1. A digital computer simulation depicting some features of cardiovascular-respiratory interaction in the salmonid fish has been developed. This embodies six controlled system relationships; three of which are mass balance equation describing oxygen transport and storage in the extra-branchial, branchial and general body phases. A Fick equation, and two relationships approximating the transport characteristics of venous and arterial blood are also included in the system equations. Three feedback equations provide for adjustments in cardiac output, ventilation and exchange surface area.

2. The model predicts, with reasonable accuracy, values for ventilation, cardiac output and blood gas tensions under conditions of routine and moderate activity in an oxygen-sufficient environment. Qualitative agreement between predicted and experimental values has been obtained in simulations of responses to progressive environmental hypoxia.

3. The study emphasizes the probable role of venous oxygen tension in the regulation of ventilatory and cardiac activity. A central role for branchiovascular responses in adaptation to elevated oxygen demand is also indicated. The existence of a depressor function, operating under conditions of acute hypoxia to dampen positive ventilatory feedback effects is suggested by simulation of responses to reduced environmental oxygen tensions.

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