Session 4 - Oxygen in blood Flashcards
How soluble is oxygen in water at 13.3 kPa?
- Will dissolve 10%
* 0.13 mmol.l-1
What is the minimum oxygen we need per minute (in mmol)
- 12 mmol
* Would require 92 litres of fluid- impossibly high
What problems do we encounter when faced with binding O2 to a solute?
• The reversibility of the reaction
Is the addition of O2 to haem a reduction reaction?
• No, it’s an oxygenation reaction
What is myoglobin?
• A haem storage unit found in muscle
What is a dissociation curve?
- Plot of amount O2 bound vs pO2
* Total content then bound and dissolved
What happens at the top end of an oxygen dissociation curve?
• Chemical binding will saturate
How do you use a dissociation curve to work out how much oxygen taken in or given up?
- Work out the difference in fractional saturations between the two pO2’s
- Multiply by the amount bound at full saturation
- Tells you how much O2 is taken or given up
What is the normal ppO2 in the lungs?
• 13.3 kPa
What is the normal ppO2 in the tissues?
• 5kPa
What is the structure of haemoglobin?
- A tetramere
- Each subunit has one haem + globin
- Variable quaternary structure
What are the two states of haemoglobin?
- Tense - strong relationships between subunits
* Relaxed - Weak relationships between subunits
How many O2 can one haemoglobin pick up?
• Four
In what state does haemoglobin pick up oxygen?
• Relaxed
Which oxygen does haemoglobin find it hardest to bind?
• The very first
What is sigmoidal binding, and why does this happen?
- S shaped dissociation curve
* Easier to bind as oxygen content increases
When is haemoglobin saturated?
• Above 8.5/9 kPa
When is haemoglobin unsaturated?
• Below 1 kPa
What is half saturation point for haemoglobin?
• 3.5/4kPa
How does the dissociation binding curve for haemoglobin demonstrate reversibility?
• Saturation changes greatly over narrow range of pO2
What is the normal amount of O2 which should be bound to Hb leaving lungs?
- 8.8 mmol/l
* 4 hb in each RBC, 2.2mmol per Hb
What is standard tissue kPa?
• 5 kPa
What problems does the standard tissue kPa pose for Hb?
• Only removes 35% O2 from Hb
How do tissues increase amount of O2 released from Hb?
- Increase capillary density
- Decrease pH
- Increase temperature
What is the state of Hb in acidic conditions?
• Tense
How do cells utilise Hb’s tense state in acidic conditions?
• Metabolically active tissue release acid (H+), which shifts oxygen dissociation curve to the right
If you can combine low pH and high temperature, what proportion of Hb loses O2?
• 70%
What is the oxygen reserve?
• Only 27% O2 in arterial blood given up to tissues
What three factors effect the haemoglobin dissociation curve?
• Increasing H+
• Increasing temperature
• Increasing CO2
○ Will shift to right
What can you do if venous pO2 is known?
• A dissociation curve can be used to calculate the percentage of oxygen that has been given up to that tissue
What are the three factors which determine gas exchange
- Area available for the exchange
- Resistance to diffusion
- Gradient of partial pressure
What can happen to CO2 in the blood?
- Dissolved in blood
- Reacts with water
- Binds directly to proteins
Why does Co2 dissolve in water?
• More soluble than O2
Why does CO form when it binds directly to proteins?
• Carbamino compounds
What does CO2 react with water to form?
• H+ and HCO3-
What does the amount of CO2 reacting with water depend on?
• Amount reacting depends on concentrations of reactants and products
What is the henderson hasselbach equation?
• pH = 6.1 + log (HCO3-/pCO2x0.23)
What are two critical determinants of pH (other than H+)?
- HCO3-
* CO2
Outline the reaction of CO2 in an RBC
- Reacts with water, forming HCO3- and H+
- H+ removed by binding to RBC
- Keeps reaction moving forward (CO2 + H20 -> H+ + HCO3-)
What happens to HCO3- in RBC?
- Released through RBC membrane
* Exchanged for CL
How is HCO3- formed in plasma?
• Slow reaction between CO2 and water
How is HCO3- formed in RBC/
- Fast reaction between CO2 and water
* Due to the presence of HCO3-
CO2 + H20 -> HCO3- + H+
In light of this equation, explain why HCO3- is rapid and constant in an RBC
• H+ consistently removed, favouring further HCO3- production
How does Hb act as buffer in RBCs?
• H+ reacts with Hb, driving the reaction between CO2 + H20 (creating further H+ and HCO3-
What do carbamino compounnds do?
• Bind directly to proteins, contributing to CO2 transport but not acid base balance
Where is the concentration of carbamino compounds the greatest?
• More in venous blood, as pCO2 highest
What is arterial blood Co2?
• 21.5 mmol/litre
What is venous blood CO2?
• 23.5 mmol/litre
What is the conc of HCO3- in arterial plasma?
• 25 mmol/l-1
What happens to blood pH at higher concentration of CO2?
• pH will fall
What is hydrogen carbonate in the blood derived from?
• Sodium hydrogen carbonate
What is the purpose of having a high HCO3- in the plasma?
• Prevents CO2 reacting with water
State the portions of CO2 travelling in particular forms
- 80% as HCO3-
- 11% as carbamino compounds
- 8% travels as dissolved CO2
