Fig. 1.

Transitions along the oxygen pathway for marine mammals breathing on the water surface (top) and diving (bottom). In this simplified model, the movement of oxygen travels sequentially through three major compartments: the respiratory system (blue box), the cardiovascular system (pink box) and into the cells of the tissues (white box) where O₂ is taken up by the mitochondria during ATP production. The driving force for this cascade is the change in O₂ partial pressures along the pathway. The size of each compartment and the red arrows denote the relative change in oxygen transport for each physiological state. Free-ranging marine mammals alternate between these states (black arrows) with each dive and interposing recovery period during sequential dive bouts as shown by a typical time-depth record for a wild narwhal (center inset). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)