Dr Andrew Murray, Cambridge University
Andrew - Oxygen is absolutely critical for almost every process in the body. Just about every tissue uses oxygen to make energy in the form of ATP, the universal energy currency of the cell. And if oxygen levels fall, the amount of ATP that can be produced and the amount of energy available to the cell is reduced. So you find it very much harder to do exercise work which requires ATP because you got less oxygen to make it.
Laura - The longer you stay at higher altitudes, the more you get used to it. How does the body begin adapting to that?
Andrew - Okay, so thereís several adaptations you see and they all occur over different timeframes. So, first of all, your heart will start beating harder. The reason for this is that it wants to deliver more blood to the muscles, the blood carrying the oxygen. So you pump more of that blood to the muscles to maintain the oxygen supply so you can do the work. The other change that youíll see is you start breathing harder to get that oxygen into your blood. Over longer terms or medium terms rather, youíll start to see an increase in red blood cells in the blood of people adapting to altitude. The red blood cell, the erythrocyte is what carries the oxygen in the blood. So by increasing the amount of blood, you increase the oxygen-carrying capacity, of course this makes the blood very thick, and so it can also be slightly detrimental and it becomes very viscous, very hard to push through the capillaries into the muscle. More long term changes that you start to see, your breathing rate returns to normal. Your red blood cell mass stays high but the signs are that thereís other changes going on. And actually, we donít know a great deal about this, whatís actually happening in the muscles themselves. We think it could be more blood vessels delivering oxygen to the muscle or it could be changes within the tissue itself, in the way that the muscle metabolism uses oxygen to make energy. It could become more efficient or it could use other metabolic pathways that are not dependent on oxygen.
Laura - Is there any height at which the human body cannot adapt to high altitude?
Andrew - Absolutely. As part of our expedition in 2007, my colleagues and I took blood samples from very high on the mountain. We took samples as high as 8,400 meters and interestingly enough, we found that even up to about 7,000 meters, the blood oxygen content was maintained by which I mean, you had more red blood cells, each of them was carrying less oxygen. But the fact that there was more of them and that the total amount of oxygen in your blood was the same. But above 7,000 meters and certainly above 8,000 meters, this began to fall. This area is known as the death zone by climbers and you really donít want to spend too much time there for obvious reasons as its name would suggest. But above around 7,000 meters, the body could no longer adapt and the amount of time you can spend there is limited.
Laura - One thing that you can test to see how your body is adapting is the alkalinity of your urine. Why does that change?
Andrew - Okay, so one of the initial changes is you start breathing very hard and this is triggered by a low oxygen level in your blood. Your chemoreceptors which monitor oxygen and pH sense that oxygen levels are falling and you start breathing hard. Now one of the problems of breathing hard as well as bringing in more oxygen is that you breathe off carbon dioxide. So you decrease the amount of carbon dioxide in your blood and carbon dioxide is acidic. If your blood then becomes very alkaline, this triggers other chemoreceptors to say, ďWoh! Stop breathing. You're breathing far too hard.Ē You get very sporadic breathing patterns and this particularly happens when you're asleep and you havenít got conscious control of your breathing. So this is known as Chain-Stokes breathing. So, you might be breathing very hard one minute and then suddenly youíll stop breathing and youíll wake yourself up, thinking that you're drowning in your sleeping bag and try to rip it off you. Can be very scary, but itís even more scary for your tent-mate who thinks you're having a heart attack and are about to die in the middle of the night. So one way the body adapts to this is to reduce the alkalinity of your blood by excreting bicarbonate which is alkaline. The kidneys do this job very effectively, putting the bicarbonates into your urine and then your urine becomes very alkaline. Of course, the side effect to this is that you need to pee a lot. So youíd be waking up a lot in the middle of the night and getting up and going to the loo especially your first night there.
Laura - A very interesting thing that changes at altitude is your sense of smell. Is there an understanding of why that occurs?
Andrew - So my understanding is that thereís two possible causes of this and we arenít absolutely sure which of these is the true explanation. Yes, your sense of smell is definitely decreased and it could be because the chemicals that are flying around in the air that are picked up by our olfactory nerves, those chemicals are more dispersed in the thin air. The other possibility is that certain neural pathways might be down-regulated and therefore, the sense is impaired. These neural pathways obviously require a huge amount of energy, which requires a huge amount of oxygen, so by downregulating these pathways, just a little bit, you might save you just enough oxygen to survive at higher altitudes.
Laura - Thereís very severe effects that can sometimes happen when people are at high altitude, altitude sickness. Why does this happen and what are the signs of that?
Andrew - So, thereís various degrees of altitude sickness. At the low-end, thereís a high altitude headache that you will get and everybody will get this at some stage. If this isnít monitored carefully, it can develop into AMS or Acute Mountain Sickness. When you start to see quite a severe headache, you get nausea, vomiting, you become disorientated. Weíre not entirely sure why this happens, but itís probably due to a lack of oxygen really being delivered to the brain and these are various neural pathways which are basically telling your body that itís not in a good place at the moment. A more severe altitude sickness can be High Altitude Cerebral Oedema or High Altitude Pulmonary Oedema. So thatís either water retention of the brain or on the lungs, in the case of pulmonary oedema, HAPE or HACE. And these are extremely severe. People do die of these. All climbers going above base camp or even at the kind of base camp level need to be monitored carefully for any signs of HAPE or HACE and if any appear, the only solution is descend, descend, descend. Why they occur and why they occur in some individuals and not others is not really clear. Certainly, aspects like physical fitness at sea level are not good predictors of altitude sickness and in fact, the only predictor of how well somebody will do with altitude is how well have you done on previous trips to altitude. If somebody has done well previously, they're likely to do well again. If they've done badly previously, they are likely to do badly again which suggests thereís certain genetic factors, what the genes are that regulate this, we donít know, but work is again ongoing.