Lecture 5: Oxygen in the blood Flashcards
Whats the solubility coefficient of oxygen?
0.01 mmol/L/kPa (at 37 degrees)
Not very soluble in water
How much oxygen is dissolved in arterial blood?
0.13 mmol/L
At PaO2 of 13.3 kPa
(13.3 x 0.01)
Why can’t we rely on dissolved oxygen only?
Cardiac output would have to be really high, as we need 12 mmol of oxygen per minute so we would need to pump 92 L/min around the body
What is the average cardiac output for a healthy person?
4.5-5 L
Why does oxygen binding need to be reversible?
- oxygen must be able to associate with the carrier at the lungs
- dissociate from carrier at tissues to supply them with oxygen
What is the structure of Hb?
Tetramer, which binds 4 oxygen molecules
- haem molecule on each chain
- haem molecule has iron at its centre
- molecular oxygen combines reversibly with iron
What are the 2 states of Hb?
T state (tense): low affinity for oxygen, difficult for oxygen to bind R state (relaxed): high affinity for oxygen, easy for oxygen to bind
What determines whether Hb is in the T or R state?
Partial pressure of oxygen
- when pO2 is low, Hb is tense
- when pO2 is high, Hb is relaxed
What is positive coorperativity?
As each oxygen binds, Hb becomes more relaxed and binding of the next oxygen molecule is easier
(opposite is true: oxygen leaving Hb makes it more tense and therefore more likely that another oxygen will dissociate)
Does the oxygen dissociation curve tell us how much Hb a person has?
It does not tell us about the amount of Hb a person has
Does saturation of Hb change in anaemia?
No, they are fully saturated, there is just less Hb.
-but O2 content will be lower so not enough oxygen present in the blood
Describe the oxygen dissociation graph:
Y axis: -Hb saturation % -oxygen conc mL/100mL X axis: -kPa (pO2)
Sigmoidal curve:
- initially, there is a shallow relationship with b/w the pO2 and binding. It is very hard for oxygen to bind to the Hb
- as some oxygen binds, this facilitates further binding (steep part of curve)
- curve steepens as pO2 rises
- curve flattens as saturation is reached
What information can we get from an oxygen dissociation curve?
- see how much oxygen will be bound or given up when moving from one partial pressure to another
- work out the difference in % saturations between 2 pO2 values
- p50: PP of oxygen where Hb is 50% saturated
Give some important partial pressures of oxygen on the Hb dissociation curve:
-Hb saturated above 9-10 kPa
-virtually unsaturated below 1 kPa
-half saturated at 3.5-4 kPa
(saturation changes greatly over a narrow range)
What are the different zones on the Hb dissociation curve?
Zone A: wide range of safe partial pressure of oxygen
Zone B: even small drops in partial pressure of oxygen are dangerous- tissue hypoxia as delivery to tissues is compromised
How do we define respiratory failure in terms of oxygen?
When the arterial partial pressure of oxygen falls below 8 kPa
What is the average Hb concentration for oxygen?
2.2 kPa
Each Hb carries 4 molecules
=8.8 mmol/L (average)
What does tissue pO2 depend on?
How metabolically active the tissue is
- usually 5 kPa
- Hb saturation drops to 65% (given off 35% of oxygen)
- at this low saturation, Hb is tense and doesn’t want to bind oxygen
What is spare capacity of Hb?
In mixed venous blood over half the oxygen is still bound: 6kPa
(in case we find ourselves in a situation where we need to extract more oxygen)
How low can tissue pO2 get?
-capillaries supplying the tissues can’t fall below 3kPa
-need a partial pressure gradient
-higher the capillary density, the lower the pO2 can fall as it doesn’t have far to diffuse
(very metabolically active tissues have a higher capillary density/high altitudes will have a higher capilarry density)
What is the Bohr effect?
-acidic conditions shift dissociation curve to the right, so it has less affinity for oxygen: promotes the T state (metabolically active tissues have a low pH)
How does the temperature effect the oxygen dissociation curve?
High temp shift curve to the right, so there is less affinity for oxygen
-as metabolically active tissues have high temperatures
What is the most metabolically active tissue?
The heart
-it removes the most oxygen from the blood
Where does maximum unloading of oxygen occur?`
-in tissues where pO2 can fall to a low level
-conditions where increased metabolic activity results in a more acidic environment and higher temp
-in tissues with a high capillary density where partial pressure gradient of oxygen can be very small
Here up to 70% of bound oxygen can be given up (on average 30% of oxygen is extracted from blood)
