Carbon Dioxide Transport Flashcards
How is carbon dioxide transported within the the blood
70% of CO2 is transported in the form of carbonic acid and bicarbonate (HCO3-)
23% of CO2 is bound to Hb
7% of CO2 is dissolved in plasma
Why is a greater proportion of CO2 transported via plasma and not Hb?
CO2 is more soluble in water than O2 is.
CO2 binds to different sites in Hb than O2.
How is CO2 converted to HCO3- (bicarbonate)
CO2 reacts with water to form H2CO3 (carbonic acid)
Reaction is catalysed by carbonic anhydrase (enzyme is in high conc in RBC)
H2CO3 then partially ionises to form H+ and HCO3-
…..This affects the pH, more acidic. This conversion has critical implications to pH homeostasis.
What is the Bohr effect/shift?
Increased CO2 levels decrease O2-Hb binding affinity
CO2 mediates this effect directly and indirectly via its relationship with blood pH.
This is because when CO2 and H+ bind to the Hb molecule, they induce a conformation change in the Hb molecule which changes the structure of the O2 binding site, altering O2-Hb binding affinity.
These effects and the shift they induce in the O2-Hb dissociation curve are known as the Bohr effect/shift.
What is the Haldane effect?
Effect of O2 on CO2 transport.
When O2 binds to Hb, a conformation change is induced to the Hb structure which reduces Hb affinity for CO2 and H+ resulting in oxygenated Hb carrying less CO2 and H+.
Degree of H+ binding to Hb has knock on effect on CO2 transport.
H+-Hb binding acts as a buffer and removes H+ from the surroundings. Causes equilibrium to shift to the right. Resulting more HCO3-
This is the Haldane effect and is the reason why venous blood can carry more CO2 than arterial blood.
Why can the Haldane effect be dangerous in patients with COPD if they are given oxygen therapy?
COPD patients chronically hypoventilate
Therefore CO2 levels rise within the body
Their blood has greater CO2 capacity due to low levels of O2 and the haldane effect.
Sudden O2 levels increase causes CO2 to be displaced from the blood as the blood can carry less CO2 bound to Hb and as HCO3-
Resulting in sudden very high levels of CO2 potentially leading to dangerous acidaemia.
A healthy person would simply increase their level of ventilation to get rid of the excess CO2.
How do the respiratory and renal systems function to maintain blood pH homeostasis via the carbonic acid - bicarbonate buffer system?
Renal regulation of HCO3- e.g. regulating reabsorption/excretion in glomerular filtrate
Respiratory regulation of PaCO2 e.g. regulating ventilation
This means that blood pH is determined by the ratio of HCO3- to PaCO2
At which pH does acidosis occur?
<7.35
At which pH does alkalosis occur?
> 7.45
