Lung Zones

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When considering the zones of the lung, it helps to visualize the zones as pictured below. 


Zone 1- PA>Pa>Pv
Zone 2- Pa>PA>Pv
Zone 3- Pa>Pv>PA


The lung can be conceptually divided into different zones based on blood flow and ventilation. Additionally, terms like PA (alveolar pressure), Pa (arterial pressure), and Pv (venous pressure) are related to the pressures in the respiratory system. Let's break down these concepts:

Lung Zones:

  1. Zone 1:

    • Found in the apex (top) of the lungs.
    • In this zone, alveolar pressure (PA) is less than both arterial pressure (Pa) and venous pressure (Pv) during the entire respiratory cycle.
    • Blood flow is intermittent and occurs mainly during systole.

  2. Zone 2:

    • Found in the middle of the lungs.
    • During systole, PA is greater than Pv but less than Pa. During diastole, PA is greater than both Pa and Pv.
    • Blood flow is continuous throughout the cardiac cycle.

  3. Zone 3:

    • Found at the base (bottom) of the lungs.
    • In this zone, PA is always greater than both Pa and Pv during the entire cardiac cycle.
    • Blood flow is continuous during systole and diastole.

Pressures in the Respiratory System:

  1. PA (Alveolar Pressure):

    • PA represents the pressure within the alveoli of the lungs.
    • It changes during the respiratory cycle, becoming negative during inhalation and positive during exhalation.
    • The pressure changes in the alveoli drive the movement of air in and out of the lungs.

  2. Pa (Arterial Pressure):

    • Pa refers to the pressure in the arteries that carry oxygenated blood away from the heart and to the body tissues.
    • In the context of the lungs, PaO2 (partial pressure of oxygen in arterial blood) is a critical measure for assessing oxygenation.

  3. Pv (Venous Pressure):

    • Pv represents the pressure in the veins returning deoxygenated blood to the heart.
    • In the context of the lungs, PvO2 (partial pressure of oxygen in venous blood) is lower than PaO2, reflecting the oxygen consumption by tissues.

Ventilation-Perfusion Matching:

  • Ventilation (V):

    • Refers to the movement of air in and out of the lungs, reaching the alveoli.

  • Perfusion (Q):

    • Refers to blood flow to the lungs, reaching the pulmonary capillaries.

For optimal gas exchange, ventilation and perfusion need to be matched. In areas where ventilation is greater than perfusion, there is wasted ventilation (dead space). In areas where perfusion is greater than ventilation, there is wasted blood flow.

Understanding lung zones and pressures is crucial for comprehending the mechanics of breathing and the physiological processes involved in gas exchange.


 

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