Expansion
Bronchial and thebesian veins drain deoxygenated blood into the systemic circulation
Expansion
Two normal right to left shunts exist:
- Bronchial veins - part of the bronchial circulation drains into the pulmonary veins rather than back to the right heart, adding deoxygenated blood to oxygenated blood
- Thebesian veins - small cardiac veins draining directly into the left heart chambers
Together they account for about 2 to 5 per cent of cardiac output, and they are the main reason the normal alveolar-arterial gradient is not zero and arterial saturation is about 97 rather than 100 per cent.
The term venous admixture describes the calculated amount of mixed venous blood that would have to be added to end-capillary blood to produce the observed arterial oxygen. It combines true shunt with the effect of low V/Q units.
Shunt is quantified by the shunt equation, and its defining feature is that it does not correct with 100 per cent oxygen, since shunted blood never contacts alveolar gas.