Haemoglobin Video Imaging (HVI) demonstrates conjunctival and episcleral blood flow in man with the resolution of a single erythrocyte. A new method for establishing vessel hierarchy in micro-circulations is described, which recognises either delivery or drainage vessels and references vessel order to the capillary. These tools have been used to characterise blood flow.Anterior ciliary arteries show pulsatile variation in diameter. The episcleral arterial circle that they supply has functional apices with pulsatile flow reversal. Perfusion fields overlap: a single delivery vessel may project to many drainage vessels and vice-versa. Some vascular pathways remained inactive throughout a 1 min angiogram.Small conjunctival delivery vessels have laminar flow, but advancing luminal constrictions are often observed within the blood column. Laminar flow is lost in low-order drainage vessels where erythrocytes aggregate, but quickly recovers, new striae being added to the blood column at each confluence. Aqueous forms a discrete column, which centralises in episcleral drainage vessels.There is strong evidence that the luminal constrictions in small delivery vessels propel blood by peristalsis: they form spontaneously, remote from bifurcations; a single vessel may have multiple constrictions; they truly narrow the lumen, rarely contributing volume to post-capillary venules; they can proceed faster than the vessel contents; they never enter the drainage system; the trailing edges of erythrocyte boluses usually taper. They are rhythmically aligned with cardiac systole.While blood is transported to the periphery by the heart, it is actively transferred through tissues by peristalsis in small delivery vessels.
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