The Role of Cardiac Shunts in the Regulation of Arterial Blood Gases

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Abstract
SYNOPSIS. The pulmonary and systemic circulations are not completely separated in reptiles and amphibians, so oxygen-rich blood returning from the lungs can mix with oxygen-poor blood returning from the systemic circuit (cardiac shunts). In these animals, the arterial blood gas composition is determined by both lung ventilation and the cardiac shunt. Therefore, changes in cardiac shunting patterns may participate actively in the regulation of arterial blood gases. In turtles the cardiac shunt pattern changes independently of ventilation and the cardiac R-L shunt (pulmonary bypass of systemic venous blood) is reduced under circumstances where the demands on efficient gas exchange are high (hypoxia, hypoxemia or exercise). We propose, therefore, that the size of cardiac shunts is regulated independently of ventilation and hypothesize that there exist at least two groups of peripheral chemoreceptors with different reflex roles.

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