Why do you feel dizzy when you stand up too quickly?

That brief head-rush is orthostatic hypotension — a momentary drop in blood pressure as gravity redistributes blood away from your head.

When you stand, something like half a litre to a litre of blood pools rapidly in the veins of your legs and abdomen. Less blood returns to the heart, so the heart has less to pump, and pressure at the level of the brain falls. Your brain is roughly 30cm above your heart when upright, and it is exquisitely sensitive to even a brief reduction in perfusion.

Normally you never notice this, because a reflex corrects it within seconds. Pressure sensors called baroreceptors in the carotid arteries and aorta detect the drop and trigger an immediate response: heart rate rises and blood vessels constrict, restoring pressure. The dizziness happens when that reflex is slower than the drop — you get a two- or three-second window of reduced blood flow before the correction catches up.

What makes it more likely: dehydration, which reduces blood volume; heat, which dilates vessels; standing after a long period lying down; alcohol; large meals, which divert blood to the gut; and a range of medications, particularly blood pressure drugs, diuretics and some antidepressants. It also becomes more common with age as the reflex slows.

Practical measures: stand up in stages, sit on the edge of the bed first, stay well hydrated, and flex your calf muscles before standing to push blood back up.

Seek medical advice if it happens frequently, causes actual fainting, occurs with chest pain, palpitations or breathlessness, follows a new medication, or comes with confusion or slurred speech. Recurrent fainting always warrants assessment, since some causes are cardiac.

General information only, not a substitute for clinical advice.