IT'S TOUGH AT THE TOP
Life at high altitude
THE overwhelming majority of theworld's population live between sea level and an altitude of 3,000 metres. Only some 15 million persons live in permanent settlements between that level and 4,800 metres, mainly on the high plateaux of the Andes in South America, and of Tibet in Central Asia.
Altitude inevitably imposes constraintson people who live in these regions. In the first century AD, Pan Kou, a Chinese historian, described in these terms the Karakoram mountain range in Central Asia: "It is necessary to cross two mountains, one large and one small, which make the head ache. The earth lies naked there, on the scorching slopes, and the human body loses its colour because of the heat; the torrid sun causes headaches and nausea, and it has the same effect on animals . . .'
Father Jose de Acosta, a Jesuit missionaryto the New World, describes acute mountain sickness in his treatise Historia natural y moral de las Indias (1590; Natural and Moral History of the Indies, 1604), and relates it to the fact that at such altitudes "the air is so subtle and so refined that it is unsuited to human respiration, which needs air that is thicker and more temperate . . .'
What exactly is meant by the term altitude?Altitude in the sense of environment consists of a number of physical factors, principally the lowering of barometric pressure1 and therefore of the partial pressures2 of respiratory gases, the wide range of variation in day and night temperatures, increased solar radiation and a greater incidence of high-energy heavy particles in the atmosphere.
1. Atmospheric pressure as indicated by a barometer,usually of mercury, and expressed in several different systems of units, such as pounds per square inch, or millimetres of mercury.
2. The pressure that a gas, in a mixture of gases, wouldexert if it alone occupied the whole volume occupied by the mixture.
The lowering of barometric pressure isby far the most important factor as far as man is concerned, and it is an unavoidable constraint to which technology has found no practical long-term solution. Consequently, the values of the partial pressure of respiratory gases, especially that of oxygen in the arterial blood, are usually taken as the basis for determining the physiological limits of altitude.
"Physiological' altitude therefore doesnot correspond to physical altitude. On the one hand, the fall in atmospheric pressure is not directly proportional to height above sea level; on the other, the oxygen content of the blood is not a simple function of the partial pressure of oxygen in the air. Three levels can be distinguished, according to the value of the partial pressure of oxygen in arterial blood:
- above 70 millimetres of mercury,changes in the partial pressure of oxygen (i.e. in altitude) have little effect on the amount of oxygen contained in the blood. (The normal arterial blood pressure at sea level is 90 millimetres of mercury.);
- between 70 and 40 millimetres of mercury,the effect of the fall in the partial pressure of oxygen becomes more noticeable;
- below 40 millimetres of mercury, theoxygen content of the blood falls considerably and more or less constantly with small changes in the partial pressure of oxygen.
Altitudes of less than 3,300 metres correspondto the first level; there are no significant effects on the oxygen transporting function of the blood.
It is impossible for man to live permanentlyat the third level--about 5,500 metres.
Altitude, in the physiological sense ofthe term, thus corresponds to a level of between 3,300 and 5,500 metres (a barometric pressure of between 500 and 370 millimetres of mercury).
The main problem for the humanorganism in such an environment consequently lies in transferring oxygen from the atmosphere to the cells.
Like the higher animals, man has twoactive transfer mechanisms: the respiratory system, and the circulatory system which distributes to the various tissues the oxygen bonded to the haemoglobin in the blood. …