You feel dizzy when you stand up too fast because gravity pulls 300 to 800 milliliters of blood into your legs and abdomen the moment you rise. Less blood returns to the heart, blood pressure dips, and for a second or two your brain receives less than it needs. The sensation is that gap, before a reflex closes it.
In most people the whole episode is over in seconds. When it is not, the pattern has a clinical name and a measurable definition.
What Gravity Does the Moment You Stand
Lying down, blood is distributed fairly evenly along the body. Standing turns that arrangement vertical.
Between 300 and 800 milliliters, somewhere between a can of soda and a large bottle of water, shifts downward into the veins of the legs and the splanchnic circulation around the gut. Veins are stretchy, so they hold it. That blood is no longer returning to the heart, which means the heart has less to pump, and cardiac output falls.
Blood pressure is the product of cardiac output and the resistance of the blood vessels. Drop one side of that equation and pressure falls with it.
The Reflex That Normally Catches It
The correction is the baroreceptor reflex, and it is quick.
Baroreceptors are stretch sensors in the walls of major arteries. When pressure drops, they fire less, and the nervous system reads the change instantly. Sympathetic activity rises, narrowing arterioles and the splanchnic vessels so that the same volume of blood fills a smaller space at higher pressure. Vagal tone falls, letting the heart rate climb.
In a healthy person these transient changes return to baseline within about 15 seconds. Diastolic pressure, the lower of the two numbers, actually rises slightly on standing rather than falling.
The brief swimming sensation some people notice is the reflex working with a short delay, not failing.
When the Dizziness Has a Name
If the drop is larger and lasts, it is orthostatic hypotension, defined as a sustained fall of at least 20 mmHg in systolic pressure or 10 mmHg in diastolic pressure within three minutes of standing.
That definition matters because it is measurable. It separates a common momentary sensation from a finding a clinician can record with a cuff.
Clinicians divide the causes in two. Neurogenic causes involve the signaling itself, where the reflex cannot mount an adequate response, and include Parkinson disease, multiple system atrophy and diabetic neuropathy. Non-neurogenic causes leave the reflex intact but give it too little to work with, including volume depletion, cardiovascular disease, alcohol, aging, and medicines such as blood pressure drugs, diuretics and some antidepressants.
Who It Affects Most
Age is the strongest pattern. US population studies put prevalence below 5 percent among people aged 45 to 49, at around 15 percent for those aged 65 to 69, and above 25 percent at 85 and older. A systematic review found roughly one in five community-dwelling adults aged 60 or over experience it, with higher rates in institutional settings.
The reason this is tracked so closely is falls. A brief loss of balance is a different event at 30 than at 80.
How It Relates to Low Blood Pressure Generally
They overlap but are not the same thing. The National Heart, Lung, and Blood Institute defines low blood pressure as a reading below 90/60 mmHg, against a normal adult reading usually under 120/80.
A resting reading can sit below that line without causing any symptoms at all, and a person with entirely normal resting pressure can still drop sharply on standing. What clinicians attend to is not the number by itself but whether organs are receiving enough blood, which is what produces symptoms such as dizziness, lightheadedness, blurry vision, confusion, fainting, fatigue, nausea and heart palpitations.
What Official Guidance Says About Warning Signs
This article explains a mechanism and is not medical advice. Anything about your own readings or symptoms is a question for a clinician who can examine you.
On one point official sources are specific. The NHLBI lists cold and sweaty skin, rapid breathing, a blue skin tone, or a weak and rapid pulse as signs of shock, and advises calling emergency services if they appear.
Beyond that, thresholds and management differ by country and guideline, which is one reason the same reading can be described differently depending on where it is taken.