Why Do Some People Faint at the Sight of Blood?

What You'll Learn

• Fainting is your body's way of quickly sending blood back to your brain
• The vagus nerve acts like an alarm button that can drop your heart rate and blood pressure
• When blood pressure drops too fast, less blood reaches the brain and you faint
• About 1 in 3 people have a reflex that makes them faint at the sight of blood
• Fainting is not dangerous and usually lasts only a few seconds

Words to Know

Faint

To suddenly pass out for a few seconds

"She felt dizzy and fainted at the sight of blood."

Vagus nerve

A super-long nerve from your brain to your belly that controls heart rate

"The vagus nerve helps slow down your heart."

Blood pressure

How hard your blood pushes through your body's tubes

"When blood pressure drops, you might feel dizzy."

Reflex

Something your body does automatically without choosing it

"Sneezing is a reflex you can't control."

Check What You Learned

Fun Facts

• Fainting (syncope) is a sudden, brief loss of consciousness caused by reduced blood flow to the brain

  Source: American Heart Association

• The vagus nerve runs from the brain stem to the abdomen and helps regulate heart rate and blood pressure

  Source: National Institute of Neurological Disorders and Stroke

• Vasovagal syncope is the most common type of fainting, triggered by stress, fear, or the sight of blood

  Source: Mayo Clinic

• When the vagus nerve is overstimulated, it can cause heart rate and blood pressure to drop rapidly

  Source: Mayo Clinic

• The brain uses approximately 20 percent of the body's total energy despite being only about 2 percent of body weight

  Source: Harvard Medical School

• About 1 in 3 people have a vasovagal reflex strong enough to cause fainting at the sight of blood or needles

  Source: American College of Cardiology