Respiratory expert session - 06.11.17 Flashcards
What are the normal PaO2, PaCO2, pH and HCO3- values?
PaO2: 10 - 13.5 kPa
PaCO2: 4.8 - 6 kPa
pH: 7.35 - 7.45
HCO3-: 23 - 27mmol/L
What is the alveolar gas equation?
PAO2 = FiO2(Patm - PH2O) - PaCO2/RER
FiO2 - 0.21 Patm - 101kPa PH2O - 6.25kPa PaCO2 - from blood analysis RER - 0.8
What is the alveolar-arterial difference and what is the normal range?
is a measure of the difference between alveolar and arterial oxygen
- 0.5-1.5kPa
What type of protein is Hb?
Allosteric protein
What is meant by “cooperative binding”?
Binding of oxygen to one haemoglobin subunit causes a conformational change that are relayed to the other subunits, making them more able to bind oxygen by raising their affinity for this molecule - oxygen binding to iron is reversible and changes the structure of the subunit - the new conformation causes the haem groups to become progressively more exposed
What is the key purpose of Hb?
increases the total blood oxygen capacity seventy fold in comparison to dissolved oxygen in the blood
What form is oxygenated Hb?
has a planar form = referred to as R or relaxed form
What does the Bohr effect?
facilitates oxygen release in the tissues
What effect does the Bohr effect have on the oxygen-dissociate?
Shifts to the right
because this decreases the pH of the blood which promotes the dissociation of oxygen
When does Hb have a higher binding affinity for H+?
Hb has proton binding sites which have a higher affinity for binding H+ in deoxyhaemoglobin than oxyhaemoglobin
- oxygen is pushed off Hb to accommodate proton binding
What conc is 2,3-BPG present in the blood?
approx. 5mmol/L
Which form of Hb does 2,3-BPG bind to with a greater affinity?
Binds with a greater affinity to deoxygenated Hb (e.g. RBCs near respiring tissue) than it does to oxygenated Hb (e.g. lungs) due to spatial changes
- 2,3-BPG fits in the non-planar de-oxygenated Hb configuration but not in the oxygenated planar formation
-
What part of Hb does 2,3-BPG interact with?
interacts with deoxygenated Hb beta subunits decreasing O2 affinity so it allosterically promotes the release of the remaining oxygen molecules bound to Hb
What are “salt bridges”?
in the presence of CO2 and H+, charged groups are formed on each subunit - charged groups form salt bridges
These form between 2 subunits and stabilise the deoxygenated form of Hb
What light does oxygenated conformation of Hb absorb and reflect?
light in the blue-green range and reflects red light, giving the red appearance of oxygenated blood
What light does deoxygenated conformation of Hb absorb and reflect?
Deoxygenated conformation of Hb absorbed light in the orange range and reflects blue light giving a bluish appearance
How does CO form?
it is the product of combustion of organic matter under conditions with reduced oxygen supply, preventing the complete oxidation to CO2
What forms when CO enters the blood?
Carboxyhaemoglobin - competitive inhibitor for oxygen - affinity is 250 times stronger than its affinity for O2 - therefore it is unlikely to dissociate and once bound it blocks the binding of oxygen to that subunit
Why is it relevant that CO is structurally similar to O2?
carboxyhemoglobin therefore favours the planar state raising the oxygen affinity of the remaining unoccupied subunits - this combination significantly reduces the delivery of oxygen to the tissues which is what makes CO so toxic
What colour is the blood in CO poisoning ?
it absorbs blue-green light and reflects red giving the Red appearance of oxygenated blood - people look pink
What are the symptoms of CO poisoning?
dull headache weakness dizziness nausea or vomiting shortness of breath confusion most of these are due to low oxygenation but nausea is due to gastric paresis
What does chronic CO poisoning cause?
Persistent CO creates a state of chronic low oxygen status
- Similar to high altitude
- HR and RR increase
- 4-5 days later RBCs release from bone marrow increases to try and increase oxygen carriage potential
When does anaemic hypoxia occur and what happens?
when the oxygen carrying ability of the blood decreases - fewer Hb molecules or oxygen binding sites are available for binding oxygen
All the principles of oxygen diffusion remain the same
Reduction in oxygen delivery to tissue will lead to an increase in capillary perfusion, arteriolar dilation and a reduction in blood viscosity (reduction in Hot)
Increase in RR and HR
- blood flow and oxygen delivery will increase until capacity is met
PaO2 will be unchanged because the Hb are saturated, the lack of O2 delivery is due to too few Hb molecules
