Author Question: What is the minimum amount of time that blood must take for pulmonary capillary transit for ... (Read 34 times)

tichca · **on:** Jul 16, 2018

What is the minimum amount of time that blood must take for pulmonary capillary transit for equilibration of O2 to occur across the alveolar-capillary membrane?

a. 0.15 sec
b. 0.25 sec
c. 0.35 sec
d. 0.45 sec

Question 2

Which of the following values corresponds most closely to the normal PO2 and PCO2 in the mixed venous blood returning to the lungs from the right side of the heart?

a. PO2 = 40 mm Hg; PCO2 = 46 mm Hg
b. PO2 = 40 mm Hg; PCO2 = 100 mm Hg
c. PO2 = 100 mm Hg; PCO2 = 40 mm Hg
d. PO2 = 100 mm Hg; PCO2 = 46 mm Hg

Silverbeard98 · **Reply #1 on:** Jul 16, 2018

Answer to Question 1

ANS: B
If blood flow increases, such as during heavy exercise, capillary transit time can decrease to as low as 0.25 sec. Even this short timeframe is adequate to ensure that equilibration takes place, as long as no other factors impair diffusion. However, in the presence of a diffusion limitation, rap-id blood flow through the pulmonary circulation can result in inadequate oxygenation.

Answer to Question 2

ANS: A
Venous blood returning to the lungs has a lower PO2 (40 mm Hg) than alveolar gas. Thus the pressure gradient for O2 diffusion into the blood is approximately 60 mm Hg (100 mm Hg 40 mm Hg). Therefore, as blood flows past the alveolus, it takes up O2, leaving the capillary with a PO2 close to 100 mm Hg. Because venous blood has a higher PCO2 than alveolar gas (46 mm Hg vs. 40 mm Hg), the pressure gradient for carbon dioxide causes diffusion of carbon dioxide in the opposite direction, from the blood into the alveolus.