The reversible reaction of CO2 with H2O to form H2CO3, with its subsequent dissociation to HCO3 − and H+ , is catalyzed by the enzyme carbonic anhydrase within red blood cells and is the major mechanism for generation of HCO3 − .
4.
The O2 dissociation curve is S shaped. In the plateau area (>60 mm Hg), increasing or decreasing PO2 has only a minimal effect on Hgb saturation from 100% •Fig. 24.9 Equilibrium Curves for CO2in Arterial and Venous Blood. VenousbloodcantransportmoreCO2thanarterialbloodcanatanygivenPCO2.IncomparisonwiththeHgbO2equilibriumcurve,theCO2curvesareessentiallystraightlinesbetweenaPCO2of20andaPCO2of80mmHg.Long dashes representthearterialequilibriumcurve;short dashes representthevenousequilibriumcurve.
at different sites, deoxygenated Hgb has greater affinity for CO2 than oxygenated Hgb. Thus deoxygenated blood (venous blood) freely takes up and transports more CO2 than oxygenated arterial blood does. The deoxygenated Hgb more readily forms carbamino compounds and also more readily binds free H+ released during the formation of HCO3 − . The effect of changes in oxyhemoglobin saturation on the relationship of CO2 content to PCO2 is referred to as the Haldane effect and is reversed in the lungs when O2 is transported from the alveoli to red blood cells. This effect is illustrated by a shift to the left in the CO2 dissociation curve in venous blood in comparison with arterial blood.
to 90%. This ensures adequate Hgb saturation over a large range of PO2 values.
5.
The CO2 dissociation curve is linear and directly related to PCO2. PCO2 is solely dependent on alveolar ventilation and CO2 production.
6.
The CO2 to HCO3 − pathway plays a critical role in the regulation of H+ ions and in maintaining acid-base balance in the body.
7.
Tissue oxygenation is dependent on Hgb within red blood cells and subsequently the number (and production) of red blood cells, which is controlled by the hormone erythropoietin. Low O2 delivery, low Hgb concentration, and low PaO2 stimulate the secretion of erythropoietin in the kidneys.
8.
Tissue hypoxia occurs when insufficient amounts of O2 are supplied to the tissue to conduct normal levels of aerobic metabolism.
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