Blood gases Flashcards
How do partial pressures relate to the amount of gas transport?
The partial pressure is the concentration gradient for the gases to diffuse down
How can gas be transported?
Gas is dissolved in solution
Gas can bind to carrier molecules (proteins) to be transported
What are the determinants of how much gas is transported?
How much gas is contained in a litre of blood
How much blood is transported
Modulation of gas content in blood
What is the equation for the amount of gas dissolved?
Amount dissolved = partial pressure x Solubility coefficient (alpha)
Amount of gas dissolved is proportional to its partial pressure
What is the normal alveolar partial pressure of oxygen and carbon dioxide?
O2 = 13 kPa CO2 = 5kPa
For every litre of blood how much 02 is dissolved?
About 2.9 ml/L is dissolved from a PO2 of 13kPa
Alternatively we have a lot of CO2 (~ 25ml/L
What are the gas carrier molecules?
Haemoglobin 4 Haem +globin chains)
Why is haemoglobin packaged into RBCs?
Haemoglobin MW = 64.5KDa and therefore is small enough to be excreted through the glomerulus.
To prevent this it is packaged into RBCs to ensure that it is not excreted.
What will the globin chains bind to?
alpha and beta chains will bind to CO2 and H+ ions
What will the Haem groups bind to/carry?
Each moiety will bind to an O2 molecule
What are the main roles of Haemoglobin?
Carries O2 (haem groups), CO2 (globin chains) and buffers pH
If there is a higher concentration of O2 in the RBCs then what will happen to the O2?
More O2 in plasma will diffuse into the RBC and therefore this will shift eqm to produce more oxyhaemoglobin.
What is the relationship between the alveolar, arterial pressure and Hb saturation?
Alveolar pressure sets the arterial pressure and therefore will be equal.
The O2 molecules in plasma will also diffuse into RBCs and equilibrate with cytoplasm.
How do you calculate the % saturation of Hb?
O2 bound / O2 capacity x100
What is the physiological significance of the haemoglobin saturation curve?
When one O2 molecule binds to Hb, it becomes easier for a second O2 molecule to bind to another site of Hb since there is a shape change in the protein structure.
This leads to a quick increase in saturation over a shorter PO2.
The curve then plateaus as the 4th O2 molecule binds
What is the P50?
The partial pressure required for 50% saturation.
If P50 is further to the left then there is a higher affinity for O2.
Why is it important for the alveolar gases to have a high P02?
This means that the % saturation of Hb is at 100% even when there are slight changes in PO2.
eg. even at a PO2 of 9, the %saturation is still around 100%
What is the difference in sensitivity of Partial pressure changes in the lungs and tissues?
Lungs (loading) - low sensitivity of PO2 changes
Tissues - High sensitivity of PO2 changes
What is the Bohr effect?
Hb affinity for O2 decreases (P50 increases) and the curve shifts to the right when there is increased:
CO2, H+, 2-3DPG, higher temperature.
What is the significance of the bohr effect?
Enables us to have local matching of O2 delivery to tissue metabolic needs.
What is 2-3-DPG?
Formed as a side reaction of glycolysis from anaerobic respiration in RBCs.
What does 2-3-DPG bind to?
Binds to the Beta chains of Hb
Binds more favourably to deoxyHb than OxyHb
Why is 2-3-DPG useful?
In cases such as chronic Hypoxia and blood alkalosis where Hb curve shifts to the left, then increasing 2-3-DPG counteracts the left shift (bringing it back right) to help maintain O2 delivery to tissues.
What happens if we decrease 2-3-DPG levels?
Decreases O2 availability in transfused blood
What is a haematocrit?
Measure of the red cell volume
What is O2 capacity?
The capacity of Hb to bind to O2 (if 100% saturated)
What is the normal amount of O2 bound to Haemoglobin?
195 - 200 ml/L
What is the normal amount of O2 dissolved in the blood?
about 3 ml/L
What is the relationship between O2 content and O2 consumption?
The rate at which oxygen is consumed by the periphery is equal to the difference between the oxygen content in arterial blood leaving the heart and in the mixed venous blood returning to the heart
VO2 = (CaO2 - CvO2 ) x CO
What enzyme speeds up formation of H2CO3?
Carbonic anhydrase
What is the main way CO2 is carried in the body?
65% as bicarbonate in the plasma
20% as bicarbonate in the RBCs
What is the main store of CO2 in the body?
As Bicarbonate ions to enable the body cope with acid base balance.
Why is Hb important regarding CO2?
For the CO2 flux in and out of the tissues and lungs
What is the significance of the Haldane effect?
HCO3 + H+ions is made and then moved into the plasma.
DeoxyHb is a strong proton acceptor
Therefore will mop up H ions
Therefore CO2 will be bound to Hb but also form more HCO3
What are the transport rates between the arterial and venous blood for O2 and CO2.
VCO2 = 200ml/min
VO2 = 250 ml/min
What is the Haldane effect?
Since deoxyhaemoglobin has a higher affinity for CO2, when PO2 falls which leads to a rising CO2.
Therefore it makes it easier to transport CO2.
What happens to the affinity Hb has for O2 in acidosis?
Affinity decreases.
