What are the 3 ways CO2 is transported in the blood?
- Dissolved (approx 10%)
- Carbamino compounds (approx 21%)
- Bicarbonate (approx 69%)
What are the approxiamte partial pressures of CO2 in:
Venous blood: 6.5kPa
Alveoli: 5.3 kPa
Arterial blood: 5.3kPa
Explain how CO2 is transported dissolved in the blood
Reacts with water to form carbonic acid (H2CO3) which then dissociates to form HCO3- and H+. This happens slowly in plasma due to the existing high concentration of HCO3-.
This is a negligible amount
What are weak and strong acids?
- Completely dissociate in water, releasing large amounts of H+
- E.g. HCl → Cl- + H+
- Incompletely dissociate in water, forming a buffer pair with their conjugate base until they reach equilibrium. Reversibly bind H+ in response to H+ concentrations.
- E.g. Carbonic acid H2CO3 → H+ + HCO3-
What is normal blood pH?
What are volatile and non-volatile acids?
- More easily vapourised
- Excreted by lungs
- 1400 mmol H+ produced per day by aerobic metabolism and CO2 production by tissues.
- Not easily vapourised
- 70-100mmol H+ produced per day by other metabolic processes
- Excreted by kidneys
How is CO2 transported in the blood as carbamino compounds?
Formed when CO2 reacts with protein amino groups
CO2 + protein NH2 → protein-NHCOOH
What determines the amount of H+ (and therefore pH) in the blood?
Amount of bicarbonate (reduces amount of H+)
Amount of CO2 (increases amount of H+)
Explain the Henderson-Hasselbach equation
pH = pK + log10 [HCO3-] / CO2 x 0.23
Allows measurement of pH using CO2 and HCO3- concentrations.
pK= constant for the reaction
What are the physiological buffer systems?
- If the body produces too much acid, H+ reacts with HCO3- ions to produce CO2
- CO2 excreted by the lungs
- If pCO2 is too high, the kidneys excrete less HCO3-
- HCO3- in plasma is raised, restoring pH
CO2 + H2O ⇔ HCO3- + H+
How is CO2 transported as bicarbonate?
In red blood cells, CO2 and water are converted to HCO3- and H+, catalysed by carbonic anhydrase.
This reaction is further promoted as the products of the reaction are removed:
- H+ is buffered by histidine residues on Hb → HHb
- Excess HCO3- leaves the red blood cell in exchange for Cl- via the Cl-/ HCO3- exchanger (chloride/hamburger shift)
The end result of these reactions is that the majority of CO2 is transported in the blood as HCO3-
Describe the buffering ability of haemoglobin
The ability of haemoglobin to buffer H+ is enhanced by deoxygenation, therefore venous blood can carry more H+ than arterial.
What happens if O2 is given up by the Hb without taking up CO2?
The pH of the cell will rise as excess H+ will be buffered by deoxygenated Hb.
The cell will become alkaline
What is the effect of Hb taking up CO2 on O2 avidity?
What is the effect of Hb giving up O2 on CO2 avidity?
Hb avidity for oxygen reduces with its uptake of CO2 by modifying making Hb more tense (Bohr shift)
Hb avidity for CO2 increases as it gives up O2 by making Hb more relaxed (Haldane effect)
In which direction does the oxygen dissociation curve shift to the left and right?
What does this mean for O2 avidity?
Shift to the left:
- Alkaline conditions
- For any given pO2, Hb binds more O2
Shift to the right:
- Acidic conditions
- For any given pO2, Hb binds less O2