人体的十二个极限

iavjssssmqee · 发表于 2010-5-4 19:03:28

9 How high can you go?

Altitude has strange effects on the body, and it's mostly down to the reduced pressure of oxygen in the air. Cells need oxygen in order to survive. At higher altitudes haemoglobin, the blood protein that transports oxygen from the lungs to cells, cannot absorb oxygen to its full capacity, creating a deficit in the body.

The brain is very sensitive to oxygen levels, which is why headaches and dizziness are the first signs of altitude sickness. With prolonged stays above 5000 metres, muscle mass deteriorates and the risk of fatal fluid accumulation in the lungs and brain increases. Go higher than around 7500 metres and the magnitude of the oxygen deficit can lead to loss of consciousness and, ultimately, death.

What about living at altitude? Well, almost half the population of Bolivia lives in the country's Altiplano region, 4000 metres above sea level. But go much higher and problems kick in. Some Chilean mine workers spend several weeks at a time at 5800 metres, but they probably couldn't reproduce there because altitude temporarily suppresses male fertility, says Mike Grocott of the University of Southampton in the UK, who studies the physiological effects of altitude.

Only a handful of people have made it to the summit of Everest without oxygen
Most people can adapt to altitude if they take time to acclimatise. A good rule of thumb is that the higher you go, the shorter you should stay. If you were suddenly transported to the summit of Everest (8848 metres) without acclimatising first, you would probably be dead in 2 minutes.

Only a handful of people have made it to the summit without supplemental oxygen. Babu Chiri Sherpa set the record for remaining at the top of Everest without oxygen for 21 hours in 1999. He was probably genetically primed to cope at altitude, says Grocott.

What is the limit beyond which unaided survival is impossible? As it happens, the summit of Everest is pretty close to it. There has been just one ascent without breathing apparatus in winter, when barometric pressure falls and therefore the amount of oxygen drops further than ever, says Grocott. "I'd guess the limit would be about 9000 metres."

9.人类究竟可以登上多高的地方？

海拔差异会对人体产生奇怪影响，在大多数情况下，高海拔地区空气里的氧气压力减小。人体细胞需要氧气才能存活。在更高海拔，把肺部的氧气输送给细胞的血液蛋白——血红蛋白无法有效输送氧气，这导致人体缺氧。大脑对氧气水平非常敏感，这也是高原病的第一反应就是头痛眩晕的原因。英国南安普敦大学的麦克·格洛科特表示，人如果长时间呆在海拔超过5000米的地方，就会导致肌肉萎缩、肺部和大脑积液的风险显著增加，不过他们在那可能无法生育，因为高海拔暂时抑制了男性的生育能力。格洛科特研究了海拔差异对生理产生的影响。

如果人们在这种环境下生活很长时间，大部分人最终都能适应。一个很好的经验是，你登得越高，呆得时间就应越短。如果你在没适应新环境前被突然送上珠穆朗玛峰(8848米)，可能不出2分钟你就会死亡。只有少数人在没有氧气补充的情况下登上珠峰。1999年，巴布·奇里·夏尔巴在珠穆朗玛峰缺氧环境下生活了21小时，打破世界纪录。格洛科特表示，也许奇里·夏尔巴天生具有适应高海拔环境的能力。人类最高能在多高的地方生存呢？可能珠峰已经接近这个高度。格洛科特说，迄今只有一个人在冬季没有氧气装置的情况下攀登珠峰，当时大气压降得更低，空气里的氧气更少。“我想人类能够承受的最高海拔可能是9000米。”

iavjssssmqee · 发表于 2010-5-4 19:03:47

10 How much can a human lift?

In the heaviest dead lift recorded, British weightlifter Andy Bolton lifted 457.5 kilograms from the floor to his thigh.

Strongmen like Bolton are perhaps five or six times stronger than the average man, who will often struggle to lift 45 kilograms over his head, says Dan Wathen, an athletics trainer at Youngstown State University, Ohio. The record for an overhead lift is 263.5 kilograms.

So what is the maximum weight a human could ever lift? Todd Schroeder at the University of Southern California in Los Angeles thinks we are already close to the maximum. "If you look over time at the records for maximal lifts, they have crept up but are starting to plateau," he says. "Today's weightlifters, including those that use steroids, are near the limit of human potential."

