Week 4 - Blood Gas Carriage

Question 1

What is the typical pO2 in lungs and in tissues?

Answer 1

• Typical pO2 in the lungs is 13.3 kPa

- Typical pO2 in the tissues is ~5kPa

Question 2

What are the effects on the oxygen saturation curve if pH falls?

Answer 2

Curve shifts to the right

• Promotes T state
• pH is lower in metabolically active tissues
• So extra O2 is given up in the tissues

Question 3

What are the effects on the oxygen saturation curve if temperature rises?

Answer 3

Curve shifts to the right

• Metabolically active tissues have a slightly higher temperature
• So extra O2 is given up

Question 4

What happens if pO2 in capillaries falls?

Answer 4

pH falls and temperature rises so that Hb will give up more oxygen
- So the saturation of Hb leaving the capillaries will be greatly reduced

Question 5

What is cyanosis?

Answer 5

Bluish colouration due to unsaturated haemoglobin

• Deoxygenated haemoglobin is less red than oxygenated
• Can be peripheral due to poor local circulation
• Can be central (mouth, tongue, lips, mucous membranes) due to poorly saturated blood in systemic circulation

Question 6

What are the reactions of CO2 in the blood?

Answer 6

• Dissolves in water
• – More soluble than O2
• Reacts with water
• – Forms carbonic acid initially
• – This quickly dissociates to form H+ and HCO3-
• – Reversible reaction depending on concentrations of reactants and products
• – Reaction is slow since there is little carbonic anhydrase in the plasma
• Binds directly to proteins
• – Forms carbamino compounds

Question 7

What happens to CO2 in plasma vs RBCs?

Answer 7

In plasma: CO2 dissolves in plasma and undergoes a slow reaction (little carbonic anhydrase) with water, creating HCO3-
In RBCs:
- CO2 also reacts with water, rapidly to form H+ and HCO3-
— Carbonic anhydrase is present
- H+ ions bind to Hb, drawing the reaction towards HCO3- production
- The amount produced depends primarily upon the buffering effects of Hb

Question 8

What is the buffering action of haemoglobin in RBCs?

Answer 8

H+ ions bind to Hb, so it acts as a buffer by ‘mopping up’ H+ ions

• This drives the reaction of CO2 with water, producing more H+ ions and HCO3-
• Hb has a large capacity for binding H+ ions
• The amount of HCO3- that erythrocytes produce depends on the binding of H+ to Hb

Question 9

How are carbamino compounds formed?

Answer 9

CO2 can bind directly to proteins

• Binds directly to amine groups on globin of Hb
• Binding contributes to CO2 transport
• More carbamino compounds are formed at the tissues
• – Because pCO2 is higher
• – Unloading of oxygen at the tissues facilitates binding of CO2 to Hb

Question 10

What is the normal content of carbon dioxide in arterial and venous blood?

Answer 10

• Arterial CO2 is determined by alveolar pCO2
• – This determines how much CO2 is dissolved
• – And hence affects pH
• – 21.5 mmol/L
• Venous CO2
• – pCO2 is higher in venous blood (comes from metabolically active tissues)
• – 23.5 mmol/L

Question 11

How does CO2 transport occur?

Answer 11

• Only 8% of the total CO2 in the blood is transported
• – The rest of the CO2 is there as part of the pH buffering system
• Approximately:
• – 60% travels as hydrogen carbonate
• – 30% as carbamino compounds
• – 10% as dissolved CO2