Blood Gases Flashcards
What are the determinants of much gas is transported?
1) how much gas is contained in a litre of blood
- gas transported in solution
- gas transport bound to carrier molecules
2) how much blood is transported
- cardiac output
3) modulation of gas content in the blood
- Bohr and Holdane effects
What is Henry’s law?
amount of gas dissolved in solution is proportional to its partial pressure
amount dissolved = partial pressure x solubility constant
How many and what type of global chains does an adult have?
- 2 α chains
- 2 beta chains
What do the α and beta chains bind to?
- CO2 and H+
- beta chains also bind 2,3-DPG
What is there role of haemoglobin?
carrying CO2 and O2 and buffering PH
What happens when you increase the partial pressure of oxygen in a RBC?
more oxygen becomes bound to haemoglobin
How do you work out saturation?
%=O2 bound/O2 capacity x 100
Why does the gradient of the saturation curve increase sharply?
-because binding the first and second O2 molecule changes the shape of haemoglobin in a way that makes it easier for the next O2 molecule to bind. (cooperativity due to allosteric changes)
Why does the gradient of the curve flatten off?
-when 3 O2s are bound the molecule is quite connected which makes it more difficult for the third molecule to bind
What is the P50?
the partial pressure required to get 50% saturation (measure of affinity to O2)
Describe the importance of the arterial PO2 in the lungs?
- high PO2
- saturated haemoglobin at about 7/8kPa
- even though we try to keep alveolar partial pressures constant, if the PO2 drops O2 being bound to haemoglobin is not being affected
Describe the importance of the venous tissue PO2?
- lower, P50 is at the steep part of the curve
- a small change in PO2 will significantly change saturation
- so at a tissue with a lower PO2 (using more O2 for metabolism) helps to deliver more O2 to that tissue
What happens if you move the curve to the right?
- increasing P50
- lowering affinity for O2
What factors lower affinity of Hb for O2?
-increase pCO2
-increase [H+]
-increase 2,3-DPG
-increase temp
(reversing this increases affinity)
Why is it useful that these factors decrease affinity for O2?
- most of these increase with metabolic rate
- so at the same PO2, more O2 can be dropped off at cells with high metabolic rates
How do RBCs respire?
-don’t have mitochondria so respire anaerobically by glycolysis
When is 2,3-DPG formed?
in a side reaction of glycolysis
When does 2,3-DPG bind to beta chains more favourably?
when it is deoxyHb
When is 2,3-DPG useful?
in chronic hypoxia and blood alkalosis (shifts Hb curve right) counteracting left shift this helps maintain O2 delivery to tissues
When does 2,-DPG decrease?
-under storage or in chronic acidosis- decreases O2 availability in transfused blood
What is the haematocrit?
the measure of red blood cell volume
What is O2 capacity?
the capacity of Hb to bind to O2
Whats the difference between a content curve and a saturation curve?
- saturation curve tells us how much O2 is bound to Hb
- content curve also puts haemoglobin conc into consideration (how much O2 you have per litre of blood)
How do you work out VO2?
(CAO2-CVO2) x cardiac output
look at calculations
on slides
How can you increase VO2?
- increase cardiac output
- desaturate blood more at veins
What happens when CO2 is in the plasma?
- can stay in the plasma
- can bind to a plasma protein like albumin
- convet to H2CO3 and then HCO3- (bicarbonate) + H+ (extremely slow without carbonic anhydrase)(v unlikely)
- CO2 could diffuse into RBC
- Once in RBC can bind to α and beta chains of Hb (deoxy better)
- form bicarbonate using carbonic anhydrase (this can only happen if there isn’t a build up of end products, this happens by exchange with a Cl- ion in the plasma)
What are the percentages of CO2 in each area?
- bicarbonate in plasma 65%
- bicarbonate in RBCs 20%
- PCO2 in plasma 4%
- PCO2 in RBC 5%
- bound to albumin in plasma 1%
- bound to α and beta chains 5%
What is the largest store of CO2?
HCO3
What is the most important mechanism for flux (moving in and out) for CO2?
Hb
What increases Hb affinity for CO2?
hypoxia
How do you prevent to build up of H+ when CO2 is converted to bicarbonate?
H+ binds to α and beta chains of deoxy Hb
What is the Bohr effect?
changes in O2 affinity causing changes in O2 content
What is the haldene effect?
changes in CO2 content
