The transfer of fluid through a tissue. For example, when blood passes through the lung tissue, dissolved oxygen perfuses from the moist air in the alveoli to the blood. Fluid may also be deliberately introduced into a tissue by injecting it into the blood vessels supplying the tissue. It is used as a sign of how adequate the circulation is at the time of illness. Poor peripheral perfusion, a sign of circulatory collapse or shock, is recognised by pressing on the skin to force blood from capillaries. The time it takes for them to refill and the skin to become pink is noted: more than 5 seconds, and the circulation is likely to be compromised.