Air from the environment enters through the nostrils, goes through the airways, and finally reaches the alveoli, the tiny air-filled sacs in the lungs.
Here, gas is exchanged between the alveoli and blood flowing through the capillaries that surround each alveolus.
And blood flows from the higher pressured arteriole (Pa) to the lower pressured venule (Pv).
Now, PA, which is the pressure within the alveoli of the lungs is relatively constant throughout the lungs.
At the end of expiration, it’s equal to atmospheric pressure, which is 0 centimeters of water (0 cmH2O) And although Pa is always greater than Pv , their values change at different vertical levels within the lungs.
Consider the fact that some blood vessels are more vertical while others are more horizontal. The horizontal ones are unaffected by gravity, but the more vertical ones are affected by gravity.
The analogy would be a cylinder filled with water - the cylinder represents a blood vessel and the water would be the blood.
As you add more and more water, the height (H) of the water increases. And when the column is completely filled, the pressure (P) from the water that’s exerted on the bottom of the cylinder, or the hydrostatic pressure, is equal to the density of water (p) multiplied by gravitational acceleration (g), multiplied by the height of the column of water above it.
Blood in vertical blood vessels in upright lungs have similar hydrostatic effects.
At the apex of the lung, Pa and Pv are relatively low, at the base of the lung, Pa and Pv are relatively high, and in the middle of the lung, Pa and Pv are somewhere in between.
Now because PA is constant, the relationship of Pa and Pv with respect to PA changes. And it’s the relationship between these three that determines the zones of the lungs.
In zone 3, at the base of the lungs, Pa is higher than Pv, and both are higher than PA.
In zone 3, blood flows through the capillaries because of the pressure difference between Pa and Pv.