Air taken into the lungs mixes with air that is already in the lungs. However, only the air in the respiratory zone can take part in gas exchange. Air in the conducting zone does not take part in gas exchange, so it is considered to be dead space. This anatomical dead space is a fixed volume of about 150 mL. With quiet breathing, anatomical dead space makes up a rather large fraction of the tidal volume, which is usually about 500 mL. This means that incoming air is significantly diluted by stale air.
With deeper breathing, such as during exercise, the tidal volume increases significantly. So does the efficiency of ventilation, as the dead space is now a smaller fraction of the tidal volume. Breathing through a tube physically increases the dead space, so reduces the efficiency of ventilation. Thus, endotracheal tubes in mechanical ventilators are inserted into the airways and become the conducting zone without dramatically increasing dead space.
Ventilation and blood flow through the alveoli are generally matched so that alveolar gases come to equilibrium with gases dissolved in the plasma of the surrounding capillaries. That is, blood flow is slower than gas exchange across the thin respiratory membrane. Gas exchange may be limited in disease states due to a thickened alveolar membrane or edema.
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