Cardiovascular responses to diving and their relation to lung and blood oxygen stores in vertebrates

Abstract

Air-breathing vertebrates generally respond to apnea during diving by adjusting cardiovascular performance (e.g., bradycardia, selective increases in peripheral resistance, reduction and redistribution of cardiac output). In mammals, and to a lesser extent in birds, the major O2 stores at the beginning of a dive reside within blood and tissues rather than in lung gas. Consequently, there is limited respiratory benefit during apnea in either maintaining or transiently restoring extensive lung perfusion to predive levels, and so cardiac output (and thus lung perfusion) remains low during the dive. In contrast, in most amphibians and reptiles the major O2 stores at the beginning of a dive reside within lung gas rather than in blood and tissues. Recent experiments on frogs and turtles reveal that pulmonary blood flow during diving can transiently increase to or above predive levels when it becomes necessary during the dive to transfer O2 from lung gas to arterial blood. In this regard, cardiovascular responses to diving in lower vertebrates are qualitatively different from those of higher vertebrates.