三分钟迅速降低体温
室外气温39度，阳光毒烈，水汽闷蒸，凉风是遥远的记忆：欢迎来到夏天！
或许相比刺骨的寒冷，有人更喜欢温暖的天气，在阳光下户外跑步和骑行，漫长的白昼也能使人心情愉悦。但事实上，热可能是一种致命的危险。越来越多的热浪让气温创下新高，在美国，每年大约有1300人死于极端高温。高热会让身体加倍努力运作以降低体温，这可能导致热衰竭和中暑，对于患有心脏和呼吸系统疾病的人来说，高温尤其危险。
即使是正常的高温，例如八九十年代的夏天，也很容易让人感到不舒服。人们汗流浃背、气喘吁吁，只想快速降温。每个人都有自己最喜欢的方法让自己凉快下来，比如用凉水洗脸、喝冷饮，或是脱掉鞋袜，赤脚走在凉爽的地板上：根据研究温度调节的生理学家的说法，这两类做法都是相当有效的策略。
我们的的脚底和手掌是快速降低体温的关键。一些运动员甚至开始使用特殊的冷却手套，以便在高热运动后迅速恢复凉爽。
克雷格·海勒（Craig Heller）是美国斯坦福大学的生物学教授，他的研究方向是人类体温调节及其对表现的作用。海勒表示：“我们的体温通常在37摄氏度左右，当达到40摄氏度时，身体就不能正常运作了。我们如今生活在非常接近这一极限的状况下。”
作为哺乳动物，我们已经演化出了维持温暖体温的能力。大多数哺乳动物都拥有一层漂亮的毛发，可以隔热。人类体表也覆盖着数百万的毛囊，只不过毛发比其他动物要细短得多，这意味着，我们的散热能力通常很糟糕。
然而，我们的身体有一种“紧急温度调解阀”，也就是海勒一直在研究的东西。这种是一种特殊类型的血管，名为动静脉吻合（arteriovenous anastomosis），简称AVAs。大多数动脉和静脉通过非常薄的毛细血管床相连，将营养和氧气送至细胞，动静脉吻合则不同，它们是动脉和静脉的直接连接点，因此血液流经它们的速度非常之快。
动静脉吻合解热功能的关键，在于它们只集中在身体的少数几个地方。
海勒说：“我们发现，在手掌、脚底和脸的上部，也就是所谓的无毛皮肤中，存在特殊的血管，这些血管可以绕过毛细血管，直接将血液从动脉分流到静脉。拥有皮毛的哺乳动物，不能非常有效地在整个身体表面散热。因此，在哺乳动物不长毛的皮肤上就存在这些特殊的血管，比如脚掌手掌的肉垫、舌头，在某些情况下还有耳朵。”
为了看看他们是否能够利用人体内的动静脉吻合，早在21世纪初，克雷格和同事丹尼斯·格拉恩（Dennis Grahn）使用了一种略显简单粗暴的装置：他们在受试者的手上套了一个有机玻璃圆筒，然后用潜水衣的袖子部分将其密封到他们的手臂上，圆筒内则有凉水流过他们的手掌。
受试者在运动之后，从身体的核心部位流入动静脉吻合的高温血液会将其中的热量传递到温度约为13摄氏度的冷水中，然后，冷却下来的血液将循环到身体的核心，降低核心处的热量。他们发现，受试者们在短短几分钟内就恢复了正常体温。
相关研究发表在《应用生理学杂志》（Journal of Applied Physiology）等期刊上。研究人员所处的斯坦福大学，是一所拥有一群精英体育运动队的大学，这种装置开始在学校健身房流传开来。运动员们往往会在训练过程中体温过高、筋疲力竭，通常要休息几小时或者修养一整天，海勒和格拉恩制作的这种降温手套有了用武之地。
运动员们会在两组训练之间戴上它们，在大约三分钟内让体温降下来，然后起来再做一组。海勒说，降温手套让一个运动员在大约20分钟内做了618个引体向上，一些女运动员则在20分钟内做了900个俯卧撑。
这种降温手套名为CoolMitts。海勒说，旧金山49人队（San Francisco 49ers）的一些职业美式橄榄球运动员也在用这种手套。这种手套可能真的很有效，但它还没有在各种人群中进行详尽的测试，而且价格大约为1500美元。
那么，当天气变得非常炎热，热到你出现热应激的迹象——大汗淋漓、皮肤粘稠、肌肉痉挛、 头晕脑胀 ，除了戴上这些手套，还有什么好方法可以快速降低体温呢？
把身体浸泡在冰水里？海勒表示，这可能会有作用，但问题在于你没办法随时随地泡冰浴。
把脚泡在一桶冰水里？这不是一个好方法，因为……[查看全文]
How to Cool Down Fast in Summer Heat
Tanya Lewis: Today we’re talking about the best way to beat the heat this summer. Your body has evolved a natural technique for cooling down rapidly, and it’s remarkably effective. We’ll discuss how to take full advantage of it.
Josh Fischman: It’s hot out. It’s sweltering. The sun beats down on your head. Breezes are distant memories. Welcome to summer!
Lewis: Hey, it’s not that bad! I prefer warm weather to the cold. I like doing more things outside. It’s easier to convince myself to go for runs and bike rides. And I love those long summer days when it stays light so late out.
Fischman: Okay, I like summer too. But the fact is heat can be dangerous. We’ve been getting more and more blistering summer heat waves. About 1,300 people in the U.S. die because of extreme heat every year.
