Question

Carbon dioxide (co2) is readily soluble in water, according to the equation co2 + h2o ↔ h2co3. carbonic acid (h2co3) is a weak acid. respiring cells release co2 into the bloodstream. what will be the effect on the ph of blood as that blood first comes in contact with respiring cells?

Answer 1

Blood is a buffer solution of bicarbonate (HCO₃⁻) and carbon dioxide (CO₂). The Henderson equation which relates the concentration of HCO₃⁻ and CO₂ is given below:

$p^{H}$=$p^{k_{a} } = p^{H} + log\frac{HCO_{3}^{-} }{CO_{2} }$.

Respiring cell releases CO₂ in blood stream and that CO₂ on reaction with water molecule produces H₂CO₃ which is a weak base and its conjugate base is  HCO₃⁻.

CO₂ + 2H₂O⇄ HCO₃⁻ + H₃O⁺

pH of the buffer solution (the blood) depends only on the ratio of the amount of CO₂ to the amount of HCO³⁻. [So, due to respiration produced CO₂ will get dissolved in water and favours the equilibrium towards forward direction. Then immediately HCO₃⁻ reacts with HCO₃⁻ and starts producing CO₂.]This ratio remains relatively constant because the concentrations HCO3- and CO2 are very large compared to the amount of CO₂ produced to the blood from respiring cells. So, $p^{H}$ of blood does not change.