Physiology 3: Gas Transport (L-4&5) Flashcards
Describe how PO2 change as oxygen travels from the lungs to the systemic capillaries
It decreases
What is the effect of partial pressure on gas solubility?
Henry’s Law
The amount of gas dissolved in a given type and volume of a liquid is proportional to the partial pressure of the gas in equilibrium with the liquid
(At a constant temp.)
What are the 2 mechanisms by which oxygen is transported in the blood?
- Dissolved oxygen (1.5%)
- Oxygen bound to haemoglobin (98.5%)
Describe the structure of haemoglobin (Hb)
- 4 haem groups (1 per subunit)
- 2 Beta chains
- 2 Alpha helices
Describe the binding of oxygen to Hb
- Each Hb group reversibly binds to one oxygen molecule
- PO2 is the primary factor determining %O2 saturation of Hb
- Binding of oxygen to one subunit increases the other subunits oxygen affinity (Co-operativity)
Describe the oxygen delivery index (DO2I)
Oxygen delivery to tissue depends on the oxygen content of arterial blood and the cardiac output
DO2I = (Arterial O2 conc.) X (Cardiac Index)
Describe the factors that determine the oxygen content of arterial blood
- Hb concentration
- Saturation of Hb with oxygen
What factors affect the partial pressure of inspired oxygen?
- Total pressure
- Proportion of oxygen in the air
Give examples of ways that oxygen delivery to tissue can be impaired
- Decreased partial pressure of inspired oxygen
- Respiratory disease (Less oxygen can bind to Hb)
- Anaemia (Less Hb so less oxygen)
- Heart failure
- Infarction/vessel occlusion
Describe co-operativity in Hb with regards to oxygen binding
When oxygen binds to one Hb subunit, the affinity of the other subunits for Hb increases
(This is positive cooperativity)
What is the reason for the sigmoid shape of the oxygen binding curve?
The positive cooperativity of Hb due to oxygen binding
Describe the effect of a change in PO2 on %Hb saturation at high PO2 values (e.g. in pulmonary capillaries), and lower PO2 values (systemic capillaries)
At a high PO2:
- A change in PO2 will have a small impact on the %Hb saturation. (Graph is flat)
At lower PO2:
- A change in PO2 will have a larger impact on the %Hb saturation. (Graph is steeper)
Explain the significance of the fact that, at a lower PO2, a change in PO2 will have a larger impact on the %Hb saturation
- PO2 lower at peripheral tissues
- Smaller change in PO2 required to release the same amount of oxygen from the Hb into the tissues
Describe myoglobin
- Present in skeletal and cardiac muscle
- One haem group per molecule (1:1 ratio)
- No cooperative effect
- Provides short term storage of oxygen for anaerobic conditions
Does myoglobin exhibit a cooperative effect?
No
Only has one subunit
Compare the oxygen association/dissociation curves of haemoglobin and myoglobin
Haemoglobin:
- Sigmoid (due to cooperativity)
Myoglobin:
- Hyperbolic (due to no cooperativity)
Describe the structure of foetal haemoglobin (HbF)
- 2 alpha subunits
- 2 Gamma subunits
Compare the interactions of HbF and Hb with 2,3-Bisphosphoglycerate (2,3-BPG)
What does this cause?
HbF interacts less with 2,3-Bisphosphoglycerate in red blood cells
Thus HbF has a higher oxygen affinity than Hb
Compare the sigmoid curves of Hb and HbF
HbF has a sigmoid curve that is shifted to the LEFT
This means it has a higher oxygen saturation at a lower PO2
Give an example of the significance of the differing oxygen affinities of Hb and HbF
Allows maternal Hb to lose oxygen to foetal HbF
List changes in environment that would increase the release of oxygen at the tissues (i.e. decrease Hb% saturation)
- PCO2 increase
- pH decrease ([hydrogen ion] increase)
- Temp increase
- 2,3-Bisphosphoglycerate increase
List changes in environment that would decrease the release of oxygen at the tissues (i.e. increase Hb% saturation)
- PCO2 decrease
- pH increase ([hydrogen ion] decrease)
- Temp decrease
- 2,3-Bisphosphoglycerate decrease
What term describes the shift in Hb oxygen affinity under different environmental conditions?
Bohr Shift
due to the Bohr Effect
List the 3 ways carbon dioxide is transported in the blood
And give their relative proportions
- Dissolved (10%)
- Bicarbonate (60%)
- Carbamino compounds (30%)
What determines the amount of carbon dioxide that is dissolved in the blood?
The partial pressure of the carbon dioxide gas (PCO2) in equilibrium with the solution
(Henry’s Law)
Which is more soluble carbon dioxide or oxygen?
Carbon dioxide is roughly 20 times more soluble
By which method is the majority of carbon dioxide transported in the blood?
As bicarbonate (HCO3)
Describe the how bicarbonate is formed in the blood
- In red blood cells
- Water and carbon dioxide react to form carbonic acid
- This is catalysed by Carbonic Anhydrase
- Carbonic acid dissociates into bicarbonate and a proton
Which enzyme catalyses the production of carbonic acid as part of the formation of bicarbonate?
Carbonic anhydrase
Describe the Chloride shift
- In order for more CO2 to diffuse into the cell the equilibrium of bicarbonate formation must shifted
- By facilitated diffusion bicarbonate ions move out of the RBC into the plasma in exchange for chloride ions
- Occurs at tissue, the reverse happens at the alveoli
Describe how carbamino compounds are formed
- Carbon dioxide reacts with the terminal amine group in blood proteins
- Especially globin of haemoglobin (forming carbamino-haemoglobin)
- Very rapid even without enzyme action
Describe how is carbamino-haemoglobin HbCO2 formed
- Carbon dioxide reacts with the terminal amine group in the globins (subunits) in haemoglobin
How many carbon dioxide molecules can each haemoglobin molecules bind to?
4
One for each subunit
Describe the Haldane Effect
HbO2 –> Hb
Increases the ability of Hb to bind to carbon dioxide and hydrogen ions
(O2 availability governs the binding of CO2 and H+ to Hb)
Which 2 effects work together to ensure:
O2 liberation and uptake of CO2 & H+ at tissues
- Bohr Effect
- Haldane Effect
Why at the lungs does:
HbCO2 –> Hb + CO2
HbH –> Hb + Hydrogen ion
The Hb binds to oxygen with a higher affinity
Haldane Effect
Provide an overview of the exchange of oxygen and carbon dioxide at tissue
- Some CO2 is dissolves in plasma, some in the RBC
- Oxyhaemoglobin releases oxygen, freeing up Hb
- CO2 binds to form carbaminos (e.g. Carbamino-haemoglobin)
- Bicarbonate ion formed
- Bicarbonate ion is moved to the blood plasma during Chloride Shift
- Free Hb binds to hydrogen ions making HbH
Provide an overview of the exchange of oxygen and carbon dioxide at the alveoli
- Carbaminos release their Hb and intracellular dissolved CO2
- HbH releases its Hb and hydrogen ions
- Free Hb binds to oxygen, forming oxyhaemoglobin
- The chloride shift is reversed
- Bicarbonate ions and freed hydrogen ions form carbonic acid
- Carbonic acid released CO2
- Intracellular and extracellular dissolved CO2 –> Alveoli
