Regulation of blood flow is necessary to adapt to different conditions. Regulation of ocular blood flow (OBF) compensates for varying perfusion pressures (autoregulation), adapts to the retinal activity (neurovascular coupling), and keeps the back of the eye at constant temperature (thermoregulation). While all vessels are under the control of the vascular endothelial cells, the retinal vessels are additionally under the control of the neural and glial cells, and the choroidal vessels are influenced by the autonomic nervous system. The optic nerve head is additionally controlled by circulating hormones. If the regulation does not occur according to the needs of the tissue, it is referred to as vascular dysregulation. Such a dysregulation can be secondary in nature, as, for example, in multiple sclerosis, in which the high level of endothelin reduces OBF. Dysregulation, however, can also occur without any underlying disease and is characterized by an inborn tendency to respond differently to various stimuli, such as cold temperatures or mechanical or emotional stress. The constellation of these features is known as primary vascular dysregulation (PVD). Subjects with PVD have disturbed autoregulation leading to an unstable OBF. This instability, in turn, induces a repeated mild reperfusion injury. The resulting oxidative stress contributes to the pathogenesis of glaucomatous optic neuropathy.