The Facts About Fainting

Fainting is a temporary loss of consciousness and muscle control. Fainting itself usually doesn't cause injury. The danger comes when someone falls down and hits her head or another body part, leading to injury.

The most common cause of fainting is a sudden drop in blood pressure. The heart is a pump and blood vessels are pipes. If the pump or the pressure in the pipes isn't strong enough, the blood won't reach your head. Since your head is at the top of your body and above your heart, it can't depend on gravity to get the blood there. Fainting is both a response to a lack of blood flow and an effective way of getting you to lie down quickly so blood can flow back to your brain. 

According to the National Institutes of Health, common causes for sudden low blood pressure and fainting are:

  • Heat, which can dilate or relax your blood vessels and lower your pressure. This means your heart has to pump harder to circulate blood.  
  • Dehydration, which concentrates the fluid in your blood vessels. As a result, there is a shortage of blood pumping through your veins and arteries to meet your brain's circulatory needs.
  • Standing up too quickly. This is a gravity issue in which the blood can't circulate to the top of your body quickly enough.
  • Certain medications, especially cardiac and blood pressure medications, which can lower your blood pressure too much or cause irregular or erratic heart rhythms. Your heart pumps in response to electrical impulses provided by your neurologic system. If your heart doesn't get the right signals, it won't beat in the correct rhythm to pump blood efficiently.
  • Low blood sugar (hypoglycemia), which decreases energy supplies your heart needs to pump properly. As a result, adrenaline is released, which causes your heart to beat quickly, but sometimes ineffectively. 
  • Heart problems, which might include anatomical, nerve-conduction, electric, or chemical problems. If the pump doesn't receive the right signals, can't squeeze the blood out, has blocked vessels, leaky valves, weak muscles or other problems, the blood won't circulate through the rest of the body properly.

Other common causes of fainting are pregnancy, stress, and Vagus nerve stimulation.

  • Pregnancy can cause low blood sugar and dehydration, but fainting can also occur when the baby's position in the uterus compresses one of the mother's main blood vessels, effectively blocking circulation to her head. It's not uncommon for a mother to faint after childbirth, due to blood loss and fatigue.  
  • Vagus nerve stimulation from stress, pain, shock, and sometimes pressure from pushing while having a bowel movement or urinating, can create an imbalance of adrenaline and acetylcholine. Adrenaline causes the heart to beat faster and acetylcholine slows it down. Vagus nerve stimulation causes a release of acetylcholine, which temporarily decreases blood flow to the brain. 

How do you know when fainting is serious and when it's not?

Fainting is one of your body's most effective ways of telling you it needs something-and fast. Call your doctor whenever you faint, just to let her know. If it happens once and you recover quickly, it's probably no big deal. If it happens again or frequently, however, it's time to get yourself checked out. Your fainting spells might require a simple fix like reducing stress, changing your diet, drinking more water, or tinkering with your prescriptions, but fainting might also indicate you have a serious medical condition.  


National Institutes of Health
Medline Plus

US Department of Health and Human Services

National Diabetes Clearinghouse