7 - Carbon Dioxide in the Blood

Question 1

How much CO₂ is in arterial blood?

Answer 1

- 21mmol/L

• Dissolved and associated with Hb
• Needed to maintain pH, not a waste product in arterial blood

Question 2

How much carbon dioxide is dissolved in plasma?

Answer 2

At 37 degrees, 5.3 kPa with solubility coefficient of 0.23 –> 1.2 mmol/L

Question 3

Why is the reaction to produce hydrogen carbonate ions from CO₂ initially slow?

Answer 3

• Little carbonic anhydrase in the blood
• Dissociation usually resisted by the high level of HCO₃- already in the blood

Question 4

How does altering the pCO₂ change the pH and what factor controls the pCO₂?

Answer 4

pCO₂ all depends on the rate of breathing, if you breathe faster it drops, breathe slower it rises

Question 5

Why is the blood pH slightly alkaline?

Answer 5

• Plasma contains 25 mmol/L HCO_3- from RBC and CO2
• This prevents nearly all the dissolved CO₂ from reacting

Question 6

How do we work out the pH of blood?

Answer 6

• Henderson Hasselbach Equation
• pH depends on ratio of HCO₃- to CO₂

Question 7

How does pCO₂ affect HCO_3- levels?

Answer 7

• In body fluids with few or no other buffer systems, e.g CSF, it is not affected over all physiological values of pCO₂.
• Depends how well the buffering effect of Hb is, higher when deoxygenated

Question 8

How does the red blood cells maintain a high bicarbonate concentration in the blood?

Answer 8

• RBC contain carbonic anhydrase
• H+ ions mopped up by negatively charged Hb- so reaction favoured in forward direction
• Bicarbonate transported into blood by chloride-bicarbonate transporter

Question 9

What is the pH of the body fluids determined by?

Answer 9

• Amount of CO₂ dissolved in plasma depending on pCO₂
• Amount of HCO3- formed from CO₂ in the red cell (controlled through levels excretion through the kidney)
• Ratio of CO₂ to HCO₃ which should be 20

Question 10

How does hydrogen carbonate act as a buffer?

Answer 10

• Reacts with acids, e.g lactate, keto and sulphuric acids, to produce CO₂
• CO₂ is just removed by breathing

Question 11

Why is there more CO₂ in venous blood than arterial blood?

Answer 11

• pCO₂ is 6kPa rather than 5.3 kPa in venous blood
• Dissolved is CO₂ and reacted is HCO_3-
• Only a small change in pH of venous as HCO₃- and CO₂ have increased

Question 12

How is CO₂ released in the lungs to be breathed out?

Answer 12

• Hb picks up O₂ and goes into R state
• Hb gives up H+
• H+ reacts with HCO₃ to produce CO₂

Question 13

Apart from being dissolved and being transformed to hydrogen carbonate, how else can CO₂ be transported?

Answer 13

- Carboamino compounds

• Bind to amine group of the globin part of Hb
• Not part of acid base balance
• More carboamino compounds at tissues an pCO₂ higher and oxygen unloading
• Given up at the lungs as oxygen loads

Question 14

How can you work out how much carbon dioxide is being transported?

Answer 14

• Difference between arterial and venous content
• 23.3-21.5 = 1.8 mmol/L
• 8% transported, the rest is part of pH buffering system

(60% hydrogen carbonate, 30% carboamino, 10% dissolved)

Question 15

What is the difference between the carbon dioxide and oxygen dissociation curve? How do they differ during hyperventilation?

Answer 15

• Carbon dioxide is linear not sigmoid
• In hyperventilation both pCO₂ AND CO₂ in the blood fall
• With oxygen the pO₂ increases but oxygen content remains equal as Hb saturated to 100% already by around 9kPa

Question 16

What would happen to the total oxygen content of blood if an individual breathes air twice the atmospheric pressure?

Answer 16

Would increase a tiny tiny amount as haemoglobin is already saturated at atmospheric pressure, only the dissolved will increase

Question 17

What are the differences beteen ABG analysis and SaO₂ and when would you use each method?

Answer 17

Question 18

A man has pneumonia in one lobe of his lung, how would you figure out the partial pressures of oxygen and carbon dioxide in the mixed blood?

Answer 18

Use dissociation curves, draw on the values of each and then work out the middle value

Question 19

What could be a cause of arterial hypoxia when you have pneumonia?

Answer 19

- Perfusion/Ventilation mismatch

• Although reduced ventilation would lead to vasoconstriction of pulmonary arterioles, there is still some adequate perfusion

Question 20

Why does this man have tissue hypoxia even though his tests reveal he is in the normal range?

Answer 20

• Oxygen saturation normal as the Hb that is there is saturated but doesn’t mean theres lots of Hb
• pO2 is normal as this refers to dissolved oxygen, which is only a minor contribution

Question 21

What is the significance of central cyanosis compared to peripheral cyanosis?

Answer 21

Central is an issue with the lungs or a whole body system whereas peripheral is just issues with a local circulation

Question 22

What will happen to the patients blood pH and how will the body work to combat this if pCO₂ doesnt change?

Answer 22

• It will drop as an increase in CO₂
• The kidney will retain more hydrogen carbonate to restore the pH and buffer it