CO2 transport in blood

Question 1

Learning outcomes

Answer 1

Describe buffers and the respiratory regulation of pH
Discuss carbonic acid as a buffer
Discuss haemoglobin as a buffer
Describe the carriage of carbon dioxide in simple physical solution in blood
Describe the carriage of carbon dioxide as bicarbonate ions by reaction with water in blood
Describe the carriage of carbon dioxide as carbamino compounds by reaction with amino groups
Especially the globin of haemoglobin
Draw a carbon dioxide dissociation curve for arterial blood venous blood
Apply the Fick principle to carbon dioxide output
Define the respiratory exchange ratio and respiratory quotient

Question 2

What are the 3 defences of (H+)?

Answer 2

• Buffers- immediate
• Respiratory regulation- minutes
• Renal regulation- days/weeks

Question 3

Buffers

Answer 3

HA <> H++ A ̄
[H+] = k×[HA][A ̄]
pH = pk+ log[A ̄][HA)

Henderson-hasselbach equation

Question 4

Buffers in blood

Answer 4

• CO2+ H2O <> H2CO3 <> H++ HCO3 ̄ ̄ (PK=6.1, most important buffer in body (HCO3 kidney, co2 lungs)
• H2PO4 ̄ ̄ <> H+ + HPO4 (pk=6.8)
• HHb <> H+ + Hb ̄ (pk=7.4)

Question 5

Acidosis and alkalosis

Answer 5

-CO2+ H2O <> H2CO3 <> H++ HCO3 ̄ ̄
•Normal pH is 7.4
•Acidosis is pH < 7.35
•Alkalosis is pH > 7.45
•Respiratory acidosis is retention of carbonic acid -↑PCO2 -may be acute or chronic
•Non-respiratory (metabolic) acidosis is increased non-carbonic acids -↓PCO2, ↓HCO3─
Reduction of PP co2 and removal of bicarbonate ions (exhaled as co2), low bicarb conc

Question 6

Carbon dioxide carriage by blood

Answer 6

In the lungs

• CO2+ H2O (carb.anhydrase) <> H2CO3 <> H++ HCO3̄
• CO2+ + HbNH2 <> HbNHCOOH (slow reaction) (Carbaminohaemoglobin)
• Hb- + H+ <> HHb (Deoxyhaemoglobin- more readily binds to H+- Haldane effect ( Oxygenation of blood in the lungs displaces CO2 from hemoglobin which increases the removal of CO2. This property is the Haldane effect. Consequently, oxygenated blood has a reduced affinity for CO2.)
• O2-Hb+ H+ <> O2HHb (Oxyhaemoglobin)

Question 7

Distribution of CO2 in blood

Answer 7

HCO3- 80%
Dissolved CO2 10%
Carbamino compounds 10%

Plasma carries 70%, RBCs 30% (less due to the amount of Hb carried by red blood cells)

Question 8

Fick principle

Answer 8

Carbon dioxide output = arterio-venous difference × cardiac output

Question 9

Comparison of O2 and CO2 transport by blood

Answer 9

Oxygen-
•carried in 2 ways–dissolved (3 mL/L)–reacts with haem
•approx 200 mL/L
•plateau
•requires 7 kPa to extract approx 50 mL/L at rest
•Bohr shift (4 factors)
Factors that move bohr shift right:
CADET, face Right!” for CO2, Acid, 2,3-DPG, Exercise and Temperature.

Carbon Dioxide •carried 3 ways–dissolved (30 mL/L)–reacts with globin–mostly reacts with water
•approx 500 mL/L
•no plateau
•requires 0.7 kPa to addapprox 45 mL/L at rest
•Haldane effect (1 factor)

Question 10

Simplified rules for diagnosing acid/base status

Answer 10

1. Acidosis/alkalosis –pH inc /dec
2. Respiratory acidosis/alkalosis –PaCO2 -/inc
3. 1 Acute/chronic respiratory acidosis –bicarbonate - /dec
4. Metabolic acidosis/alkalosis –bicarbonate inc/dec PLUS respiratory compensation –PaCO2 inc/dec
5. 1 Mixed metabolic and respiratory –no respiratory compensation for metabolic disorder