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Summary: Vascular pressure separation by virtue of a two-chambered
ventricle evolved independently in mammals and birds from a
reptilian ancestor with a single ventricle. Complete separation
of systemic and pulmonary blood flows allows for high
systemic perfusion pressure and a more effective oxygen
transport, while protecting the lungs from oedema (Burggren,
1982; Hicks, 1998; Burggren et al., 1998). Ventricular pressure
separation and high systemic blood pressure have also evolved
in varanid lizards and this trait is normally considered to be a
unique adaptation to their active predatory life style and high
metabolic rates (Burggren and Johansen, 1982; Burggren et al.,
1998). Pressure separation within the heart of varanid lizards
is accomplished by a well developed muscular ridge that
separates the systemic side of the heart (cavum arteriosum,
CA) from the pulmonary side of the heart (cavum pulmonale,
CP) early in systole (Burggren and Johansen, 1982).
Ventricular pressure separation does not occur in the snakes
Vipera, Natrix and Thamnophis (Johansen, 1959; Burggren,
1977), but the cardiac anatomy of pythons resembles that of
varanid lizards and many descriptions of the heart from Python
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