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Histological examination of the pulmonary arteries of four species of sea turtles revealed the presence of a muscular sphincter just distal to the origin of the ductus Botalli. This structure has not previously been described. Its presence suggests that right to left intra-cardiac blood shunts may be a feature of diving in sea turtles; the sphincter providing a mechanism for the control of blood flow through the heart.
The comparative anatomy of the pulmonary arteries of selected terrestrial reptiles suggests that a similar mechanism exists in non-diving species. The possible significance of this is discussed.