The human diving response, its function, and its control

The purpose of this review is to outline the physiological responses associated with the diving response, its functional significance, and its cardiorespiratory control. This review is separated into four major sections. Section one outlines the diving response and its physiology. Section two provides support for the hypothesis that the primary role of the diving response is the conservation of oxygen. The third section describes how the diving response is controlled and provides a model that illustrates the cardiorespiratory interaction. Finally, the fourth section illustrates potential adaptations that result after regular exposure to an asphyxic environment. The cardiovascular and endocrine responses associated with the diving response and apnea are bradycardia, vasoconstriction, and an increase in secretion of suprarenal catecholamines. These responses require the integration of both the cardiovascular system and the respiratory system. The primary role of the diving response is likely to conserve oxygen for sensitive brain and heart tissue and to lengthen the time before the onset of serious hypoxic damage. We suggest that future research should be focused towards understanding the role of altered ventilatory responses in human breath-hold athletes as well as in patients suffering from sleep-disordered breathing. A lack of oxygen even for short periods can be detrimental to most birds, mammals, and humans. However, many diving birds, mammals, and humans have adapted to endure hypoxia or anoxia for extended periods (Hermes-Lima & Zenteno-Savin, 2002). The major physiological adaptation allowing animals to endure the lack of oxygen during apnea is the diving response. The diving response has been demonstrated across a variety of diving birds, mammals (Butler & Jones, 1997), and humans (Hong, 1989). It is qualitatively similar between species and involves both bradycardia and peripheral vasoconstriction triggered in response to respiratory arrest (Daly, 1997) and stimulation of facial cold receptors (Elsner & Gooden, 1983). For the purpose of this review, we will use the following terms: diving response, apnea, and face immersion and define them as follows. The diving response is a reduction in heart rate because of an increase in cardiac parasympathetic nerve activity, peripheral vasoconstriction on the arterial vascular tree, and an increase in sympathetic activity triggered in response to the cessation of respiration and, but not necessarily including, the stimulation of facial cold receptors. We will refer to the cessation of respiration as apnea and the stimulation of facial cold receptors by water will be referred to as face immersion. The human diving response involves bradycardia (Asmussen & Kristiansson, 1968), vasoconstriction