1. 1.

    The oxygen and carbon dioxide gas tensions in lung gas and blood from the central and peripheral arteries and veins have been measured in unrestrained, undisturbed turtles (Pseudemys scripta) and tortoises (Testudo graecd).

  2. 2.

    Lung and blood gas composition fluctuates widely with intermittent and irregular lung ventilation. The pulmonary gas exchange ratio, which progressively falls during apnoea to as low as 0.2-0.3 in Pseudemys, rises dramatically to over 1.5 during lung ventilation in both species. It is postulated that CO2, which has been passively stored by entering the tissues along gas tension gradients during apnoea, becomes rapidly eliminated into the lungs during ventilation. In diving Pseudemys the lung has only a limited function as a CO2 sink compared to the tissues, but the lung acts as a large oxygen store, which can be drawn upon during apnoea through periodic increases in lung perfusion.

  3. 3.

    Blood gas tensions in the various systemic arches reflect the proximity of the arch's origin to the systemic or pulmonary venous blood streams in the ventricle. Thus, the brachiocephalic artery and right aorta have identical blood gas compositions while the composition of the left aorta is intermediate between these and that in the pulmonary. These relationships are unaffected by normal intermittent breathing. However, this does affect both the origin and composition of systemic and pulmonary arterial blood, such that the greatest proportion of oxygenated blood perfuses the systemic vascular bed during lung ventilation, while the greatest proportion of deoxygenated blood perfuses the lungs during apnoea.

  4. 4.

    These data are discussed in the light of the marked cardiovascular adjustments to intermittent breathing, which occur in chelonian reptiles.

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