Chapter 3

Question 1

What is Fick’s Law of Diffusion? (conceptually)

Answer 1

• Rate of diffusion of a gas through a tissue slice is proportional to the surface area and the partial pressure difference
• Diffusion Rate is inversely proportional to the thickness of the tissue slice
• Diffusion rate is proportional to the solubility of the gas in the tissue but inversely proportional to the square root of the molecular weight

Question 2

What is the rate of diffusion of gas through a tissue slice proportional to?

Answer 2

• surface area
• partial pressure difference
• solubility of the gas in the tissue

Question 3

What is the diffusion rate inversely proportional to?

Answer 3

• thickness of the tissue slice
• square root of the molecular weight

Question 4

What is the equation for fick’s law of diffusion?

Answer 4

Vgas/min = A/T x D x (P1-P2)
A= area of diffusion
T = Thickness of membrane
D = Diffusion Constant
(P1-P2) = Difference in partial pressure

D=Solubility/Square root of Mol. Wt.

Question 5

How long do Red Blood Cells spend in the pulmonary capillary at rest?

Answer 5

• 0.75seconds

Question 6

At rest, how long does it take the PO2 of blood to reach that of alveolar gas?

Answer 6

• 0.25seconds
• 1/3 of time in capillary

Question 7

Why is it important that, at rest, it only takes blood 1/3 of the time in the capillary to reach PO2 of alveolar gas?

Answer 7

• Considerable reserve capacity for diffusion across the blood-gas barrier

Question 8

What does exercise do to the red blood cell transit time in the pulmonary capillary?

Answer 8

• Reduces it

Question 9

What is the diffusion process challenged by?

Answer 9

• exercise
• Alveolar Hypoxia
• Thickening of the blood-gas barrier

Question 10

Why is Carbon Monoxide used to determine diffusion capacity?

Answer 10

• it’s uptake is diffusion limited

Question 11

What is a normal diffusion capacity?

Answer 11

• 25ml/min/mmHG

Question 12

What happens to diffusion capacity with exercise?

Answer 12

• Increases

Question 13

How can the reaction rate of O2 with hemoglobin become a limiting factor in diffusion?

Answer 13

• Short time in capillary

Question 14

What does diffusion across the blood-gas barrier and O2-HG reaction rate have in common?

Answer 14

• Similar resistance to uptake of O2

Question 15

How can the reaction rate of CO be altered?

Answer 15

• Changing the alveolar PO2.

Question 16

What is a diffusion-limited gas?

Answer 16

• carbon monoxide

Question 17

what is an example of a perfusion-limited gas?

Answer 17

• Nitrous Oxide

Question 18

What limitation does Oxygen transfer normally have? when can this change?

Answer 18

Normally
- Perfusion limitation
Changes to Diffusion limitation
- intense exercise
- thickening of the blood-gas barrier
- Alveolar hypoxia

Question 19

How much faster does CO2 diffuse than O2? why?

Answer 19

20x
- differences in solubility not molecular weight