tissue perfusion

Drugs travel to tissues by hitching a ride in the blood, so the more blood a tissue receives, the faster a drug arrives. Tissue perfusion is the rate of blood flow delivered to a given tissue, and it is one of the chief factors deciding how quickly a drug reaches each part of the body.

Organs are not equal. The brain, heart, liver, kidneys and endocrine glands are richly perfused and receive drug within seconds to minutes; skin, resting muscle and especially fat are sparsely perfused and equilibrate over many minutes to hours. For tissues with good blood supply, delivery is fast and the limiting step is how easily the drug crosses membranes; for poorly perfused tissues, blood flow itself is the bottleneck.

Perfusion changes with physiology and disease. Exercise, shock, heart failure, sepsis and local vasoconstriction all alter regional blood flow and therefore drug delivery. In shock, for instance, poor perfusion of muscle and fat can make an intramuscular or subcutaneous injection absorb erratically, and slowed delivery can delay both onset and offset of effect.