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In Figure 12.6 showing blood carbon dioxide dissociation curves:
- A fall in blood PO2 would shift the curve from A to B.
- If point X represents the situation at the venous end of systemic capillaries, then point Y represents the situation of the same blood at the venous end of the pulmonary capillaries.
- A rise in blood PCO2 would shift the curve from B to A.
- The decrease in the slope of the curves as PCO2 rises is related to the saturation of plasma with CO2 as PCO2 rises.
- At a PCO2 of 50 mmHg, the amount of CO2 in solution is lower in curve B than in curve A.
Medicine
a. False The reverse is true; deoxygenated blood can carry more CO2 than oxygenated blood at a given carbon dioxide pressure.
b. True In the lungs the blood oxygen saturation rises shifting the curve from A to B and the CO2 content and pressure fall from point X to point Y.
c. False It would merely shift the position on a given dissociation curve.
d. False Plasma does not become saturated with CO2; CO2 content remains proportional to PCO2; the initial sharp rise is due to formation of carbamino compounds – this falls off sharply as the number of free amino groups declines.
e. False The amount of CO2 in solution is the same for both curves for any PCO2; differ-ences in total CO2 content are due to differences in bicarbonate and carbamino content; at low oxygen pressures, the desaturated haemoglobin is increasingly more effective in buffering hydrogen ions and, by the law of mass action, favours formation of bicarbonate ions from carbon dioxide.
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