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BrainStuff: What Causes Certain People to Faint When They See Blood? MytourImagine watching a cooking show where the chef accidentally cuts her finger—would that make you uncomfortable? How would you react if this happened right in front of you?
Around 15% of people faint when they see blood, and for up to 4%, this reaction turns into a full-blown phobia. As host Josh Clark discusses in the BrainStuff video, it’s fascinating that some people can handle their own injuries but still faint when they see their own blood.
These fainting episodes are rooted in anxiety, which researchers suggest triggers a rapid rise and fall in blood pressure. This sudden drop reduces blood flow to the brain, leading to a loss of consciousness.
A painting from 1830 by Louis Leopold Boilly shows a woman fainting in the middle of a crowd.
DEA/G. Dagli Orti/Getty ImagesIt’s natural for blood pressure to increase during moments of real or perceived danger, part of the sympathetic nervous system’s "fight or flight" reaction. However, issues with consciousness occur when this spike in blood pressure suddenly reverses. This phenomenon is linked to the vagus nerve, which connects the brain’s nucleus of the solitary tract (NST) to parts of the body controlling involuntary functions like breathing, swallowing, and heart activity.
The NST functions like a switch, alternating between fight-or-flight responses and the calming effects that follow. One theory suggests that as the NST shifts from the sympathetic response (preparing you for action) to the parasympathetic response (calming you down), the vagus nerve receives mixed signals, attempting to lower blood pressure while simultaneously increasing heart rate. This conflict diverts blood away from the brain, leading to fainting.
Another theory proposes that the NST switches too rapidly between sympathetic and parasympathetic responses, causing the body to temporarily shut down by inducing a blackout. Researchers also believe that since the NST controls the disgust response, the combination of fear and disgust becomes intertwined, triggering fainting. But why does this confusion happen? Could it potentially serve as a survival mechanism?
It’s hard to picture our ancestors frequently fainting at the sight of their own blood while still evading predators. Despite the unlikelihood that the human family tree thrived with such tendencies, researchers believe fainting might have served as a survival mechanism—both then and now. For example, if you were attacked by a bear and fainted upon seeing your own blood, the bear might lose interest and leave you alone, battered but alive. (Fainting goats, anyone?)
A more plausible explanation is that fainting at the sight of blood, triggered by a sudden drop in blood pressure, might act as a protective response to prevent excessive bleeding.
Regardless of the reason behind fainting, collapsing into a horizontal position helps the heart restore blood flow to the brain. By the time you regain consciousness, the bleeding may have already stopped.
Syncope, commonly known as fainting, affects roughly one-third of the population and can be triggered by various factors, from heart issues to dehydration.
