Regional distribution of blood flow during swimming in the tufted duck (Aythya fuligula).

The distribution of blood flow to a number of organs and tissues of the tufted duck was determined (by the microsphere technique) before and while the birds were swimming at close to their maximum sustainable velocity (i.e. at 0.69 +/- 0.01 ms-1). During swimming, oxygen uptake was twice the pre-exercise value. Cardiac output increased by 70%, there was no significant change in arterial blood pressure and total systemic conductance increased by 44%. There were no significant changes in blood flow to the brain, liver, adrenal glands, spleen and respiratory muscles. Not surprisingly, there were increases in blood flow to the heart (30% increase) and to the muscles of the hindlimbs (to 3.1 times the pre-exercise value). Significant reductions in flow occurred to various parts of the gastrointestinal tract (although not to the gastrointestinal tract as a whole), to the pancreas and to the pectoralis muscles. In the case of the flight musculature as a whole, the reduction was to approximately 40% of the values in the ducks before exercise. Thus, despite the fact that cardiac output was some three times lower than it would have been during flight, there was a clear redistribution of blood away from some visceral organs and inactive muscles during surface swimming in the tufted duck. This lends support to the suggestion that blood is selectively directed to the legs, as well as to the brain and central nervous system (CNS) and away from the visceral organs and inactive muscles during voluntary diving in these birds.