Analysis of Gas Exchange in The Ophthalmic Rete of Pigeons

Abstract

Many birds have ophthalmic retia (OR), paired vascular networks in the head, that cool arterial blood flowing toward the brain. They thus independently regulate brain and core body temperatures (Bernstein et al., 1979; Kilgore et al., 1979). The system works because evaporation from nasal and oral membranes cools venous blood before it flows through the OR. This blood then takes up heat from warmer arterial blood flowing countercurrent to it (Midtgård 1983; Clair, 1985). The vascular and neural anatomy of the OR suggests that the sympathetic nervous system controls both total blood flow and the proportioning of flow between heat-exchange and shunt vessels (Midtgård, 1985). Birds routinely tolerate body temperatures of 45°C, indicating an extreme capacity for heat storage. This and the ability to cool the brain serves during heat exposure and exercise by reducing the need for evaporative cooling.