Christina Kiel, Alexander James E Foss
The retina is both metabolically active and exposed to light, resulting in persistent heat generation. However, the quantitative contributions of metabolic and irradiative heat sources, and the mechanisms responsible for dissipating this thermal load, remain incompletely characterized. Here we develop a steady-state quantitative thermal balance model of the retina that integrates heat production from metabolic activity and light absorption with multiple heat dissipation pathways into a unified energy balance framework. Metabolic heat was estimated from reported ATP consumption rates under dark- and light-adapted conditions, while irradiative heat input was quantified based on environmental luminance, pupil size, and ocular optics. These inputs were incorporated into a lumped energy balance in the form of Q_"in" =Q_"out" , allowing calculation of the retinal temperature elevation relative to blood, ∆T. Heat dissipation pathways, including conduction to surrounding tissue, choroidal blood perfusion, convection, and radiation, were expressed as thermal conductances, G, and analysed within the same framework,∆T=Q_out/G. Under sunlight, total retinal heat input reached approximately 10 mW, approximately threefold higher than at night. Despite this, the predicted steady-state retinal temperature elevation remained very small (~ 10-3 K), suggesting that heat is dissipated highly efficiently. The analysis shows that conduction and choroidal blood perfusion dominate retinal heat removal under physiological conditions, whereas radiation and convection contribute negligibly. The results suggest that retinal heat redistribution via conduction plays a major role in retinal thermal homeostasis, whereas choroidal blood perfusion primarily contributes to subsequent systemic heat removal rather than acting as a dominant limiting mechanism itself. This analysis provides quantitative support for the long-standing hypothesis that the choroid functions as a heat sink for the retina.