Hypovolaemia is a state of decreased blood volume, the reduction of which reduces tissue perfusion which may in turn lead to cellular hypoxia and end-organ damage. In blood donation, 500ml of blood is collected in ten minutes. This equates to 10% reduction in whole blood volume. This study investigates the changes in ocular parameters following blood donation in healthy individuals. Seventy-six participants (48 study group, 28 control) were included. Following blood donation, there was no difference in axial length, central corneal thickness, retinal nerve fibre layer thickness and intraocular pressure measurements. Ocular perfusion pressure (OPP) was measured using mean arterial pressure and intraocular pressure. In this study, there was a significant reduction in OPP after blood donation (0.494µm systolic OPP). There was also a significant reduction in choroidal thickness (CT), which was lowest 10 minutes after blood donation, and returned to baseline values at three hours. CT is influenced by numerous extrinsic factors including caffeine, smoking and water drinking, whereas retinal circulation is more efficient in maintaining an almost constant blood flow. This may explain why retinal thickness was not affected by hypovolaemia. The results of this study may help understand choroidal thickness changes in disease.