perfusion limited?

Answer 13

perfusion limited. it reaches alveolar pressure about of third of the way across the barrier, about 0.25 seconds

Question 14

Under what conditions is oxygen diffusion limited? [3]

Answer 14

• Disease of the blood/gas barrier that causes thickening such as interstitial fibrosis
• exercise where blood flow is increased so there is less time along the blood/gas barrier
• decrease in alveolar pressure such as at high altitude