Pulmonary Zonal Blood Flow and Alveolar Pressure: Zone 1 to Zone 3 in Upright Lungs

Summary

This Osmosis video explains how air moves from the environment to the alveoli and how gas exchange occurs across the alveolar capillary membrane. It then shows that alveolar pressure PA remains relatively constant across the lungs while arterial and venous pressures vary with gravity, creating distinct zones (zone 1 to zone 3) in upright lungs. Using simple analogies, like a water filled cylinder and a clothespin on a straw, the talk illustrates how hydrostatic pressure and pressure gradients drive perfusion through capillaries. At the apex, pressures are lower and perfusion is limited; at the base, pressures are higher and blood flow increases; zone 2 lies in between, with alveolar pressure sometimes compressing venules. The piece concludes with a concise recap of zone relationships and when dead space can occur in non physiological conditions.

Introduction

This video from Osmosis describes the journey of air from the environment into the nostrils, through the airways, and into the alveoli where gas exchange occurs with the surrounding capillaries. A key point is that PA, the pressure within the alveoli, is relatively constant across the lungs, and at end expiration it equals atmospheric pressure. Blood moves from arterial to venous vessels driven by pressure differences, and those arterial and venous pressures vary with vertical position due to gravity.

Alveolar and Vascular Pressures

The narration emphasizes the relationship between the alveolar pressure and the pressures in the arterial and venous blood as the lung is oriented vertically. The constant alveolar pressure interacts with the gravity affected hydrostatic pressures in the blood vessels, shaping how blood flows through the capillaries that surround each alveolus. The bloodstream tends to move from higher arterial pressure to lower venous pressure, but this is modulated by lung geometry and position.

Gravity and Hydrostatic Effects

Gravity creates hydrostatic pressure differences along vertical blood vessels. The apex of the lung experiences relatively low arterial and venous pressures, while the base experiences higher pressures. The middle of the lung shows intermediate pressures. The analogy of a cylinder filled with water illustrates how increasing height increases bottom pressure due to density and gravity, a concept applied to the vertical vessels in upright lungs.

Zone 3 at the Base

In zone 3, arterial pressure is higher than venous pressure, and both are higher than alveolar pressure. Blood flow through capillaries is driven by the gradient between arterial and venous pressures despite alveolar pressure being elevated. The alveolar walls resist collapse, preserving vessel patency under these conditions.

Zone 2 in the Middle to Apex

Zone 2 features arterial pressure higher than alveolar pressure, which in turn is higher than venous pressure. Blood still flows through capillaries due to the arterial venous gradient, and alveolar pressure does not crush the vessels completely. The venules may be compressed by alveolar pressure because capillaries share a basement membrane with alveoli, increasing resistance in the venular side while arteriolar inflow continues.

Zone 1 at the Apex

At the apex, alveolar pressure is greater than arterial pressure, which is greater than venous pressure. In this zone, blood flow through the capillaries is effectively halted because the nearby alveoli compress the arterial segments. The vascular structure normally resists complete collapse, so zone 1 perfusion is typically absent only in non physiologic conditions such as a drastic drop in arterial pressure during hemorrhage or markedly increased alveolar pressure during positive pressure ventilation.

Non Physiologic Conditions and Dead Space

The video notes that dead space can occur when alveolar pressure crushes capillaries or when arterial pressure falls significantly. Normal lung architecture generally prevents this collapse, but extreme conditions can disrupt blood flow and alter gas exchange efficiency.

Recap

Pulmonary blood flow across the upright lung is uneven and governed by pressure gradients in the vessels and the alveolar pressure. In zone 3, the order is arterial pressure, venous pressure, then alveolar pressure. In zone 2, arterial pressure, alveolar pressure, then venous pressure. In zone 1, alveolar pressure, arterial pressure, then venous pressure, with zone 1 perfusion typically absent under physiologic conditions.