Arterial blood may be less well oxygenated than blood leaving the alveoli for several reasons, for example:
• Venous blood may bypass the lungs entirely (e.g. intracardiac shunts, thebesian veins, bronchial circulation)
• Blood may pass through parts of the lung which are not ventilated adequately, in which case < 1 (e.g. pneumonia)
CoO. ■» oMencii onyflfli conrsm
CfOj - rnuad i/onaus contani
CcQ, ■ pulmonary capillary o*)rgin content
QT »rdiflC rjyipul
Cis ikignl lldw
(Qt-Qs) - pulmciMjrf rtipillnry blood flow
Figure RR.21 Diagram of shunt in the lung
• Blood may pass through areas of the lung which are not ventilated at all, in which case = 0. This is sometimes referred to as "true' shunt
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