What determines if a capillary is open?

The openness of a capillary is primarily determined by the local tissue's metabolic needs and the regulation of local blood flow. Factors such as oxygen levels, carbon dioxide levels, and the presence of various signaling molecules (like nitric oxide) influence the dilation or constriction of pre-capillary sphincters, which control blood flow into capillaries. Additionally, pressure gradients within the circulatory system and the overall health of the vascular endothelium can also affect capillary perfusion.