A staple of many comedy anime, a character getting a nosebleed when they see an attractive person is almost as old as the genre itself. If you grew up watching anime in the '90s and early '00s, you've probably seen it happen at least once, especially if you watch shows with a lot of fan service. But why do the anime characters' noses start bleeding? And can it really happen in real life?
The trope comes from an old wives' tale that when a teenager filled to the brim with hormones sees something arousing, their blood pressure rises to the point that it causes a nosebleed. Mangaka Yasuji Tanioka's humorous comics from the '60s are thought to be the trope's earliest inclusion in manga. Usually, it's usage is accompanied by a panty shot or when a character looks more attractive than normal. Yuru Yuri's Chitose is notorious for getting them whenever she fantasizes about her friends getting together, mainly Kyouko and Ayano.
Master Roshi is also associated with nosebleeds, being the old lecherous man that he is. Ever since Dragon Ball was first released, he has been used as a prime example of the nosebleed trope. His first bloody nose was caused by Bulma flashing him. Since then, he's usually the character that comes to mind when someone thinks about anime and nosebleeds. One Piece's Sanji is another example of a lecherous man getting them frequently.
Another example of the anime nosebleed, albeit an extreme case, is Karin Maaka from Chibi Vampire. While not necessarily stemming from arousal, she has some of the heaviest nosebleeds in anime history. Her waterfalls stem from the fact that she is a "reverse vampire," a vampire that produces an excess amount of blood rather than craving it. She targets unhappy people, and when she bites them, she gives them an injection of her blood, which brightens their moods.
While they are usually depicted as a normal red color, sometimes instead white steam is shown. Usually, these are shown in shows for smaller kids. It is used to express general excitement rather than arousal. Pokemon's Brock is a good example of this alternate version. Many times, the blood is censored for international audiences, and steam is used instead.
So, how possible is it to get a nosebleed from sexual arousal? To answer this, let's look at the two most common causes. The first is nose picking, and the second is being exposed to dry air for extended periods of time. The second reason is why they are more common during the winter months. High blood pressure is another cause of nosebleeds. There is also anecdotal evidence of people who have gotten them from having overly zealous sex.
On a scientific level, however, there is no evidence for sex-caused nosebleeds. Dr. Kouichirou Kanaya, who is an ear, nose and throat specialist, says in a NicoNico News interview that while high blood pressure and an increase in heart rate are definitely caused by sex, nosebleeds are something that have no connection to it. In short, an average person cannot get a nosebleed from having sex. There is evidence, though, that men who take performance-enhancing drugs such as Viagra can have nosebleeds as a result of the increased blood flow.
Overall, the nosebleed trope in manga and anime is a perfect way to express the pent-up hormones of teenagers and lecherous men (and sometimes women), even if it has no basis in science. Unless someone is taking specific medication that induces a bloody nose as a side effect, the likelihood of someone getting one from excitement or arousal is slim to none. That doesn't mean we can't laugh when it happens to a character because they got too excited because they saw a pair of panties, though, or when a vampire produces a waterfall of blood instead of consuming it.