Why do some people faint when they see blood?
Why do some people faint when they see blood?
by jay ingram(science of why book)
A LOT OF SCIENTISTS WILL answer this question by simply saying, “Nobody has a clue.” That might be short and to the point, but it’s far from satisfying. There are answers, but they’re contentious, and one in particular has the uncanny ability to make some scientists’ blood boil.Lowered blood pressure, an irregular heartbeat or low blood sugar can all result in a momentary loss of consciousness called fainting—or in medical terms, syncope (rhymes with “canopy”). Humans can faint for all sorts of odd reasons—you may hear of people fainting after coughing, after urinating or after stretching. Sometimes, you can faint simply by getting out of a chair too quickly. In most cases, standing up suddenly causes blood to pool in the legs, lessening flow to the brain. If that dip in blood pressure is extreme, it knocks you out for a short time. You recover because blood flow is re-established to your brain, either because you fall down or because you have the presence of mind to tuck your head between your legs just before you black out. In both cases, your head is positioned below, or at least no higher than, your heart, so refilling the brain with blood is easier. But all these versions of fainting are straightforward physiological events triggered by physical stimuli, not mental conditions. How do we explain what happens when fainting is induced by pain, anxiety, emotional stress or fear? Some people faint at the sight of a needle. We assume that’s brought about by the expectation that pain will soon be inflicted, but that fear has little connection to the physiological consequences of, say, standing up too quickly. Instead of explaining these cases with physiology, we turn to evolution. In 2005, Rolf Diehl, at the Krupp Hospital in Essen, Germany, suggested that fainting, with its accompanying drop in both blood pressure and heart rate, was a protective response exhibited by wounded animals. He reasoned that if an animal starts bleeding (and we are animals, remember!), its initial response is to constrict blood vessels and jack up blood pressure and heart rate, so as to maintain circulation in the face of blood loss. But if the bleeding doesn’t stop and the animal’s blood loss reaches a critical value—roughly a third of its total blood volume—the reverse kicks in: blood vessels loosen, the heart rate drops and the animal’s circulatory system slows until it eventually loses consciousness. (This isn’t the same as playing dead. An animal playing dead still has its heart pumping and its nervous systems on high alert, exactly the opposite of being in a faint.) But while passing out may leave an animal vulnerable, Diehl argued that the drop in both blood pressure and heart rate buys precious time, allowing blood to clot and thereby reducing total blood loss. If the animal maintained a normal blood pressure in this critical situation, he argued, it would actually
hasten death, not prevent it. What’s fascinating is that this straightforward physiological mechanism in humans happens not only in response to the loss of one’s own blood but also to someone else losing blood. You’d think the emotion in play here would be fear—seeing someone else’s blood raises fears that you’ll see your own depleted next, and so your body shuts down and prompts clotting. But Diehl found that in cases where people fainted at the sight of blood, the feeling was one of disgust rather than fear. That feeling was followed by lowered blood pressure, slower heart rate and passing out. If the body’s response to heavy bleeding makes survival sense, the mind’s response to disgust in these situations certainly does not—falling into a dead faint at the feet of a predator because you’re disgusted is not a sensible survival strategy. Did You Know . . . Human beings have a long history of reacting with disgust to bad smells, such as feces and rotting flesh. Not only that, our disgust reflex has evolved to be a reaction to unpopular politicians, unpunished criminals and even a jacket once worn by Hitler. The most interesting twist on this already peculiar reaction is that in the doctor’s office,the disgust and fear that people feel is directed at the needle itself, not at the blood that it might draw. A needle might suggest the looming presence of blood, but really, most injections are close to 100 percent blood-free. So, something more must be at work. One psychiatrist, Stefan Bracha, has proposed a controversial theory about needle fear. Bracha claims that this particular phobia dates back to Pleistocene times—as much as two hundred thousand years ago—an era he characterizes as extremely violent. Archeologists have found definitive evidence that tribe-on-tribe and band-on-band disputes were frequent and gruesome. The weapons of choice were typically spears and axes. Deaths were gory and crude. The mere sight of blood was bad news. At some point in those conflicts, the best opportunity for survival—at least for noncombatants, such as children and young women—might have been