. A text-book of human physiology . even course when the partial pressure of oxygen israised from twenty-one to sixty, seventy-five, and ninety per cent. There isan increase in the absorption only during the first three minutes of respira-tion in air rich in oxygen, and this is due to the physical effect of a higher THE BLOOD GASES 337 partial pressure in the alveoli. A storage of oxygen in the tissues does nottake place under such circumstances (Falloise, Durig). Neither does the absorption of oxygen suffer any change in consequenceof a fall in the partial pressure to 86 mm. or lower. Only wh
. A text-book of human physiology . even course when the partial pressure of oxygen israised from twenty-one to sixty, seventy-five, and ninety per cent. There isan increase in the absorption only during the first three minutes of respira-tion in air rich in oxygen, and this is due to the physical effect of a higher THE BLOOD GASES 337 partial pressure in the alveoli. A storage of oxygen in the tissues does nottake place under such circumstances (Falloise, Durig). Neither does the absorption of oxygen suffer any change in consequenceof a fall in the partial pressure to 86 mm. or lower. Only when the atmos-pheric pressure sinks to 380 mm. (partial pressure of oxygen, 80 mm.) doesa decline in the oxygen content of the blood become evident; at a partialpressure of 55 mm. the decline is marked (Loewy). The absorption of oxygen becomes less as the carbon-dioxide tension in theblood increases. At an oxygen tension of 50 mm. Hg. and a carbon-dioxidetension of 5 nun., the absorption of oxygen was ninety-three per cent; with the. Fig. 131. -The absorption of oxygen by horses blood, after Krogh. The abscissa- represent thepartial pressures of oxygen and the ordinates percentages of saturation. same oxygen tension and a carbon-dioxide tension of 40 mm. it was seventy-eight per cent. As the blood flows through the capillaries the oxygen is gradu-ally used up and at the same time the carbon-dioxide tension increases; the lat-ter has the effect of conferring a greater tension on the oxygen present, as aconsequence of which a larger quantity of oxygen can be placed at the disposalof the tissues. The influence of this factor is especially great in asphyxiation(Bohr, Hasselbach and Krogh). C. CARBON DIOXIDE Where carbon dioxide occurs in the blood and how it is combined aremuch more complicated questions than in the case of oxygen, and notwith-standing many investigations directed to this end, the matter is not to be 338 RESPIRATION considered as by any means settled. The difficulty
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