Oxygen

The shortage of oxygen is the most important physiological factor that mammals must overcome at high altitudes. At sea level, 100 ml of blood contain about 45 ml of red blood cells and 15 g hemoglobin, and can bind 20 ml of oxygen. Because of the decreased partial pressure at an altitude of 4000 m, every breath contains less than half of the oxygen it does at sea level.

More than 22 million people worldwide live at an altitude of 4000 m or higher. How can these people survive in this thin-air environment?

Generally we can observe following mechanisms:

• Increased generation of red blood cells


The increase of red blood cells can have negative effects on health, since it leads to a higher viscosity which can cause congestions in the vessels. On the other hand, the higher viscosity slows down the blood flow and increases the absorption of oxygen by the body. Thus a higher exploitation of the inhaled oxygen can be achieved.

• Higher affinity of oxygen to hemoglobin

• Enlarged blood vessels

• Enlarged pulmonatory volume

Recent studies on people living constantly at high altitudes have revealed the interesting result that different populations have evolved srikingly different adaptations to high altitude. Comparing people of the Andes, Tibet and the Ethiopian highlands revealed different manifestations of the mechanisms listed above.

Andean people are relatively small-sized but they possess a relatively large chest volume. So their respiratory volume per minute is greatly increased. Furthermore, they show high levels of oxygen-carrying hemoglobin in their blood (but with low saturation levels).

Tibetans, on the other hand, show only slightly higher levels of hemoglobin, but have developed a genetic allele for higher oxygene saturation of hemoglobin. Furthermore, Tibetans were found to exhale nearly twice the amount of nitric oxide (NO) that Andean people do. Nitric oxide causes the relaxation of blood vessels, decreases blood pressure and leads to improved oxygen absorption in the lungs.

Ethiopian people show neither enlarged chests, nor increased oxygen saturation of the hemoglobin. All the same, their oxygen saturation in the blood is much higher than that of Andean and Tibetans and does not differ from that of people living at sea level. The saturation of oxygen in the blood of healthy sea-level dwellers varies between 92 and 100%. While Andean and Tibetan oxygen saturation is in the mid to heigh 80s, the oxygen saturation of Ethiopian people averages 95%. The mechanism by which Ethiopians reach that high saturation of oxygene in their blood without any of the adaptations known from Andean and Tibetan populations is still the subject of current research.

1a and b - Residents of the Semien Plateau in the Ethiopian highland (82K and 138K)

Temperature and UV-radiation

In high altitudes the radiation maxima are much higher than in lower altitudes. In some circumstances this means an animal can be exposed to overheating and high UV radiation. For animals that are adapted to a generally cold environment the short term overheating can be a problem. Physiological possibilities of heat emittance are poorly developed in marmots. They do not pant and have hardly any perspiratory glands. To avoid overheating of the body, the marmot (Marmota marmota) arranges its daily activity rhythm. On hot days, activity peaks are found in the morning and at dusk, while at noontime most animals are below ground. An adaption of the daily activity rhythm according temperature and radiation is also found in the Rock ptarmigan (Lagopus mutus)

2 - Daily activity rhythm of marmots on cold and hot days.

Beall CM (2000) Tibetan and andean patterns of adaptation to high-altitude hypoxia. Human Biology 72(1):201-228.

Türk & Arnold (1988) Thermoregulation as a limit to habitat use in alpine marmots (Marmota marmota). Oecologia 76:544-548.

29 August 2011
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