to faint. A body lying in a heap on the ground could easily be overlooked, while one still upright remained a prime target. A dig that began in 2012 on the shores of Lake Turkana in Kenya found evidence of a ten- thousand-year-old massacre, where it was obvious that sharp, pointed weapons like arrows and spears had been used in the slaughter. Of twelve almost-complete skeletons, ten had clearly died a violent death. Of course, this isn’t proof of Bracha’s thesis, but it does at least set the scene. If this fainting response has been passed down through generations, it might be genetic. At the same time, this hypothetical gene can’t have been too widespread. Having entire tribes collapse on the ground at the sight of a spear would have been suicidal, if not genocidal. So if the gene exists at all, it is probably present in only a minority of people. Bracha’s argument assumes that this reaction is a human-only phenomenon, so the first time that a gorilla or chimp faints at the sight of a needle, the theory will have to be tossed out. So far, not surprisingly, no such experiment has been conducted. It’s also true that there is more fainting when an expert (say, someone who’s been doing it for thirty years) draws blood rather than someone less experienced. Apparently, the expert wastes no time coddling the patient and therefore appears to be more threatening than the less experienced practitioner. Did You Know . . . Evidence suggests there’s a hereditary component to fainting at the sight of a needle. More than half of patients who have a blood or needle phobia that prompts fainting have a parent or sibling who exhibits the same symptom. And the number of females who experience needle fainting is more than double that of males. Theories like Bracha’s make some scientists feel faintly uncomfortable. The broad field into which Bracha’s theory falls is called evolutionary psychology, and its proponents have been accused of applying genetic explanations to modern-day phenomena when there’s no justification for doing so. Take, for instance, the doubtful claim I once heard that the reason girls dream of monsters under the bed and boys dream of monsters coming in the window is that back in the days of our australopithecine ancestors, females roosted in trees at night while the males slept at the base; danger, therefore, came from different directions. There’s zero evidence for any part of this claim, though it is entertaining. While Stefan Bracha’s theories remain just that, he has at least applied evidence to a puzzle that otherwise lacks explanation.
by jay ingram(science of why book)
A LOT OF SCIENTISTS WILL answer this question by simply saying, “Nobody has a clue.” That might be short and to the point, but it’s far from satisfying. There are answers, but they’re contentious, and one in particular has the uncanny ability to make some scientists’ blood boil.Lowered blood pressure, an irregular heartbeat or low blood sugar can all result in a momentary loss of consciousness called fainting—or in medical terms, syncope (rhymes with “canopy”). Humans can faint for all sorts of odd reasons—you may hear of people fainting after coughing, after urinating or after stretching. Sometimes, you can faint simply by getting out of a chair too quickly. In most cases, standing up suddenly causes blood to pool in the legs, lessening flow to the brain. If that dip in blood pressure is extreme, it knocks you out for a short time. You recover because blood flow is re-established to your brain, either because you fall down or because you have the presence of mind to tuck your head between your legs just before you black out. In both cases, your head is positioned below, or at least no higher than, your heart, so refilling the brain with blood is easier. But all these versions of fainting are straightforward physiological events triggered by physical stimuli, not mental conditions. How do we explain what happens when fainting is induced by pain, anxiety, emotional stress or fear? Some people faint at the sight of a needle. We assume that’s brought about by the expectation that pain will soon be inflicted, but that fear has little connection to the physiological consequences of, say, standing up too quickly. Instead of explaining these cases with physiology, we turn to evolution. In 2005, Rolf Diehl, at the Krupp Hospital in Essen, Germany, suggested that fainting, with its accompanying drop in both blood pressure and heart rate, was a protective response exhibited by wounded animals. He reasoned that if an animal starts bleeding (and we are animals, remember!), its initial response is to constrict blood vessels and jack up blood pressure and heart rate, so as to maintain circulation in the face of blood loss. But if the bleeding doesn’t stop and the animal’s blood loss reaches a critical value—roughly a third of its total blood volume—the reverse kicks in: blood vessels loosen, the heart rate drops and the animal’s circulatory system slows until it eventually loses consciousness. (This isn’t the same as playing dead. An animal playing dead still has its heart pumping and its nervous systems on high alert, exactly the opposite of being in a faint.) But while passing out may leave an animal vulnerable, Diehl argued that the drop in both blood pressure and heart rate buys precious time, allowing blood to clot and thereby reducing total blood loss. If the animal maintained a normal blood pressure in this critical situation, he argued, it would actually
