Blood Gas Transport Flashcards
What mechanism does gas exchange occur through?
Diffusion
What is diffusion in gas exchange dependent on?
- Diffusion surface area (many moist alveoli)
- Diffusion distance for gases (short alveolar and capillary walls)
- Concentration between alveolar air and blood (large difference in partial pressure)
- Solubility of gases
- Coordinated blood flow and airflow to allow the passage of both of the gases efficiently
What is Dalton’s law of partial pressures?
the total pressure exerted by a mixture of gases is the sum of the pressures exerted independently by each gas in the mixture
What is partial pressure?
the pressure exerted by each gas. Directly proportional to its percentage in the total gas mixture
If there is 20.9% of oxygen in the atmosphere and the atmospheric pressure at sea level is 760 mmHg what will be the partial pressure of oxygen?
20.9% (0.209) x 760mmHg = 159 mmHg
What are the partial pressure differences in the pulmonary circuit and what does this mean?
In the alveolus there is a partial pressure of 100 of oxygen and 40 of CO2. In the pulmonary capillary the partial pressure of oxygen is 40 and carbon dioxide is 45
This pressure difference allows oxygen to diffuse out of the alveolus and CO2 to diffuse in
What are the partial pressure differences in the systemic circuit and what does this mean?
The partial pressure of O2 in the systemic capillary is 95 and CO2 is 40. The partial pressure of O2 in the interstitial fluid is 40 and CO2 is 45.
Therefore, oxygen can diffuse into the cells and carbon dioxide can diffuse out due to the differences in partial pressure
What is Henry’s law and why?
- at a given temperature the amount of a particular gas in solution is directly proportional to the partial pressure of that gas
- this is because when a gas under pressure contacts a liquid the pressure tends to force gas molecules into solution
What does the amount of gas dissolved in solution depend on as well as partial pressure?
Solubility
Is oxygen soluble?
It isn’t very soluble. CO2 is more soluble
If oxygen isn’t very soluble how comes there is so much in the blood?
Because of haemoglobin
What is the haemoglobin structure?
- Conjugate protein
- 4 globular protein subunits (2 alpha + 2 beta)
- Each subunit
Protein (globin)
Non-protein group (haem) - Haem: Fe2+ in a porphyrin ring
- Fe2+ is the binding site for oxygen
What happens when Hb binds oxygen?
- Hb + O2 HbO2
- Deoxyhaemoglobin oxyhaemoglobin
- Rapid and reversible
How many molecules of oxygen can each Hb molecule bind?
four
What happens as Hb starts to bind oxygen?
- Deoxyhaemoglobin has lower affinity for oxygen but when one oxygen binds the structure changes and it’s affinity for oxygen increases allowing us to bind the oxygen more easily
- After binding with O2 Hb changes shape to facilitate further uptake – positive feedback
How is oxygen transported?
- Approx. 97% of O2 transported in blood in combination with Hb
- Remainder in plasma
What is O2-Hb saturation?
The percentage of haem units in a Hb molecule that contain bound oxygen (4 molecules of O2 bound to Hb – fully (100%) saturated)
What is O2 binding to haemoglobin affected by?
PO2 (partial pressure of oxygen) of blood
Blood pH
Temperature
State of O2 binding of the Hb molecule (how many molecules are bound to the Hb at any onetime)
What is the oxygen-Hb dissociation (saturation) curve?
- Relates the saturation of Hb (y axis) to the PO2 (partial pressure of oxygen – x-axis)
- About 75% Hb saturation in the systemic tissues when there is a partial pressure of 40 of oxygen
- In the alveoli the partial pressure of oxygen is 100 and the percentage of Hb saturation is 100%
- The higher the partial pressure of oxygen the more combines with haemoglobin
What aids haemoglobin unloading of oxygen?
- A lower pH with increased carbon dioxide leads to better unloading of oxygen: ‘Bohr shift’. This would happen for example when you are exercising
- A high temperature leads to more unloading. This happens in active tissues
- Increased 2,3 -DPG (product of anaerobic metabolism is red blood cells) leads to better unloading. Binds much more strongly to deoxyhaemoglobin than oxyhaemoglobin.
- increased BPG production (anaerobic product) due to increased pH
What happens as atmospheric pressure gets lower and lower as you increase in altitude?
- Impact on the partial pressure of oxygen and amount that can be delivered to body.
- Faster, deeper breathing at a high altitude
- Hypoxia (blood oxygen concentration very low) at a very high altitude
- ‘foggy’ brain, weak muscles at an extremely high altitude
What happens with carboxyhaemoglobin?
CO binds tighter than O2 (200x greater)
Dramatically reduce ability of O2 to bind to Hb
What happens with methaemoglobin?
Fe2+ oxidised to Fe3+ by drugs etc.
Unable to carry O2
Slowly converted back
What is foetal haemoglobin?
2 alpha and 2 gamma subunits
Higher affinity for O2
Important in transferring O2 across the placenta
Makes it easier for foetus to get oxygen from it’s mother
How are CO2 molecules transported?
70% converted to carbonic acid formation H2CO3- in haemoglobin and transported in plasma as bicarbonate ion HCO3-
Bound to haemoglobin: carbaminohaemoglobin (23%
Dissolved in plasma (7%)
What does 93% of Carbon dioxide produced from the cells do?
diffuses into RBCs to either become carbonic acid or to bind to haemoglobin
What is the chloride shift?
- CO2 + H2O
- with carbonic anhydrase ->
- H2CO3
- Which dissociates into
- H+ and HCO3-
- The bicarbonate ions (HCO3-) move into the plasma with the aid of a counter-transport mechanism that exchanges intracellular bicarbonate ions for extracellular chloride ions
- H+ removed by buffers
How is carbaminohaemoglobin formed?
- Hb is attached to NH2
- NH2 combines with CO2 to become NHCOOH and this binds to Hb (different binding site to oxygen)
How does buffering in RBCs take place?
- Every CO2 -> HCO3- yields an H+
- pH would become very acidic
- haemoglobin minimises the size of pH changes by consuming or releasing H+
- best buffers in red cells: imidazole groups of histidine residues in haemoglobin
What is the Haldane effect in the lungs and in the tissues?
- In the lungs:
- Oxygenation of Hb -> lower affinity for H+ ions -> decreased buffering power -> release of H+
- Aids unloading of CO2 in lungs
- In the tissues
- Deoxygenation of Hb -> higher affinity for H+ ions -> increased buffering power -> H+ uptake
- Aids CO2 transport from tissues
Why is diffusion easier due to the fact that CO2 and O2 are lipid soluble?
they can diffuse through the surfactant layer and the alveolar and endothelial plasma membranes
When do gases diffuse in/ out until?
until the partial pressure either side of the membrane have reached equilibrium
Why can you still survive at high altitudes?
because even when there is a lower PO2 because of the fact that haemoglobin changes shape as it binds more O2 it will still have a fairly high affinity for oxygen due to it’s curve: Hb binding is not linear
Why does venous blood still have a relatively large oxygen reserve?
Because haemoglobin still retains 75% of it’s oxygen
What is Bohr’s effect?
• When the pH decreases the shape of haemoglobin molecules changes and the molecules release their oxygen more readily