It is the muscles that set the limit. Most failures to lift a given amount do not damage the body: the weightlifter simply cannot overcome a load. But in cases where something does give way, it is usually the muscle fibres that tear, often near the tendon.Similarly, it is control of the muscles that gives weightlifters their advantage. The body has natural inhibitory mechanisms designed to keep us from hurting ourselves by trying to lift too much. These work by controlling how many muscle fibres are activated at any one time.

Weightlifters learn to suppress these signals, enabling them to use a larger fraction of the muscle's potential in lifting.Beyond that, the key to success is training, though genetics plays a role. Short limbs favour strength, says Wathen, a former weightlifter, and some people have more muscle fibres than others.

　　
10.人类最多能够举起多大的重量？

世界硬举纪录由英国举重达人安迪·博尔顿创造。他将457.5公斤的重物从地面提到大腿部位。美国俄亥俄州扬斯敦州立大学体育教练丹·瓦瑟表示，像博尔顿这样的大力士力量可能是普通的人的5倍或者6倍，普通人将45公斤的重物举过头顶就已经很吃力了。过顶举重纪录为263.5公斤。

人类所能举起的最大重量是多少呢？美国洛杉矶南加州大学的托德·斯库罗德认为，我们已经接近所能达到的极限。他说：“回顾以往的举重纪录你会发现，虽然成绩不断提高，但已开始达到稳定水平。现在的举重运动员，包括那些服用类固醇的运动员已经接近人类体能的极限。”

最终能够举起多大重量取决于肌肉。在举重比赛中，绝大多数试举失败都不会让身体遭受损伤，因为举重运动员因无法承受所要的重量选择放弃。但如果强行试举，失败则经常造成肌肉纤维撕裂，通常是腱附近的纤维。

能够有效控制肌肉让举重运动员获得优势。人体拥有天然的抑制机制，保护身体因所举重量过重受到损伤。这项工作通过控制一次参与举重的肌肉纤维数量加以实现。举重运动员经过训练懂得如何抑制这些信号，进而在更大程度上发挥肌肉潜能。除了这种控制外，成功的另一个关键因素就是训练。当然了，遗传因素也扮演着重要角色。瓦瑟曾经是一名举重运动员。他说：“四肢短小的人力量更大，一些人的肌肉纤维数量也要超过其他人。”

iavjssssmqee · 发表于 2010-5-4 19:04:04

11 How much radiation can we take?

In September 1987, two men entered an abandoned medical clinic in Goiânia, Brazil, and dismantled what they thought was a piece of valuable equipment. Within a day, both were vomiting. Diarrhoea and dizziness struck next. Unbeknown to them, the "scrap" contained a highly radioactive source used to treat cancer patients.

Scrap dealer Devair Ferreira, who bought the source, was intrigued by the blue glow it emitted in the dark. He kept the mug-sized canister of powder in his dining room and invited friends and family around to marvel at it. They touched the powder and daubed it on their bodies like carnival glitter, taking fragments of the radioactive caesium chloride salt home. Within a month, Ferreira's wife, his 6-year-old niece and two of his employees had died from acute radiation syndrome. In total, 249 people were contaminated.

Radiation doses, measured in sieverts, are calculated by taking into account the type of radiation and the area of the body that has been irradiated. All of the fatalities received between 4.5 and 6 sieverts in a matter of days. That's a huge dose when you consider that each year we receive an average of 2.4 millisieverts from natural sources such as radon.

The threshold for an early death is around 2 sieverts, and death is highly likely at 6 sieverts, though Ferreira received 7 and survived. He died in 1994 as a consequence of alcoholic liver cirrhosis. No one knows for sure why Ferreira survived the radiation. The most likely explanation is that he spent more time out of the house than his wife. This gave the cells in his body time to repair some of the damage done.

11.人体最多能够承受多强的辐射？

1987年9月，两名男子走进巴西戈亚尼亚一个废弃的医务所，拆下一个他们自认为非常贵重的设备。一天之内，两人均出现呕吐症状，随后又出现腹泻和眩晕。他们根本不知道，这个废弃的设备实际上是一个高辐射源，用于治疗癌症患者。

这个辐射源能够在黑暗中发出蓝光。废品商德瓦尔·费莱拉对它产生浓厚兴趣，最后花钱买下。费莱拉将这个杯子大小装有粉末状物质的罐子放在饭厅，并邀请好友和亲戚参观。他们将粉末涂抹在身上，让自己变成了会发光的人。但令他们万没有想到的是，神奇的粉末居然是放射性物质氯化铯。一个月内，费莱拉的妻子、6岁大的侄女以及2名员工都死于急性放射综合症。在这起事故中，总计共有249人被这种放射性物质污染。