hasten death, not prevent it. What’s fascinating is that this straightforward physiological mechanism in humans happens not only in response to the loss of one’s own blood but also to someone else losing blood. You’d think the emotion in play here would be fear—seeing someone else’s blood raises fears that you’ll see your own depleted next, and so your body shuts down and prompts clotting. But Diehl found that in cases where people fainted at the sight of blood, the feeling was one of disgust rather than fear. That feeling was followed by lowered blood pressure, slower heart rate and passing out. If the body’s response to heavy bleeding makes survival sense, the mind’s response to disgust in these situations certainly does not—falling into a dead faint at the feet of a predator because you’re disgusted is not a sensible survival strategy. Did You Know . . . Human beings have a long history of reacting with disgust to bad smells, such as feces and rotting flesh. Not only that, our disgust reflex has evolved to be a reaction to unpopular politicians, unpunished criminals and even a jacket once worn by Hitler. The most interesting twist on this already peculiar reaction is that in the doctor’s office,the disgust and fear that people feel is directed at the needle itself, not at the blood that it might draw. A needle might suggest the looming presence of blood, but really, most injections are close to 100 percent blood-free. So, something more must be at work. One psychiatrist, Stefan Bracha, has proposed a controversial theory about needle fear. Bracha claims that this particular phobia dates back to Pleistocene times—as much as two hundred thousand years ago—an era he characterizes as extremely violent. Archeologists have found definitive evidence that tribe-on-tribe and band-on-band disputes were frequent and gruesome. The weapons of choice were typically spears and axes. Deaths were gory and crude. The mere sight of blood was bad news. At some point in those conflicts, the best opportunity for survival—at least for noncombatants, such as children and young women—might have been to faint. A body lying in a heap on the ground could easily be overlooked, while one still upright remained a prime target. A dig that began in 2012 on the shores of Lake Turkana in Kenya found evidence of a ten- thousand-year-old massacre, where it was obvious that sharp, pointed weapons like arrows and spears had been used in the slaughter. Of twelve almost-complete skeletons, ten had clearly died a violent death. Of course, this isn’t proof of Bracha’s thesis, but it does at least set the scene. If this fainting response has been passed down through generations, it might be genetic. At the same time, this hypothetical gene can’t have been too widespread. Having entire tribes collapse on the ground at the sight of a spear would have been suicidal, if not genocidal. So if the gene exists at all, it is probably present in only a minority of people. Bracha’s argument assumes that this reaction is a human-only phenomenon, so the first time that a gorilla or chimp faints at the sight of a needle, the theory will have to be tossed out. So far, not surprisingly, no such experiment has been conducted. It’s also true that there is more fainting when an expert (say, someone who’s been doing it for thirty years) draws blood rather than someone less experienced. Apparently, the expert wastes no time coddling the patient and therefore appears to be more threatening than the less experienced practitioner. Did You Know . . . Evidence suggests there’s a hereditary component to fainting at the sight of a needle. More than half of patients who have a blood or needle phobia that prompts fainting have a parent or sibling who exhibits the same symptom. And the number of females who experience needle fainting is more than double that of males. Theories like Bracha’s make some scientists feel faintly uncomfortable. The broad field into which Bracha’s theory falls is called evolutionary psychology, and its proponents have been accused of applying genetic explanations to modern-day phenomena when there’s no justification for doing so. Take, for instance, the doubtful claim I once heard that the reason girls dream of monsters under the bed and boys dream of monsters coming in the window is that back in the days of our australopithecine ancestors, females roosted in trees at night while the males slept at the base; danger, therefore, came from different directions. There’s zero evidence for any part of this claim, though it is entertaining. While Stefan Bracha’s theories remain just that, he has at least applied evidence to a puzzle that otherwise lacks explanation.
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