Zones of pulmonary blood flow

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.

Now, even though the pressure in the arterioles and venules exceeds the pressure in the adjacent alveoli, the protein and cells that make up the walls of the alveoli are strong enough to resist collapsing.

In zone 2, in the middle to top, or apex, of the lungs, Pa is higher than PA, but PA is higher than Pv. In zone 2, as before, blood flows through the capillaries because of the pressure difference between Pa and Pv.

And also as before, the alveoli don’t get crushed even though there are higher pressures in the nearby arterioles. Now - here the alveolar pressure is greater than the venule pressure, and the blood vessels feel the pressure directly because they share a basement membrane with the alveoli.

This causes the venules to get slightly crushed, decreasing their diameter, and increasing in their resistance, which leads to a decrease in blood flowing through them.

But arteriolar blood continues to flow into the venule, overcoming that external pressure. It’s a bit like a clothespin pinching down a bit on a straw.

If you blow air into one end of the straw, the clothespin will make it harder for air to get through to the other side. But if there’s enough pressure, then the air will overcome the resistance and make it through.

In zone 1, at the extreme apex of the lungs, PA is higher than Pa, which is still higher than Pv. And in this zone, blood does not flow because the arterioles get crushed by the relatively higher pressures in the nearby alveoli.

Now, normally, the structure of blood vessels and alveoli allow them to resist completely collapsing. So this only occurs in non-physiologic conditions, like when Pa decreases drastically, like during a hemorrhage, or when PA increases drastically, like during positive pressure ventilation.

During these times, the PA can crush the capillaries and completely cut off blood flow, creating dead space.##SummaryAlright, as a quick recap… Pulmonary blood flows through the four zones of the lungs is unequal, and it’s driven by the effects of pressure gradients in blood vessels as well as the pressure within alveoli.In zone 3, in decreasing order: there’s Pa, then Pv, and then PA; in zone 2, Pa, then PA, and then Pv; in zone 1 at the extreme apex, which normally doesn’t happen under physiologic conditions, PA, then Pa, then Pv.

alveoli can crush the capillaries and completely cut off blood flow creating dead space All right As a quick recap pulmonary blood flow through the four zones of the lungs is unequal and it is driven by the effects of pressure gradients in blood vessels as well as the pressure within alveoli In zone three In decreasing order there is pressure in the arterioles then pressure in the ul and then pressure in alveoli in zone two pressure in the arterioles then pressure in alveoli and then pressure in the venues in zone one at the extreme apex which normally doesn't happen under physiologic conditions You have pressure in alveoli then pressure in arterioles then pressure in the