The Head Rush

The short answer: Standing moves half a litre to a litre of blood into the legs within seconds. Two different things get called the head rush: a brief, dramatic drop in the first 15 seconds that recovers on its own and is actually more common in young people, and a slower, milder sustained drop that becomes more common with age and matters considerably more.

Up too fast out of a chair, and the room goes briefly grey.

A few seconds of nothing much. Vision narrowing at the edges, a hand out to the back of the couch, and then it passes and you carry on and never think about it again.

The assumption is that this is an ageing thing. It arrived at some point, it did not used to happen, and it joins the list.

That assumption is half wrong, and the half that is wrong is the interesting part. The dramatic version of this is, if anything, more common in young people. The version that genuinely tracks with age is milder, slower, and much easier to ignore.

What Is Actually Happening

Standing is a hydraulic problem.

When you get up, gravity moves roughly half a litre to a litre of blood out of the chest and down into the veins of the legs and abdomen. Less blood returning to the heart means less going out of it, and blood pressure falls.

The body has a correction for this, and it is fast. Pressure sensors called baroreceptors, sitting in the carotid arteries and the aorta, detect the drop within a heartbeat or two. They trigger a sympathetic response: heart rate rises, and the blood vessels in the periphery clamp down to push pressure back up.

Done properly, the whole thing is invisible. You stand up and nothing happens, which is the point.

Sitting for a long stretch and then standing is the version that catches most people out., because the pooling has had longer to establish. Standing up after a long car trip can redistribute 500 to 1000ml downward in one movement.

The Fast One

Here is the first of the two things.

Initial orthostatic hypotension is a transient drop of more than 40 mmHg systolic, or more than 20 diastolic, within 15 seconds of standing. It recovers on its own, usually within 20 to 30 seconds.

Those numbers are large, much larger than the threshold for the other kind, which is why the experience is so noticeable. The mechanism is a timing mismatch: the blood moves faster than the correction arrives. Cardiac output drops before the vessels have clamped down, and for a few seconds the brain is underperfused.

Two things about it are worth knowing.

It occurs more frequently in adolescents and young adults, and has been described in healthy people with completely normal autonomic function. It is generally regarded as benign.

And it is almost impossible to measure in a normal consultation, because a standard blood pressure cuff takes 30 to 60 seconds to inflate and read. By the time the number appears, the event is over. Detecting it requires beat-to-beat monitoring. It also shows up on actively standing but not on a passive tilt table, which is part of how it was identified in the first place.

So the dramatic grey-out, on its own, is not evidence of anything going wrong. It is a brief lag in a correction that then works.

The Slower One Is the One That Matters

Classic orthostatic hypotension is defined differently: a sustained drop of at least 20 mmHg systolic or 10 diastolic within three minutes of standing.

Smaller numbers, but sustained rather than transient, and that is the distinction that counts.

This one does track with age. Prevalence is around 6 per cent in the general population, roughly 5 per cent under 50, and rises to somewhere between 15 and 30 per cent in people over 70. Medical references put it at 15 to 20 per cent of older adults.

The reason is the correction itself. Baroreceptor responsiveness declines with age, which delays both the heart rate response and the peripheral vasoconstriction. The sensors still work. The reply is slower and smaller.

It also sits alongside the other systems that keep people upright, which are changing on their own timetable.

Several other factors load onto it: multiple medications, diabetes, low body mass, and dehydration.

And one that reads as a contradiction. High blood pressure is a risk factor for this, not a protection against it. Hypertension appears to blunt baroreceptor sensitivity, which makes the drop on standing worse. Treated or untreated, it increases vulnerability.

Telling Them Apart

The practical distinction is duration and recovery.

The fast one: hits immediately, feels dramatic, and is gone within half a minute without you doing anything. Worse after prolonged sitting, in hot weather, or standing up from a squat.

The slower one: builds over the first minute or two of standing rather than hitting at once, is milder moment to moment, and persists while you remain upright. It often presents as vague unsteadiness or fogginess rather than a definite grey-out, which is why it gets attributed to tiredness.

The second is the one worth raising with a GP, and it is also the one people are least likely to mention, because it does not feel dramatic enough to be a symptom.

That is worth being direct about. Sustained orthostatic hypotension is associated with falls, and the conversation is a straightforward one: a GP can check it by measuring lying and standing blood pressure, and medications are among the most common contributors, which makes it a very fixable thing to have identified.

Anyone who has actually fainted, who gets this repeatedly, or who has noticed it starting after a medication change should take it in rather than wait.

The Short Version

Half a litre of blood moves the moment you stand, every time, at every age. What changes is how quickly the correction arrives.

The brief grey-out that makes you grab the bench is mostly a timing lag, and not the thing to worry about.

The one to notice is quieter: the first minute after standing feeling slightly off, slightly foggy, slightly unreliable, for long enough that you sit back down.

That one is not a head rush. It is the correction not finishing.

This article is general information only and isn't medical advice.