Carbon Dioxide in the Blood

Question 1

Describe the equations of carbon dioxide dissolving in the blood. (4)

Answer 1

CO2 + H2O H2CO3 H+ + HCO3-
But the second step is so fast it’s almost
CO2 + H2O H+ + HCO3-
Catalysed by carbonic anhydrase.

Question 2

Describe the pH changes related to carbon dioxide dissolving in the plasma. (2)

Answer 2

If p(CO2) rises, pH falls
If p(CO2) falls, pH rises.

Question 3

Describe the Henderson Hasselbach equation. (7)

Answer 3

pH = pK + Log ([HCO3-] / p(CO2) x solubility)
pH = 6.1 + Log ([HCO3-] / p(CO2) x 0.23)
Normally 20x as much HCO3- as there is dissolved CO2 so
pH = 6.1 + Log (20/1) = 6.1 + 1.3 = 7.4

Question 4

Explain why it can appear that H+ binding to Hb controls blood pH, but why this is bollocks. (7)

Answer 4

Because H+ produced by carbonic anhydrase binds to the naturally negative Hb, meaning there’s lots of HCO3- to be excreted into the blood by the anion exchanger (HCO3- for Cl-), and then this HCO3- controls pH, so it appears like the binding of H+ to Hb (which stops this reaction reversing) controls pH.
However, the kidney controls the excretion of HCO3-, so actually the p(CO2) needed for the production of more H+ and HCO3- can become the limiting factor.

Question 5

Describe how the state of haemoglobin related to the H+ binding capacity and the p(CO2). (10)

Answer 5

More O2 binds to Hb > R state > less H+ ions bind > HCO3- binds these extra H+ > produces CO2 - but this is fine, because if the blood has lots of oxygen, it’s in the lungs and the CO2 can be breathed out straight away.

Less O2 binds to Hb > T state > more H+ ions bind > more HCO3- produced in RBC to replace the H+ bound > more CO2 used up > higher p(CO2) can exist without changing pH - needed because this occurs in venous blood.

Question 6

Describe carbamino compounds. (3)

Answer 6

CO2 binding directly to amino groups on proteins. Not part of the acid base balance but just a way of transporting CO2. Small amount though.

Question 7

Describe the ways CO2 can exist in the blood. (3)

Answer 7

Dissolved
As bicarbonate
Carbamino compounds

Question 8

Describe why hyperventilation is physiologically helpful in exercise. (2)

Answer 8

In exercise p(O2) drops and p(CO2) rises.

Hyperventilation increases p(O2) and lowers p(CO2)

Question 9

Describe the peripheral chemoreceptors. (4)

Answer 9

In the carotid and aortic bodies.
Large fall in p(O2) stimulates increased Resp rate and heart rate to try to raise it again. This is called the respiratory drive.

Question 10

Describe the central chemoreceptors. (5)

Answer 10

In the brain medulla.
Detects changes in plasma pH by checking CSF pH.
CSF HCO3- is determined by choroid plexus cells, so this means that if the medulla senses a short term altered pH (determined by p(CO2)) the choroid plexus can change the amount of HCO3- released to alter it.
This means that they “set” the p(CO2) associated with normal CSF pH, so they can be reset by persistent changed to p(CO2).

Question 11

Describe two other respiratory control techniques. (2)

Answer 11

Pulmonary and joint/muscle stretch receptors.

Question 12

Describe the changes that occur in persisting hypoxia. (5)

Answer 12

Hypoxia detected by peripheral chemoreceptors - increase ventilation
This decreases p(CO2) - want to decrease ventilation but this is not possible.
CSF composition changes to have a higher pH due to lower p(CO2).
Choroid plexus secretes less HCO3-
Central chemoreceptors accept new p(CO2) as normal.

Question 13

Describe the changes that occur in persisting hypercapnia. (4)

Answer 13

Respiratory acidosis occurs
Peripheral and central chemoreceptors stimulating breathing.
Choroid plexus adds HCO3- to CSF to combat acidosis.
They accept the new lower pH as normal and stop driving for respiration.