Lewis: Yeah, and that’s because high heat makes your body work extra hard to cool down. That can lead to heat exhaustion and heat stroke. Heat can be especially harmful for people with heart and respiratory diseases.
Fischman: Even when it’s just normally hot, say in the high 80s and 90s, it’s pretty easy to get uncomfortable. You sweat, you pant, and you just want to cool down fast. Everybody’s got their favorite tricks for doing that. After you go running, Tanya, what’s your go-to cool-down method?
Lewis: I like to splash water on my face and drink some cold water.
Fischman: Mine is to ditch my shoes and socks as fast as possible, and walk barefoot on a cool floor. And it turns out, according to physiologists who study temperature regulation, both our techniques are actually pretty effective strategies.
Lewis: Wow, the cold floor technique really helps?
Fischman: Yeah, I didn’t know this, but the soles of your feet and the palms of your hands are keys to fast cool-downs. Some athletes have even started using special cooling gloves to recover quickly after a hot workout.
Lewis: Hmm, your palms? That’s not very much surface area. It doesn’t seem like they would cool your whole body down, right?
Fischman: I agree. It’s a bit weird. So I turned to one of the scientists working in this area to explain it.
Craig Heller: I’m Craig Heller. I’m a professor of biology at Stanford. I study human temperature regulation and its role in performance.
Fischman: Quick heads up: Craig talks about temperatures using the Celsius scale. To get to Fahrenheit, multiply his number by 1.8. Then add 32.
Lewis: Or you can just remember that when he says 37 degrees, that’s 98.6 Fahrenheit. And 40 Celsius is 104 Fahrenheit.
Heller: Our body temperatures are regulated normally around 37 degrees. By the time we get to 40 degrees, we’re not functioning normally. We live very close to the edge.
Lewis: This is because we’re mammals—we’re warm-blooded. We’ve evolved to be good at maintaining a warm body temperature. And most mammals have a nice blanket of insulating hair all over their bodies. Even people are covered with millions of hair follicles. The hairs are just a lot thinner and shorter than they are in other animals.
Fischman: Which means that when it comes to losing heat, we generally suck.
Our bodies do, however, have a kind of emergency temperature relief valve. Craig has been studying it.
That valve is a special type of blood vessel. This week it’s my turn to get stuck with the hard science word, so here goes: they’re called arteriovenous anastomoses.
Lewis: Very nice.
Fischman: Well thank you. I practiced. A lot. But let’s call them AVAs from now on.
Most arteries and veins connect through a bed of very thin capillaries that bring nutrients and oxygen to cells.
AVAs, though, are different. They are direct junctions of arteries and veins, so blood flows through them pretty quickly.
And the real key to their heat relief function is they are concentrated in just a few places in the body. Here’s Craig again:
Heller: We found that in the palm of the hand, the soles of the feet, and the upper part of the face, which are called non hairy skin, there are special blood vessels, and those blood vessels can shunt the blood from the arteries to the veins directly, bypassing the capillaries.
You know mammals have fur. If you have fur you can't dissipate heat over your overall body surface very efficiently. So mammals have these special blood vessels in their non hairy skin, the pads of their feet, the tongue, the ears in some cases.
Fischman: To see if they could take advantage of AVAs in people, back in the early 2000s Craig and his colleague Dennis Grahn basically McGuyver’d this goofy device.
They put a Plexiglass cylinder around someone’s hand and sealed it around their arm with part of a wetsuit sleeve. Inside the cylinder, cool water ran over their palm.
After a person exercised, those AVAs pulled in hot blood from the core of the body. The blood gave off its heat to the cooler water, which was at about 56 degrees. Then, cooled down, the blood would circulate back to the body’s core and lower the heat there. People returned to regular body temperatures in just a few minutes.
Heller: We couldn’t believe it.
Fischman: This stuff gets published in places like the Journal of Applied Physiology. And since these guys are at Stanford, a university with a bunch of elite sports teams, it starts getting attention in the gym. Because athletes work out hard, get overheated and exhausted, and normally have to quit for the day, or several hours. But Craig and Grahn built a few more versions of this cooling mitten and handed them out.
Athletes would put them on between workout sets, cool down in about three minutes and jump up and do another set. Craig tells a story of one guy who did 618 pullups in about twenty minutes. Some women athletes did 900 pushups in that short time period.
Lewis: Wow, that’s about 899 more pushups than I can do. And he’s selling the gloves now, right?
Fischman: Yeah, they’re called CoolMitts. Heller says some pro football players on the San Francisco 49ers also adopted the gloves.
Tanya: I wouldn’t mind a pair of those on the New York subway in summer, just saying. But we should be clear that we’re not endorsing the product.
Fischman: No, we’re really not. It’s probably a fine device. But it hasn’t been exhaustively tested in a variety of people. And it costs about $1,500 bucks. But product aside, there is some cool science behind it. Literally.
Lewis: Ha-ha. So when it gets really hot, and I feel signs of heat stress—heavy sweating, clammy skin, muscle cramps, dizziness—what’s a good way to cool down if I’m not putting on one of those gloves? Should I dunk my body in an ice bath?
Fischman: Heller says that could work. The problem is it’s not very convenient. I don’t have a giant ice bath handy. Do you?
Lewis: No, but I did use to stand in an ice bath after high school cross-country practices. But seriously, could I just stick my feet in a bucket of ice water?
Fischman: Not so much...[full transcript]