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From page 239... ... Both have been exposed to the opportunity for natural selection for traits that offset the unavoid able environmental stress of severe lifelong high-altitude hypoxia. This paper presents evidence that Tibetan and Andean high altitude natives have adapted differently, as indicated by large quantitative differences in numerous physiological traits com prising the oxygen delivery process. Read the entire page →
From page 240... ... . The two high-altitude populations can be viewed as the current outcome of separate replications of a natural experi ment in which an ancestral founding population moved from low to high altitude, and its descendents have been exposed for millennia to the opportunity for natural selection to improve function under high-altitude hypoxia. Read the entire page →
From page 241... ... At 4,000-m elevation, every breath of air contains only ≈60% of the oxygen molecules in the same breath at sea level. This is a constant feature of the ambient environ ment to which every person at a given altitude is inexorably exposed. Read the entire page →
From page 242... ... and at the high altitude of 4,540 m (dotted line) illustrates the oxygen levels at the major stages of oxygen delivery and suggests potential points of functional adaptation (data from hurtado, 1964) Read the entire page →
From page 243... ... in contrast to acutely exposed lowlanders and despite the equally low level of oxygen pressure in the air and lungs, both Andean and Tibetan highlanders display the standard low-altitude range of oxygen delivery from minimal to maximal. Both populations have the normal basal meta bolic rate expected for their age, sex, and body weight (Picon-reategui, 1961; Mazess et al., 1969; Beall et al., 1996) Read the entire page →
From page 244... ... . Tibetans express a normal hvr as compared with sea-level populations in their native altitude, whereas Andean highlanders have hvrs generally lower than sea-level values. Read the entire page →
From page 245... ... Two Routes to Functional Adaptation / 2 FiGUre 13.3 Boxplots comparing pairs of Tibetan and Andean samples, measured at ≈4,000-m altitude by using the same recruiting and measurement protocols, illustrate the marked quantitative differences in resting ventilation, hvr, hemoglobin concentration, and percent of oxygen saturation (recalculated from data reported in Beall et al., 1997a, 1997b, 1998, 1999) Read the entire page →
From page 246... ... Fig. 13.4 illustrates that the calculated arterial oxygen content in a sample of Tibetans is substantially lower than among Andean highlanders, who actually have higher arterial oxygen content than sea-level natives at sea level. Read the entire page →
From page 247... ... . Because blood flow is a function of the diameter of blood vessels, dilating factors could, in principle, improve the rate of oxygen delivery. Read the entire page →
From page 248... ... . Thus, pulmonary blood flow is another element of oxygen delivery for which Tibetans differ from Andean highlanders in the direction of greater departure from the ancestral response to acute hypoxia. Read the entire page →
From page 249... ... . FiGUre 13.5 high-altitude native Tibetans have higher capillary density than their Andean counterparts or populations at low altitude; Tibetan and Andean highlanders both have lower mitochondrial volume than low-altitude populations (data from hoppeler et al., 1990, 2003; Desplanches et al., 1996; Kayser et al., 1996) Read the entire page →
From page 250... ... By this criterion, each of the following contrasts (described above) is large; the higher Tibetan mean for ventilatory traits, the lower Tibetan mean for hematological traits, particularly for hemoglobin for which the mean dif ferences are >2 sD, the higher Tibetan mean for exhaled no and muscle capillary density, and the lower Tibetan mean values for arterial oxygen level, oxygen saturation, and pulmonary artery pressure (Table 13.1) Read the entire page →
From page 251... ... Table 13.1 shows that Tibetan samples generally have higher h2 and thus greater potential for natural selection on many of the oxygen delivery traits described above. The presence among Tibetan, but not Andean, high-altitude natives of significant h2 for resting ventilation and oxygen saturation is indirect evi dence of population genetic differences, because it reflects the presence of at least two alleles in the Tibetan sample (but not the Andean sample) Read the entire page →
From page 252... ... Trait d resting ventilation, liters/min Male, 1.0 Female, 1.1 Tidal volume, ml Male, 1.1 Female, 0.8 respiration rate, breaths per minute Male, −0.2 Female, −0.2 hvr, ↓liters/min per saturation, % Male, 0.8 Female, 0.8 oxygen saturation of hemoglobin, % Male, −0.9 Female, −0.5 hemoglobin concentration, g/dl Male, −2.2 and −0.7 Female, −2.4 2,3-Bisphosphoglycerate mutase concentration Male, −0.7 erythropoietin concentration, milliunits/ml Male, −0.2 exhaled no, nmhg Male, 1.2 Female, 1.1 Partial pressure of o2 in arterial blood, mmhg Male, −0.8 Mean pulmonary artery pressure, mmhg Male, −1.3 Pulmonary artery systolic pressure, mmhg Male and female, −1.3 Calf muscle capillary density, no/mm2 Male, 1.3 Mitochondrial volume density, % Male, −0.3 negative effect sizes reflect lower Tibetan mean values. Read the entire page →
From page 253... ... Two Routes to Functional Adaptation / 2 Percent nonoverlap h2 of observations refs. ≈55% (n = 320 Tibetan, Tibetan, 0.32 Beall et al. Read the entire page →
From page 254... ... . Candidate genes for pulmonary vasodilators have been examined, based on the reasoning that alleles for high levels could improve blood flow and oxygen diffusion in the lung (Wilkins et al., 2002) Read the entire page →
From page 255... ... in summary, measures of oxygen transport reveal that Andean and Tibetan populations have large quantitative differences in numerous physiological and molecular traits involved in oxygen delivery. The hypothesis is that evolutionary processes have tinkered differently in the two founding populations and their descendents, with the result that the two populations followed different routes to the same functional out come of successful oxygen delivery. Read the entire page →

From page 239...

... Both have been exposed to the opportunity for natural selection for traits that offset the unavoid able environmental stress of severe lifelong high-altitude hypoxia. This paper presents evidence that Tibetan and Andean high altitude natives have adapted differently, as indicated by large quantitative differences in numerous physiological traits com prising the oxygen delivery process.