Blood flow

Blood flow is the volume of blood passing through a vessel, tissue, or organ per unit of time. In pharmacology, two sites matter most: the liver, where drugs are cleared, and the brain, where psychoactive effects are felt. Regional blood flow shapes both how long a drug stays in the body and where in the brain it acts.

The liver receives roughly a quarter of cardiac output at rest. For drugs with high hepatic extraction — compounds the liver strips almost completely on each pass — clearance depends on how much blood arrives, not on enzyme capacity. Anything that changes circulation, from liver disease to a co-administered drug, changes how much of these compounds actually reaches systemic blood.

How it works · its role

Blood flow through any tissue is governed by two forces: the pressure difference driving blood in, and the resistance of the local vessels. The autonomic nervous system and local chemical signals can widen vessels (vasodilation) or narrow them (vasoconstriction), shifting distribution between tissues minute to minute.

The brain runs a tighter control system called cerebral autoregulation. Over a broad range of blood pressures it keeps cerebral blood flow relatively constant by adjusting vessel diameter. This protection breaks down under extreme conditions — severe hypertension, hypoxia, or certain toxins — at which point brain perfusion tracks systemic pressure more directly.

At the liver, blood arrives by two routes: the portal vein, carrying nutrient-rich blood from the gut, and the hepatic artery. Total hepatic blood flow falls significantly in cirrhosis, heart failure, or haemorrhage. When it falls, high-extraction drugs accumulate — their half-lives extend not because enzymes change, but because less blood is delivered for processing.

Relevance to substances & effects

Stimulant drugs — cocaine and amphetamines — cause systemic vasoconstriction, raising blood pressure and redirecting flow away from peripheral tissues. Acute cocaine use is associated with reduced cerebral blood flow in several cortical regions, a pattern thought to contribute to the neurotoxic potential seen with chronic heavy use.

Cannabis shows a more complex picture: acute intoxication increases regional cerebral blood flow in some frontal and temporal areas while reducing it in others. The perceptual and cognitive shifts of intoxication map loosely onto these regional differences, though the relationship is not linear or fully understood.

Serotonergic psychedelics produce distinctive patterns of altered regional cerebral blood flow — shifts in sensory cortices and default-mode network regions that correlate with subjective state. These vascular signatures are secondary to receptor activation rather than a primary mechanism, but they help explain the neuroimaging fingerprints of psychedelic experience.

For high-extraction drugs — certain opioids and some beta-blockers among them — hepatic blood flow is the rate-limiting step for clearance. Conditions that reduce cardiac output or hepatic perfusion, including advanced liver disease and heart failure, can raise plasma concentrations and meaningfully prolong effects.