Regional distribution of blood flow during diving in the duck (Anas platyrhynchos)

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Abstract
The regional distribution of blood flow, both before and during forced diving, was studied in the duck using radioactively labelled microspheres. Cardiac output fell from 227 ± 30 to 95 ± 16 mL kg−1 min−1 after 20–72 s of submergence and to 59 ± 18 mL kg−1 min−1 after 144–250 s of submergence. Mean arterial blood pressure did not change significantly as total peripheral resistance increased by four times during prolonged diving. Before diving the highest proportion of cardiac output went to the heart (2.6 ± 0.5%, n = 9) and kidneys (2.7 ± 0.5%, n = 9), with the brain receiving less than 1%. The share of cardiac output going to the brain and heart increased spectacularly during prolonged dives to 10.5 ± 3% (n = 5) and 15.9 ± 3.8% (n = 5), respectively, while that to the kidney fell to 0.4 ± 0.26% (n = 3). Since cardiac output declined during diving, tissue blood flow (millilitres per gram per minute) to the heart was unchanged although in the case of the brain it increased 2.35 times after 20–75 s of submergence and 8.5 times after 140–250 s of submergence. Spleen blood flow, the highest of any tissue predive (5.6 ± 1.3 mL g−1 min−1, n = 4), was insignificant during diving while adrenal flow increased markedly, in one animal reaching 7.09 mL g−1 min−1. The present results amplify general conclusions from previous research on regional distribution of blood flow in diving homeotherms, showing that, although both heart and brain receive a significant increase in the proportionate share of cardiac output during diving only the brain receives a significant increase in tissue blood flow, which increases as submergence is prolonged.