When you stand up, gravity pulls about half a liter of blood downward into your legs and abdomen. Normally your body corrects for this in seconds, but if that correction fails or falls short, your brain briefly loses blood flow and you faint. This is called orthostatic syncope, and it’s one of the most common reasons people pass out. About one in five older adults experiences clinically significant blood pressure drops upon standing, though it can happen at any age.
What Happens Inside Your Body
The moment you go from sitting or lying down to standing, blood pools in your lower body and your blood pressure dips. Sensors in your neck and chest (called baroreceptors) detect that drop within a heartbeat. They send signals to your brain, which responds by speeding up your heart rate and tightening blood vessels throughout your body. The result: blood pressure bounces back to normal, and your brain stays fully supplied with oxygen.
When any part of that chain breaks down, blood pressure stays low after you stand. Your brain, sitting at the top of your body, is the first organ to feel the shortage. Within seconds you may feel lightheaded, your vision may go gray or tunnel, and if the pressure stays low long enough, you lose consciousness. The good news is that falling down (or even just sitting) brings your head level with your heart, restoring blood flow almost immediately.
Orthostatic Hypotension vs. Vasovagal Syncope
These two conditions are the most common causes of fainting, but they work differently. Orthostatic hypotension is defined as a drop of at least 20 mmHg in systolic blood pressure (the top number) or 10 mmHg in diastolic pressure within three minutes of standing. It happens right away, usually within seconds to a couple of minutes after you get up. Sitting or lying back down relieves symptoms quickly.
Vasovagal syncope, by contrast, is a nervous system overreaction to a trigger like pain, fear, the sight of blood, or standing still for a long time. With vasovagal episodes, your blood pressure often holds steady for ten minutes or more before it crashes suddenly. The triggers matter here: if you faint during a blood draw or while standing in a hot, crowded room, that points toward vasovagal. If you faint within moments of getting out of bed or rising from a chair, orthostatic hypotension is the more likely explanation.
Common Causes
Dehydration and Low Blood Volume
Your body needs enough circulating blood to maintain pressure when gravity shifts it downward. Dehydration from not drinking enough water, heavy sweating, vomiting, or diarrhea shrinks your blood volume and makes it harder for your cardiovascular system to compensate. This is one of the most frequent and most fixable causes, especially in younger people.
Medications
Drugs are one of the most common causes of orthostatic blood pressure drops. Several categories are known culprits:
- Diuretics (water pills) increase urine output and can deplete your fluid volume, particularly in older adults.
- Blood pressure medications including alpha-blockers, beta-blockers, and nitrates can reduce vascular tone or blunt your heart’s ability to speed up when you stand.
- Antidepressants are a major and underappreciated cause. Older tricyclic antidepressants cause orthostatic drops in 10 to 50 percent of patients. SSRIs roughly double the risk, and SNRIs have been linked to a fivefold increase in risk among older adults prone to falls.
- Anti-anxiety medications (benzodiazepines) can significantly impair the blood pressure response in the first seconds after standing.
If your fainting started after beginning a new medication or changing a dose, that connection is worth investigating with whoever prescribed it.
Nervous System Conditions
Conditions like Parkinson’s disease, diabetes (which can damage the nerves controlling blood vessels), and other forms of autonomic nerve damage can permanently impair the reflex that corrects blood pressure when you stand. In these cases, the problem tends to be chronic rather than occasional.
POTS
Postural orthostatic tachycardia syndrome, or POTS, is a related condition that disproportionately affects younger people. Instead of a large blood pressure drop, POTS causes your heart rate to jump by 30 beats per minute or more (40 or more in teenagers) within ten minutes of standing. Blood pressure may stay mostly normal, but the racing heart, lightheadedness, and sometimes fainting can be debilitating. POTS is diagnosed only when significant orthostatic hypotension has been ruled out.
Age Makes a Difference
Orthostatic hypotension becomes dramatically more common with age. A large meta-analysis found that about 22 percent of older adults living independently have it, and nearly 24 percent of those in long-term care facilities. The prevalence rises with each decade of life. Aging stiffens blood vessels, reduces the sensitivity of baroreceptors, and often coincides with the use of multiple medications that each contribute to the problem. In younger adults, dehydration, POTS, and vasovagal triggers are far more common explanations.
Physical Maneuvers That Help
Simple muscle contractions can meaningfully raise your blood pressure and prevent fainting. A combined analysis of over 600 subjects found that counter-pressure maneuvers increased systolic blood pressure by nearly 15 mmHg on average, which is often enough to prevent symptoms. The most effective movements include:
- Leg crossing with tensing: Cross your legs at the ankles and squeeze your thigh and calf muscles. This showed some of the strongest blood pressure increases across multiple studies.
- Squatting: If you feel faint, squatting down immediately compresses leg veins and pushes blood back toward your heart. It consistently raised blood pressure in both healthy subjects and people with orthostatic hypotension.
- Tensing before you stand: Clenching your leg and abdominal muscles before rising was more effective than tensing after you’re already upright. It helps prevent the initial blood pressure dip rather than trying to recover from it.
- Marching in place or doing calf raises: Both activate the muscle pump in your legs and improved blood pressure in people with orthostatic hypotension.
If you know you’re prone to this, get into the habit of tensing your legs for a few seconds before standing, and rise in stages: sit up first, pause, then stand.
Fluid and Salt Intake
For people with confirmed orthostatic hypotension, clinical guidelines recommend a daily salt intake of 6 to 10 grams, which is well above what most dietary guidelines suggest for the general population. The extra sodium helps your body retain fluid and maintain blood volume. If increasing salt through food isn’t practical, salt tablets taken with meals are an alternative. This recommendation is specifically for people with a diagnosed condition, not a general health tip, since excess salt raises cardiovascular risk in people who don’t need it.
Staying well hydrated matters too. Drinking a glass or two of water 15 to 30 minutes before getting up in the morning can help, since blood pressure regulation tends to be worst after a night of sleep. During sleep, your body excretes sodium through urine in response to lying flat, which means you can wake up with a meaningfully lower blood volume than you had when you went to bed.
When Fainting Is More Serious
Most fainting upon standing is caused by the mechanisms described above and, while disruptive, isn’t dangerous beyond the risk of injury from falling. However, some fainting episodes signal a cardiac problem. Fainting during exercise, while lying down, or with no warning at all (no lightheadedness, tunnel vision, or nausea beforehand) warrants prompt evaluation. The same is true if fainting is accompanied by chest pain, a pounding or irregular heartbeat, shortness of breath, or if it happens repeatedly without an obvious trigger like dehydration or medication changes. A family history of sudden cardiac death or unexplained fainting also raises the stakes.