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How does a lung shunt lower the oxygen level in arterial blood?

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Medical illustration of the human chest showing the lungs, diaphragm, ribs, trachea, bronchus, and bronchioles, with one lung cut away to reveal internal structures. — human respiratory system The lungs serve as the gas-exchanging organ for the process of respiration.

A lung shunt lowers the oxygen level in arterial blood because venous blood enters the systemic circulation without passing through functioning lung tissue to undergo gas exchange.1

This reduction occurs because of the specific shape of the oxygen-dissociation curve. In healthy areas of the lung, increased ventilation can raise the partial pressure of oxygen in the blood, but the oxygen-dissociation curve reaches a plateau at normal alveolar oxygen levels. Consequently, even if healthy portions of the lung are overventilated, they cannot increase their oxygen content enough to compensate for the deoxygenated blood arriving from the shunted, unventilated areas.1

When blood from the healthy, oxygenated portion of the lung mixes with the deoxygenated blood from the shunt, the resulting composite arterial blood has a lower oxygen content than normal. Unlike carbon dioxide, which can be effectively compensated for by increased excretion in healthy lung areas due to its steeper, more linear dissociation curve, oxygen levels remain depressed because the plateau effect prevents healthy alveoli from "over-oxygenating" the blood to offset the shunt.1