辐射剂量单位为西弗特，根据辐射类型和被辐射的身体部位加以计算。计算结果显示，所有死者在几天内受到的辐射剂量为4.5到6西弗特。我们每年因氡等天然辐射源受到的辐射剂量平均为2.4毫西弗特。也就是说，4.5到6西弗特已经是相当大的剂量了。

2西弗特左右的辐射剂量便可导致早逝，6西弗特则很有可能致人死亡。幸运的是，虽然受到7西弗特辐射，费莱拉还是活了下来。1994年，他因酒精性肝硬化离开人世。没有人确切知道费莱拉为何能在受到如此高剂量辐射情况下依然幸存下来。最有可能的一种解释是，他与妻子的大部分时间都在室外度过，让体内细胞有时间修复一些损伤。

iavjssssmqee · 发表于 2010-5-4 19:04:23

12 How long could you hold your breath?

Most people find it hard to hold their breath for more than a minute, so imagine the extreme self -control Stephane Mifsud mustered on 8 June last year when he held his breath for 11 minutes and 35 seconds, setting a new world record for stationary breath-holding, or "static apnoea".

Competitors float face down in a chilled pool, not to stop them cheating but to induce the mammalian diving reflex: when your face is submerged in cold water, outer blood vessels constrict, directing blood away from the extremities and towards the heart and brain. Your heart rate slows, reducing the rate at which oxygen is pumped around the body. With training, experienced breath-holders can drop their heart rate by twice that of non-divers upon immersion in cold water.

Hyperventilation before a prolonged breath-hold is also vital. This is because the brain monitors the amount of carbon dioxide in the blood to decide when to trigger the gasp reflex. Purging this CO2 with rapid deep breathing increases how long you can go before the urge to gasp becomes irresistible. Having large lungs is a natural advantage.

So have we reached the breath-holding limit yet? Not at all, says physiologist Johan Andersson at Lund University in Sweden, who studies the effects of breath-holding in divers. "Elite breath-hold divers expect the limit to be extended to about 15 minutes before record-setting will level off."

They may be dicing with danger, though. Andersson found that static apnoea attempts on dry land caused a brief 37 per cent rise in the level of a protein called S100B in the blood, which is used as a marker for hypoxic brain damage. Andersson stresses that this rise is "well below" the levels seen in actual hypoxia, but warns that those competing in static apnoea events may accumulate damage in the long term.

12.人类闭气最长能闭多长时间？

绝大多数人的闭气时间都很难超过一分钟，相比之下，法国人斯蒂凡·米弗苏却拥有超强的自控能力。2009年6月8日，米弗苏上演了一次壮举，闭气时间达到11分35秒，就此创造一项新的静止闭气世界纪录。

挑战中，参赛选手将面部浸泡在一个冰冷的水池中。这么做并不是防止他们***，而是激起哺乳动物的本能潜水发射。当把面部浸泡在冷水中时，外部血管收缩，血液从肢体末端流向心脏和大脑。这样一来，心率就会减慢进而减少氧气向全身扩散的几率。经过训练，闭气高手在浸泡在冷水中时的心率只有非潜水者的一半。

在挑战闭气时间纪录前强力呼吸也同样非常重要。这是因为大脑监视着血液中的二氧化碳含量，并依此来决定何时触发呼吸反射。迅速而深度的呼吸能够将二氧化碳排出体外，进而在达到身体极限前尽可能延长闭气时间。所以说，拥有较大的肺是一种天然优势。

人类是否已经达到闭气极限了呢？瑞典隆德大学生理学家约翰·安德森给出了否定答案。这位一直对潜水者闭气影响进行研究的生理学家说：“在闭气纪录趋于稳定前，出色的潜水者有望将闭气时间延长至15分钟左右。”

但在长时间闭气过程中，他们也可能面临危险。安德森发现在干燥陆地上进行静止闭气可导致血液中S100B蛋白质含量提高37%，这种蛋白质是缺氧性脑损伤的标记物。他表示虽然这种提高幅度远低于在缺氧症中发现的水平，但挑战静止闭气的人遭受的损伤可能在长期内不断积聚。

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