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三分鐘迅速降低體溫

秘密就在手掌 @Unsplash

室外氣溫39度，陽光毒烈，水汽悶蒸，涼風是遙遠的記憶：歡迎來到夏天！

或許相比刺骨的寒冷，有人更喜歡溫暖的天氣，在陽光下戶外跑步和騎行，漫長的白晝也能使人心情愉悅。但事實上，熱可能是一種致命的危險。越來越多的熱浪讓氣溫創(chuàng)下新高，在美國，每年大約有1300人死于極端高溫。高熱會讓身體加倍努力運作以降低體溫，這可能導致熱衰竭和中暑，對于患有心臟和呼吸系統(tǒng)疾病的人來說，高溫尤其危險。

即使是正常的高溫，例如八九十年代的夏天，也很容易讓人感到不舒服。人們汗流浹背、氣喘吁吁，只想快速降溫。每個人都有自己最喜歡的方法讓自己涼快下來，比如用涼水洗臉、喝冷飲，或是脫掉鞋襪，赤腳走在涼爽的地板上：根據(jù)研究溫度調節(jié)的生理學家的說法，這兩類做法都是相當有效的策略。

我們的的腳底和手掌是快速降低體溫的關鍵。一些運動員甚至開始使用特殊的冷卻手套，以便在高熱運動后迅速恢復涼爽。

克雷格·海勒（Craig Heller）是美國斯坦福大學的生物學教授，他的研究方向是人類體溫調節(jié)及其對表現(xiàn)的作用。海勒表示：“我們的體溫通常在37攝氏度左右，當達到40攝氏度時，身體就不能正常運作了。我們?nèi)缃裆钤诜浅＝咏@一極限的狀況下?！?/span>

作為哺乳動物，我們已經(jīng)演化出了維持溫暖體溫的能力。大多數(shù)哺乳動物都擁有一層漂亮的毛發(fā)，可以隔熱。人類體表也覆蓋著數(shù)百萬的毛囊，只不過毛發(fā)比其他動物要細短得多，這意味著，我們的散熱能力通常很糟糕。

然而，我們的身體有一種“緊急溫度調解閥”，也就是海勒一直在研究的東西。這種是一種特殊類型的血管，名為動靜脈吻合（arteriovenous anastomosis），簡稱AVAs。大多數(shù)動脈和靜脈通過非常薄的毛細血管床相連，將營養(yǎng)和氧氣送至細胞，動靜脈吻合則不同，它們是動脈和靜脈的直接連接點，因此血液流經(jīng)它們的速度非常之快。

動靜脈吻合解熱功能的關鍵，在于它們只集中在身體的少數(shù)幾個地方。

海勒說：“我們發(fā)現(xiàn)，在手掌、腳底和臉的上部，也就是所謂的無毛皮膚中，存在特殊的血管，這些血管可以繞過毛細血管，直接將血液從動脈分流到靜脈。擁有皮毛的哺乳動物，不能非常有效地在整個身體表面散熱。因此，在哺乳動物不長毛的皮膚上就存在這些特殊的血管，比如腳掌手掌的肉墊、舌頭，在某些情況下還有耳朵?！?/span>

為了看看他們是否能夠利用人體內(nèi)的動靜脈吻合，早在21世紀初，克雷格和同事丹尼斯·格拉恩（Dennis Grahn）使用了一種略顯簡單粗暴的裝置：他們在受試者的手上套了一個有機玻璃圓筒，然后用潛水衣的袖子部分將其密封到他們的手臂上，圓筒內(nèi)則有涼水流過他們的手掌。

受試者在運動之后，從身體的核心部位流入動靜脈吻合的高溫血液會將其中的熱量傳遞到溫度約為13攝氏度的冷水中，然后，冷卻下來的血液將循環(huán)到身體的核心，降低核心處的熱量。他們發(fā)現(xiàn)，受試者們在短短幾分鐘內(nèi)就恢復了正常體溫。

相關研究發(fā)表在《應用生理學雜志》（Journal of Applied Physiology）等期刊上。研究人員所處的斯坦福大學，是一所擁有一群精英體育運動隊的大學，這種裝置開始在學校健身房流傳開來。運動員們往往會在訓練過程中體溫過高、筋疲力竭，通常要休息幾小時或者修養(yǎng)一整天，海勒和格拉恩制作的這種降溫手套有了用武之地。

運動員們會在兩組訓練之間戴上它們，在大約三分鐘內(nèi)讓體溫降下來，然后起來再做一組。海勒說，降溫手套讓一個運動員在大約20分鐘內(nèi)做了618個引體向上，一些女運動員則在20分鐘內(nèi)做了900個俯臥撐。

這種降溫手套名為CoolMitts。海勒說，舊金山49人隊（San Francisco 49ers）的一些職業(yè)美式橄欖球運動員也在用這種手套。這種手套可能真的很有效，但它還沒有在各種人群中進行詳盡的測試，而且價格大約為1500美元。

那么，當天氣變得非常炎熱，熱到你出現(xiàn)熱應激的跡象——大汗淋漓、皮膚粘稠、肌肉痙攣、頭暈腦脹，除了戴上這些手套，還有什么好方法可以快速降低體溫呢？

把身體浸泡在冰水里？海勒表示，這可能會有作用，但問題在于你沒辦法隨時隨地泡冰浴。

把腳泡在一桶冰水里？這不是一個好方法，因為……[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]

“

論文信息

Grahn, D.A., Cao, V.H. and Heller, H.C. (2005) 'Heat extraction through the palm of one hand improves aerobic exercise endurance in a hot environment’, Journal of Applied Physiology, 99(3), pp. 972–978.

DOI: 10.1152/japplphysiol.00093.2005

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