Oxygen In Blood Flashcards
What two states can haemoglobin have?
How does the affinity differ?
- T state: tense |no O2 bound | low affinity for O2
- R state: relaxed | O2 bound | higher affinity for O2
Define cyanosis
Bluish colouration due to unsaturated haemoglobin
(Deoxygenated Hb is less red than oxygenated Hb)
What is the pO2 in alevolar air?
13.3 kPa
What is the pCO2 in alevolar air?
5.3 kPa
What is the pO2 in venous blood?
6kPa
What is pO2 in alevolar blood?
13.3kPa
What is pCO2 in alevolar blood?
5.3kPa
What is pCO2 in venous blood?
6kPa
Why is haemoglobin needed to transport oxygen?
- Oxygen’s solubility in water is very low
- there is not enough dissolved O2 in blood to meet the body’s demand
- Hb is needed to meet the demand + transport oxygen
Why is arterial blood brighter red than venous blood?
- Hb molecules with more O2 bound are brighter red
- oxygenated arterial blood is brighter than deoxygenated venous blood
What is formed when O2 binds to Hb?
Oxyhaemoglobin
Structure of haemoglobin
- 2 alpha + 2 beta subunits
- Each subunit has 1 haem group containing an iron ion to which O2 binds to
Describe the shape of the haemoglobin-oxygen dissociation curve
Sigmoidal shape
First O2 is hard to bind
Easier to bind subsequent O2
Plateaus after 4 bind
What is the cooperatively concept of haemoglobin?
- once the first O2 has bound, a small change in the structure of Hb is made
- this change makes it easier for O2 to bind
- it is hard to bind the first O2 molecule but subsequent binding is progressively easier
- as O2 dissociates, changes in the shape prompts remaining O2 to be released quickly
How do you calculate the total O2 content of the blood?
Amount chemically bound + amount dissolved
How much O2 is bound to Hb in arterial blood leaving the lungs?
8.8mmol/L
Normal conc of Hb
2.2mmol/L
Define hypoxaemia
Low pO2 in arterial blood
Define hypoxia
Low O2 levels in body/tissues
How does severe anaemia change oxygen in the blood?
O2 sats normal
pO2 normal
Reduced O2 content (less Hb)
How low can tissue pO2 get?
Cannot fall below 3kPa
What does a shift in the haemoglobin oxygen dissociation curve mean?
- R ightward shift: R educed affinity > O2 unloads
- leftward shift: increased affinity > O2 held on tighter
What causes a rightward shift of the Hb-O2 curve?
How does this affect affinity?
Increased H+
Increased 2,3-DPG
Increased temp
reduces affinity
What causes a leftward shift in the Hb-O2 curve?
How does this affect affinity?
Decreased H+
Decreased 2,3-DPG
Decreased temp
CO
increases affinity
Why are metabolically active tissues at a lower pH?
- decreased pH shifts curve to right
- reduced pH promotes the T state of Hb
- lower affinity for O2
- O2 unloads
What affect does 2,3 DPG have on affinity of Hb for O2?
Decrease affinity
Rightwards shift
What is the Bohr effect?
The rightwards shift of the Hb-O2 dissociation curve when H+ or CO2 bind to Hb causing the release of O2
What is the haldane effect?
CO2 binding to Hb is higher with deoxygenated than oxygenated Hb
What is the mechanism of the haldane effect?
Deoxygenated Hb has higher affinity for CO2
Due to allosteric modulation of CO2 binding sites
What affect does carbon monoxide have on arterial pO2?
No affect
Dissolved levels of O2 remains the same
Clinical presentation of carbon monoxide poisoning
- Headache
- Nausea + vomiting
- Slurred speech
- Confusion
Who are at an increased risk of CO poisoning?
Children
How does carbon monoxide poisoning occur?
- CO binds to Hb
- Hb has 200x affinity for CO than O2
- reduced O2 transport
- increases affinity of unaffected subunits for O2
- leftward shift in Hb-O2 dissociation curve
- reduced O2 release to peripheral tissues
What is peripheral cyanosis due to?
Poor local circulation
What is central cyanosis due to?
Poorly saturated blood in systemic circulation
Ways to measure O2 saturation
Pulse oximetry
Arterial blood gas
Outline pulse oximetry
Non invasive way to measure percentage of Hb saturated with O2
How does a pulse oximetry work?
Detects difference in absorption of light between oxygenated + deoxygenated Hb
Downfalls of pulse oximetry
- Less accurate in darker coloured skin
- Less accurate if patient has poor circulation to peripheries
- affect by severe anaemia
- may not be able to differentiate between Hb-CO or mathaemoglobins
Outline an arterial blood gas
- Invasive
- Sample of blood taken form artery (often radial)
- analysed by machine to assess pO2, pCO2, bicarbonate + pH
What cases have high levels of 2,3-DPG?
Anaemia
High altitude
Difference between partial pressure of O2, Hb saturation of O2 + content of O2
- partial pressure: pressure exerted by O2(in kPa)
- Hb saturation: how saturated a Hb molecule is with O2 (as a %)
- content of O2: conc of O2 (in mmol/L)
Where do you take blood from in an arterial blood gas?
Radial artery
If you cant take blood from the radial artery for a ABG, where else could you take blood from?
Why is this not as good of an option?
- Brachial or femoral artery
- They are less superficial and have delicate structures in close proximity which you do not want to damage such as nerves.
