VI: Regional Distributions of Perfusion

Question 1

Why is zone 1 of the lung apex a no-flow zone?

Answer 1

This is due to the inability of the weak RV to push blood up into the apical region of the lung.

Question 2

Describe the arterial (a), alveolar (A), and venous (v) pressures in each zone of the lung.

Answer 2

Zone 1 (no-flow zone): PA>Pa>Pv
Zone 2 (waterfall-flow zone): Pa>PA>Pv
Zone 3 (normal distensive-flow zone): Pa>Pv>PA

Question 3

Why do partial pressures of all soluble gases sum to barometric pressure in the arterial blood of normal individuals?

Answer 3

This is because arterial blood is an OPEN system in contact with the atmosphere through functioning alveoli.

Question 4

Why do partial pressures of all soluble gases sum to sub-barometric pressure in the venous blood of normal individuals?

Answer 4

This is because venous blood is a CLOSED system not in contact with the atmosphere.

Question 5

During inspiration, what happens to alveolar resistance vs. extraalveolar resistance?

Answer 5

Alveolar resistance increases because alveolar vessels are compressed as the alveoli expand. Extraalveolar resistance decreases because negative intrapleural pressure stretches out extraalveolar vessels, thus increasing their diameter and decreasing their resistance.
(keep in mind that all of the above are referring to vascular resistances)