Respiratory - from West's 8th ed Chapter 3 - Diffusion

Question 1

1. Using Fick’s law of diffusion of gases through a tissue slice, if gas X is 4 times as soluble and 4 times as dense as gas Y, what is the ratio of the diffusion rates of X to Y?

A. 0.25

B. 0.5

C. 2

D. 4

E. 8

Answer 1

C is correct. The law states that the diffusion rate is proportional to the solubility but inversely proportional to the square root of the density. Therefore, the ratio of X to Y is 4/root(4) or 4/2, that is, 2.

Question 2

2. An exercising subject breathes a low concentration of CO in a steady state. If the alveolar P_CO is 0.5 mm Hg and the CO uptake is 30 ml/min, what is the diffusing capacity of the lung for CO in ml/min/mm Hg ?

A. 20

B. 30

C. 40

D. 50

E. 60

Answer 2

E

Question 3

3. In a normal person, doubling the diffusing capacity of the lung would be expected to:

A. Decrease arterial PCO2 during resting breathing.

B. Increase resting oxygen uptake when the subject breathes 10% oxygen.

C. Increase the uptake of nitrous oxide during anesthesia.

D. Increase the arterial PO2 during resting breathing.

E. Increase maximal oxygen uptake at extreme altitude.

Answer 3

E

Question 4

4. If a subject inhales several breaths of a gas mixture containing low concentrations of carbon monoxide and nitrous oxide:

A. The partial pressures of carbon monoxide in alveolar gas and end-capillary blood will be virtually the same.

B. The partial pressures of nitrous oxide in alveolar gas and end-capillary blood will be very different.

C. Carbon monoxide is transferred into the blood along the whole length of the capillary.

D. Little of the nitrous oxide will be taken up in the early part of the capillary.

E. The uptake of nitrous oxide can be used to measure the diffusing capacity of the lung.

Answer 4

C

Question 5

5. Concerning the diffusing capacity of the lung:

A. It is best measured with carbon monoxide because this gas diffuses very slowly across the blood-gas barrier.

B. Diffusion limitation of oxygen transfer during exercise is more likely to occur at sea level than at high altitude.

C. Breathing oxygen reduces the measured diffusing capacity for carbon monoxide compared with air breathing.

D. It is decreased by exercise.

E. It is increased in pulmonary fibrosis, which thickens the blood-gas barrier.

Answer 5

C is correct. Breathing oxygen reduces the measured diffusing capacity for carbon monoxide because the oxygen competes with carbon monoxide for Hb, and therefore, the rate of reaction of CO with Hb is reduced.

Question 6

6. The diffusing capacity of the lung for carbon monoxide is increased by:

A. Emphysema, which causes loss of pulmonary capillaries.

B. Asbestosis, which causes thickening of the blood-gas barrier.

C. Pulmonary embolism, which cuts off the blood supply to part of the lung.

D. Exercise in a normal subject.

E. Severe anemia.

Answer 6

D is correct. Exercise increases the diffusing capacity because of recruitment and distension of pulmonary capillaries. Emphysema, asbestosis, pulmonary embolism, and severe anaemia reduce the diffusing capacity because of a reduction in surface area of the blood-gas barrier, an increase in its thickness, or a reduction of the volume of blood in the pulmonary capillaries.