Carbon dioxide equilibration curve and hypoxia
CO2 equilibration curve
The total quantity of CO2 combined with blood in all forms of transport of CO2 depends on PCO2, which can be expressed through the CO2 equilibration curve.
The PCO2 ranges between 40mm. Hg in the atrial blood and 45mm. Hg in venous blood. At the capillary bed the concentration of CO2 rises to 52 volumes percent and the level falls to 48 volumes percent as the blood passes though lungs, hence 4 volumes percent of this is actually exchanged in the process of transporting CO2 from tissues to lungs.
The combining of CO2 with Hb is influenced by degree of oxygen saturation of Hb. Greater the O2 saturation, lesser is the CO2, carrying power, more the displacement of CO2 referred as Haldane effect .
On oxygenation Hb becomes a stronger acid. (Reduced Hb being a weaker acid and displaces CO2 from carbamino Hb. Moreover, increased acidity of oxy-Hb causes increased release of H+ ion which combines with HCO3– to form H2CO3. In the lungs H2CO3 on dissociation releases CO2 and H20.
Hypoxia
Hypoxia is a state of inadequate O2 supply to tissue. Absence of O2 is referred as anoxia. The Cerebral effects seen will be mostly as excitement, hallucination, restlessness and unconsciousness. When blood O2 below 13%, breathing is stimulated. The anoxia can be classified into the following types.
Anoxic hypoxia (Ambient hypoxia)
This is due to reduced alveolar ventilation and reduced O2 tension in blood caused due to obstruction of air passage, paralysis of respiratory muscle, pulmonary disease or congenital diseases of heart. Ambient anoxia is caused by low pO2 in environmental air in high altitudes or closed space. Symptoms seen are dyspnoea, alkalemia, high cardiac output, increased pulse pressure, dilatation of peripheral vessels.
Anemic anoxia
Decrease in O2 carrying capacity of Hb due to low concentration of Hb seen in haemorrhages, anaemia, CO poisoning. In this case the partial pressure of O2 is normal but insufficient O2 delivery to tissue results in increased cardiac output and rapid circulation time.
Stagnant hypoxia
The O2 content of arterial blood is normal, but the tissues receive low O2 supply because general or local circulation failure.
Histotoxic hypoxia
The tissue oxidation is interfered due cyanide poisoning. Paralysis of cytochrome oxidase system is responsible for this condition. The amount of O2 and pO2 are normal in arterial blood and above normal in venous blood.