Abstract: The human retina is supplied by two vascular systems: the highly vascular choroidal, situated behind the retina; and the retinal, which is dependent on the restriction that the light path must be minimally disrupted. Between these two circulations, the avascular retinal layers depend on diffusion of metabolites through the tissue. Oxygen supply to these layers may be threatened by diseases affecting microvasculature e.g., diabetes and hypertension, which may ultimately cause loss of sight. An accurate model of retinal blood flow will therefore facilitate the study of retinal oxygen supply and, hence, the complications caused by systemic vascular disease. Here, two simple models of the blood flow and exchange of hydrogen with the retina are presented, and compared qualitatively with data obtained from experimental measurements. The model captures some interesting features of the exchange and highlights effects that will need to be considered in a more sophisticated model and in the interpretation of experimental results.