Low blood pressure happens when your heart, blood vessels, or the volume of blood in your body can’t maintain enough force to push blood effectively through your circulation. A reading below 90/60 mmHg is generally considered low. For some people that’s perfectly normal and causes no symptoms at all, but when blood pressure drops enough to reduce blood flow to your brain and organs, it causes dizziness, fainting, fatigue, and in severe cases, shock.
Understanding why it happens starts with how your body keeps blood pressure stable in the first place, and the many points where that system can break down.
How Your Body Regulates Blood Pressure
Your body has a real-time pressure monitoring system built into the walls of your major arteries. Specialized sensory neurons called baroreceptors form claw-like structures around the aortic arch, the large vessel leaving your heart. These neurons contain pressure-sensitive channels that physically detect how much the artery wall is stretching with each heartbeat. When they sense a drop, they signal your brain within seconds.
Your brain responds by speeding up your heart rate, tightening blood vessels to increase resistance, and triggering hormone signals that tell your kidneys to retain water and salt. When everything works, this system keeps your blood pressure remarkably stable whether you’re lying down, standing up, exercising, or digesting a meal. Low blood pressure happens when one or more parts of this chain fail: the heart isn’t pumping strongly enough, the blood vessels are too relaxed, or there simply isn’t enough fluid in the system.
Not Enough Blood Volume
The most straightforward cause of low blood pressure is not having enough fluid in your bloodstream. Your cardiovascular system is a closed loop, and pressure depends on how full that loop is. When fluid drops, pressure drops with it.
Dehydration is the most common culprit. Not drinking enough water, sweating heavily, vomiting, diarrhea, or fever can all pull fluid out of your circulation faster than you replace it. Blood loss works the same way, whether from an injury, surgery, or internal bleeding. In its most severe form, called hypovolemic shock, your heart literally can’t fill with enough blood between beats to maintain pressure, and organs start losing their blood supply.
Even mild dehydration can cause noticeable drops in blood pressure, especially in older adults or people who are already on the lower end of normal. Hot weather compounds the effect because your blood vessels dilate to release heat, reducing pressure further while you’re already losing fluid through sweat.
Heart Problems That Lower Pressure
Your heart is the pump that generates blood pressure, so anything that weakens the pump directly lowers the pressure it creates. Several heart conditions can do this:
- Abnormally slow heart rate (bradycardia): Fewer beats per minute means less blood pushed through the system each minute, which can drop pressure if the heart can’t compensate by pumping more per beat.
- Heart valve problems: Leaky or narrowed valves reduce the efficiency of each heartbeat, so less blood reaches your arteries.
- Heart failure: A weakened heart muscle can’t contract forcefully enough to maintain adequate pressure.
- Heart attack: Damage to the heart muscle impairs its pumping ability, sometimes suddenly and severely.
These conditions don’t always cause dramatically low readings. Sometimes they produce a slow, gradual decline that shows up as persistent fatigue and lightheadedness before anyone checks a blood pressure cuff.
Blood Vessels That Open Too Wide
Even with a strong heart and plenty of blood, pressure will drop if your blood vessels relax too much. Think of it like water pressure in a garden hose: widen the hose and the pressure at the nozzle drops, even though the same amount of water is flowing.
This is exactly what happens during sepsis, a life-threatening response to infection. Your immune system floods the body with inflammatory signals that force blood vessels to dilate. Blood pools in expanded veins instead of circulating efficiently. On top of that, the walls of tiny blood vessels become leaky, allowing fluid to seep out of the bloodstream into surrounding tissues. The result is a dangerous combination: the pipes are too wide and the fluid is escaping. Even though the total blood volume started out normal, it’s suddenly not enough to fill the expanded space.
Anaphylaxis, a severe allergic reaction, works through a similar mechanism. A massive release of histamine and other chemicals causes widespread vessel dilation and fluid leakage, crashing blood pressure within minutes. Both septic shock and anaphylactic shock are medical emergencies precisely because of how rapidly and severely they drop blood pressure.
