AJP - Regulatory, Integrative and Comparative Physiology, Vol 244, Issue 2 292-R298, Copyright © 1983 by American Physiological Society
Role of venoconstriction in the cardiovascular responses of ducks to head immersion
B. L. Langille
Central venous pressure of ducks rose from resting values of 0.31 +/- 0.16
(SE) to 1.75 +/- 0.20 kPa during forced head immersion. Because a similar
increase in mean circulatory pressure (Pmc) was also observed (0.71 +/-
0.16 to 2.15 +/- 0.20 kPa) the rise in central venous pressure was
attributed to a venoconstrictor mechanism. When this
venoconstrictor-induced rise in central venous pressure was prevented by
graded withdrawal of venous blood, then immersion bradycardia was
inhibited, and the reduced cardiac output associated with head immersion
was largely the result of reduced stroke volume. When compared with normal
dives, this intervention resulted in greater myocardial energy
requirements, as assessed by the pressure-rate product. It is concluded
that venoconstriction increases central venous pressure during head
immersion. The increase in central venous pressure alters cardiac function
through the Frank-Starling mechanism such that myocardial energy
requirements are minimized during this period of low oxygen availability.
