Introduction 70 The two main challenges to life at high altitude come from hypobaric hypoxia and the low ambient temperatures. Temperature decreases about 1°C for each 150 m elevation, so that at 4,500 m temperature is roughly 30°C lower than at sea level. Barometric pressure falls progressively with increasing altitude. Up to about 2,500 m there are few if any effects of hypoxia. Above 3,000 m some effects of hypoxia are likely to be experienced and above 4,000 m adverse effects would be experienced by most unacclimatized visitors. However, many people live and work at altitude with no apparent adverse effects. One such example is Cerro de Pasco a busy mining town of 72,000 population at 4,300 m in the Peruvian Andes, where much high altitude research has been undertaken. At this altitude barometric pressure is 450 mmHg and without hyperventilation alveolar oxygen tension would be only 34 mmHg. Autonomic control in visitors Most adaptive changes occur in the first days and weeks following arrival at altitude, and this is the period when acute mountain sickness with cerebral and/or pulmonary oedema may occur. Recent studies in animals and man have highlighted the role of the autonomic nervous system in adaptation and in particular the importance of sympathetic activation following high altitude exposure. Cardiovascular effects 5 68 68 69 34 68 69 21 23 54 49 65 72 24 2 + 2+ 2+  48 24 57 Sympathetic activity 39 56 28 60 59 18 40 41 59 29 71 10 2 54 30 67 35 1 48 1 31 56 2 19 2 53 2 22 55 11 8 Muscle sympathetic nerve activity 17 27 27 Heart rate variability 9 32 16 17 Arterial baroreflex 61 25 26 5 6 7 63 15 Mechanisms for sympathetic activation at high altitude 39 38 46 Autonomic function in high altitude residents Healthy highlanders 1 4 2 2 13 33 62 51 52 20 5 45 47 12 13 12 14 Patients with chronic mountain sickness 43 13 42 37 64 44 64 3 62 36 21 6 33 45 Cerebrovascular control 66 14 58 50