Hormonal and Endocrine Causes
Hormones play a quieter but critical role in blood pressure regulation. The adrenal glands, which sit on top of your kidneys, produce two hormones that matter most here. Cortisol helps maintain blood vessel tone and the body’s response to stress. Aldosterone controls how much sodium and potassium your kidneys retain, which directly determines blood volume.
In Addison’s disease, the adrenal glands don’t produce enough of either hormone. Without adequate aldosterone, your kidneys flush out too much sodium and water, shrinking blood volume. Without enough cortisol, blood vessels lose some of their ability to constrict. The combined effect is chronic low blood pressure that gets worse with standing. In a crisis, such as during illness or injury, the adrenal glands can’t ramp up cortisol production the way a healthy body would, and blood pressure can plummet alongside blood sugar.
Thyroid disorders can also contribute. An underactive thyroid slows the heart rate and reduces the force of each contraction, while an overactive thyroid can cause blood vessels to dilate. Either can shift blood pressure readings lower in some people.
Medications That Lower Blood Pressure
Medications are one of the most common reasons people experience unexpectedly low blood pressure. This goes beyond blood pressure drugs, though those are the most obvious offenders. Diuretics reduce fluid volume. Beta blockers slow the heart rate. Alpha blockers relax blood vessel walls. All of them can overshoot their target, especially when doses change or when combined with each other.
Several other medication classes cause low blood pressure as a side effect:
- Parkinson’s disease medications containing levodopa affect the nervous system in ways that reduce blood vessel constriction.
- Tricyclic antidepressants can interfere with the signals that tighten blood vessels when you stand up.
- Erectile dysfunction drugs work by dilating blood vessels, and the effect isn’t limited to one area of the body. Combining them with nitrate heart medications can cause a dangerous pressure drop.
If you’ve recently started a new medication or changed a dose and notice dizziness or lightheadedness, the timing is rarely a coincidence.
Drops After Standing Up
Orthostatic hypotension is a specific pattern where blood pressure falls when you go from sitting or lying down to standing. Gravity pulls blood toward your legs the moment you stand, and your body normally compensates within a couple of heartbeats by tightening leg veins and speeding up the heart. When that reflex is too slow or too weak, blood pressure drops and you feel dizzy or see spots.
This type of drop is more common in older adults because the baroreceptor reflex slows with age. Dehydration, prolonged bed rest, and many of the medications listed above all make it worse. Addison’s disease specifically causes this pattern because of the fluid loss from low aldosterone. Some people experience orthostatic drops only in the morning or after hot showers, both situations where blood vessels are already more relaxed.
Drops After Eating
Postprandial hypotension is a blood pressure drop that occurs within 30 to 60 minutes of eating, sometimes lasting up to two hours. When you eat, your digestive system demands a surge of blood flow. Normally, your heart rate increases and blood vessels elsewhere in your body tighten to compensate. In people with postprandial hypotension, the heart rate doesn’t rise enough and the vessels don’t constrict as they should, so blood pressure falls by 20 mmHg or more.
Large meals, high-carbohydrate meals, and alcohol all make postprandial drops worse. Not getting enough salt, being dehydrated, and hot weather compound the problem. This form of low blood pressure is most common in older adults and in people who already have other conditions that affect their autonomic nervous system, such as Parkinson’s disease or diabetes.
Putting the Causes Together
What makes low blood pressure tricky is that causes often stack. An older adult taking a beta blocker who skips water on a hot day and then stands up quickly is hit by three factors at once: medication slowing the heart, reduced fluid volume, and gravity pulling blood to the legs. Each cause alone might not produce symptoms, but together they can cause a faint.
The pattern of when your blood pressure drops tells a lot about why. Drops only on standing point toward orthostatic causes. Drops after meals suggest postprandial hypotension. Persistent low readings regardless of position raise questions about heart function, blood volume, or hormonal issues. Sudden, severe drops with other symptoms like confusion, rapid breathing, or skin changes signal emergencies like sepsis or anaphylaxis where the body’s ability to maintain pressure has failed on a systemic level.