Blood pressure

Blood pressure is the force that circulating blood exerts against the walls of the arteries, recorded as two numbers in millimetres of mercury (mmHg). The higher figure — systolic pressure — is the peak force generated each time the heart contracts and ejects blood outward. The lower figure — diastolic pressure — is the residual force present when the heart relaxes between beats.

A reading near 120/80 mmHg is considered typical for a healthy adult at rest, though normal varies with age, body size, and individual physiology. Blood pressure is classed as a vital sign because it reflects how efficiently the circulatory system is meeting the body's demands — and because both sustained highs and sharp lows carry real risk.

How it works · its role

Blood pressure is determined by two interacting forces: how hard the heart pumps (cardiac output) and how much the blood vessels resist that flow (peripheral vascular resistance). When vessels constrict, resistance rises and pressure climbs; when they dilate, resistance falls and pressure drops.

The autonomic nervous system is the primary regulator. Its sympathetic branch — the "fight-or-flight" arm — releases norepinephrine, which constricts blood vessels and speeds the heart. The parasympathetic branch does the opposite, slowing the heart and allowing vessels to relax.

Hormones add a longer-range layer of control. Epinephrine amplifies the sympathetic response rapidly. The renin–angiotensin–aldosterone system adjusts blood volume through the kidneys, providing a slower, sustained influence over resting pressure.

Relevance to substances & effects

Blood pressure is one of the most common physiological targets of psychoactive substances, because many drugs act directly on the same pathways the nervous system uses to control it.

Stimulants — including amphetamines, cocaine, and MDMA — raise blood pressure by triggering norepinephrine release or blocking its reuptake. The result is vasoconstriction and increased cardiac workload. At high doses or in people with underlying cardiovascular conditions, these increases can become clinically significant.

Classic psychedelics such as LSD and psilocybin produce modest blood pressure elevation, thought to arise partly from agonism at adrenergic receptors alongside their primary serotonergic action.

Cannabis produces a more variable picture. The initial phase of intoxication often brings a transient rise in heart rate and sometimes blood pressure, followed in some users by a drop on standing — a common cause of cannabis-related dizziness or brief fainting.

Opioids and most sedative–hypnotic drugs tend to lower blood pressure through central depression of sympathetic output and vasodilation. Alcohol acutely dilates peripheral vessels and can reduce pressure; chronic heavy use is associated with elevated resting pressure over time.

Because many substances shift blood pressure in different directions, combinations can produce compounding or opposing effects that are difficult to predict — a risk the Interactions section of each substance record addresses.