Fainting at the sight of blood may be an ancient trick for playing dead
Roughly 15 percent of people react to blood, needles or injury by feeling faint. One evolutionary idea holds that collapsing once helped our ancestors survive attackers and slowed bleeding, much like an opossum playing dead. Another suggests fainting signalled to rival groups that a person posed no threat.
Syncope, the medical term for fainting, is a brief loss of consciousness and muscle strength that comes on fast and resolves by itself. It happens when blood flow to the brain drops, usually because blood pressure falls. People often notice warning signs first: lightheadedness, sweating, pale skin, blurred vision or nausea. When the collapse is only partial, it is called presyncope. The word comes from a Greek term meaning a cutting up or striking off.
Causes fall into three groups. The most common is reflex or neurally mediated syncope, in which blood vessels widen and the heart slows at the wrong moment. Triggers can be emotional, like fear or the sight of blood, or oddly specific: coughing fits, urinating, swallowing, even cold fizzy drinks or belching in the rare swallowing-induced form. Pressure on the carotid sinus in the neck can do it too. A second group involves a drop in pressure on standing up. The third, the heart and its vessels, accounts for about 10 percent of cases and is usually the most serious, including abnormal rhythms that can turn dangerous.
In the common vasovagal type, a person already short on circulating blood meets a fright, and the brain calls on the heart to pump harder. When it cannot, a feedback signal through the vagus nerve overcorrects and slows the heart too much. The pathway was first traced in animal experiments by Bezold in Vienna in the 1860s, and a 2023 study pinpointed a network of vagal sensory neurons and a brain region involved in fainting and recovery. Genetics seems to play a part as well, with a first risk locus found near the ZNF804A gene.
Fainting affects about three to six people in every thousand each year, more often older people and women. Up to half of women over 80 and a third of medical students recall at least one episode, and it accounts for one to three percent of emergency visits.
Source: Syncope (medicine)
