The exercise response and the "classical" diving response during natural submersion in birds and mammals

When swimming at the surface with ready access to the air, aquatic birds and mammals appear to show the typical exercise response. There are increases in oxygen uptake, heart rate, and pulmonary ventilation. These are presumably accompanied by rises in cardiac output and perfusion to the active muscles, together with a reduction in overall peripheral vascular resistance. When restrained animals are submerged under water involuntarily, total oxygen consumption is reduced below the resting level as a result of a large reduction in perfusion of most parts of the body (including skeletal muscles) except the brain and heart. The underperfused parts metabolise anaerobically, producing lactic acid, while only the brain and heart consume oxygen. The large increase in peripheral vascular resistance is compensated by a proportionally similar reduction in cardiac output which results almost entirely from a reduction in heart rate below the resting level. This is the classical diving bradycardia. Evidence is presented from a number of aquatic birds and mammals that suggests that during the vast majority of natural dives metabolism is largely if not completely aerobic with the active skeletal muscles as well as the brain and heart receiving an adequate supply of oxygen. There may, however, be a reduced blood supply to other parts of the body including to the inactive skeletal muscles. Thus during most natural dives stored oxygen is used at an elevated rate, compared with resting, and is quickly replaced upon surfacing. It is possible for the animals to perform a number of such dives in fairly quick succession. In tufted ducks, at least, there seems to be a balance between the cardiovascular response to exercise and that to involuntary submersion with the bias towards the former. This balance can be shifted in the direction of the response to involuntary submersion when the birds swim long horizontal distances for their food (e.g., under ice in winter) or if they are briefly unable to surface (e.g., if rising vertically beneath ice).