perfusion

Perfusion is the actual delivery of blood flow to a tissue or organ—the whole reason the circulation exists. It is not about pressure for its own sake but about whether enough blood, and therefore enough oxygen, is reaching the cells that need it.

Perfusion to an organ depends on the pressure driving blood across it (roughly the difference between arterial inflow and venous outflow pressure) divided by the resistance of that organ’s vessels. Many organs autoregulate—adjusting their local vessel tone to keep flow fairly constant despite changing pressures—so within limits, perfusion is protected. But if mean arterial pressure falls below that autoregulatory range, or if vessels are blocked, perfusion drops and tissues become ischemic.

Clinicians judge perfusion at the bedside through clues like skin color and warmth, capillary refill, urine output, and mental status, since adequate blood pressure does not always guarantee adequate flow. Shock, in essence, is a state of inadequate perfusion—organs not getting the oxygen they need—even though blood pressure may at first be deceptively maintained.