Carriage of O2 and CO2 in the Blood Flashcards
What are the 2 main ways in which oxygen is carried in the blood?
Dissolved in the blood or combined with haemoglobin
What equation is used to determine the amount of oxygen that dissolves in the blood?
volume of oxygen = 0.0232 x pO2
0.29 = 0.0232 x 12.5
By the time the oxygen reaches the alveoli, the pO2 is 12.5 kPa
What equation is used to determine partial pressure?
partial pressure = fractional concentration x barometric pressure
What is barometric pressure in the atmosphere?
101.3 kPa
How much oxygen is dissolved in one decilitre of blood?
0.29 ml of oxygen is dissolved in each 100 ml of blood
In cold water, how does the amount of gas dissolved differ than at 37 degrees?
There is more oxygen and CO2 dissolved in cold water
More gas will dissolve at lower temperatures
What is the significance of the value 0.0232 in the equation?
Is it a constant?
It describes the solubility of oxygen
Calculations are always at 37oC so it does not change
The value changes if the calculations are worked out at a different temperature
What is oxygen saturation?
It describes how much of the haemoglobin in the blood is bound with oxygen, compared to how much haemoglobin there is in total
How is oxygen saturation calculated?
Amount of oxyhaemoglobin divided by the total amount of haemoglobin
HbO2 / (HHb + HbO2)
What do SO2 HbO2 HHb stand for?
SO2 - oxygen saturation
HbO2 - oxyhaemoglobin
HHb - deoxyhaemoglobin
What equation is used to determine how much oxygen is being carried by haemoglobin?
volume of O2 = SO2 x [Hb] x 1.39
19 = 0.97 x 14 x 1.39
What is the Hüfner constant and what does it describe?
1.39
it describes the number of ml of oxygen that each gram of Hb can carry
What is normal haemoglobin concentration?
14 grams per 100 ml
How much oxygen is carried by Hb per 100 ml of blood?
Hb can carry 19 ml of oxygen per 100 ml of blood
How many amino acids are in each of the 4 polypeptide chains that make up Hb?
141 - 146 amino acids per chain
What is the tertiary structure of Hb?
A globular structure determined by the order of amino acids in the primary structure
It forms a crevice in which the haem group will sit
What are the 2 forms in which Hb exists?
What does this describe?
Relaxed and tense forms
This describes how tightly packed the globin is within the chain
What does Hb act/look like in the relaxed state?
The globin is more spread out
The crevices containing the haem group becomes slightly wider
The oxygen can access the haem group more easily
What does Hb act/look like in the tense state?
The globin is more tightly condensed
The crevices become too small for the oxygen to fit into easily
What happens upon binding of the first oxygen molecule to Hb?
As soon as one molecule of O2 binds to the haem group, the shape of the globin chain changes
This alters how tense the adjacent globin chains are
The globin chains relax a bit more with each O2 that binds
Why does pO2 of the blood have to be high before Hb begins to be oxygenated?
The first molecule of oxygen does not bind to the Hb easily
When deoxygenated blood returns to the heart, how saturated is the Hb and why?
Hb is still 75% oxygenated
As blood circulates around the body, only 25% of O2 that is bound to Hb is actually consumed
Under normal circumstances, what part of the Hb molecule is used to carry oxygen?
Only the final binding site on each Hb molecule
This is the site that works the most efficiently
What happens as partial pressure of oxygen becomes lower?
It becomes harder to carry oxygen in the blood
What is meant by the term ‘cooperativity’?
The way in which altering the state of one part of the protein affects the other components
What causes the shape of the oxygen dissociation curve?
The cooperativity between polypeptide chains in the Hb molecule
What happens at the bottom of the oxygen dissociation curve?
It is difficult to get the oxygen to bind to Hb
The partial pressure must be increased significantly before oxygen starts to bind
What happens at the top of the oxygen dissociation curve?
All of the oxygen binding sites are occupied and the curve levels off
What does the arterial point on the oxygen dissociation curve show?
The saturation of normal arterial blood
Saturation is around 97% and partial pressure of oxygen is 12.5 - 13 kPa
What does the venous point on the oxygen dissociation curve show?
The saturation of normal venous blood
Saturation is around 75% and pO2 is 6.3 kPa
What is shown by P50 on the oxygen dissociation curve?
This is the point on the curve where Hb is 50% saturated
pO2 is normally 3.5 kPa
What is the P50 point on the oxygen dissociation curve used for?
To see whether a curve is normal or if it has been shifted
What 3 factors can cause the oxygen dissociation curve to shift?
- temperature
- pH change (Bohr effect)
- 2,3 diphosphoglycerate in the RBC
What is 2,3 diphosphoglycerate and how is it produced?
It is produced by anaerobic metabolism
It is a signalling mechanism to RBCs to show that there is not enough oxygen being delivered
What happens as the oxygen dissociation curve shifts to the right?
The further to the right the curve moves, the more easily the Hb will give up oxygen
It will be harder for Hb to take up oxygen in the lung
What causes the oxygen dissociation curve to shift to the left?
increased pH
decreased temperature
decreased 2,3 DPG
What causes the oxygen dissociation curve to shift to the right?
decreased pH
increased temperature
increased 2,3 DPG
What are the 4 ways in which a Hb molecule may be abnormal?
- absent globin chain
- defective globin chain
- defective iron atom
- wrong ligand
How can an abnormal Hb have an absent globin chain?
The globin chains may be absent from an individual’s genes
What causes thalassaemia?
The gene for one or more of the globin chains is very abnormal
The gene is so abnormal that none on the globin chain is produced
What causes alpha thalassaemia?
The alpha chain of Hb is not produced
What is significant about alpha thalassaemia?
Both foetal and adult Hb contain alpha subunits
The foetus often dies before it reaches full development
What is the consequence of beta thalassaemia?
It is less severe than alpha
Patients will keep foetal Hb for their whole life
What causes sickle cell disease?
A single amino acid substitution on the beta globin chain which interferes with the interactions of this globin chain with the other subunits
What happens when Hb becomes deoxygenated in sickle cell disease?
The Hb molecules in the RBCs begin to stick together
This changes the shape of the RBC
When does sickling occur in sickle cell disease?
What is significant about this?
At low pO2
this is usually when patients develop infections, become hypoxic or cold
How may an iron atom become defective in Hb?
Fe2+ is oxidised to Fe3+ which does not carry oxygen
What happens when Hb contains Fe3+?
What % of Hb in the blood has Fe3+ instead of Fe2+?
This creates methaemoglobin which doesn’t carry oxygen
0.5%
How is methaemoglobin usually produced?
Usually drug-induced
Drugs used to treat leprosy, local anaesthetics
How many times more strongly does CO bind to Hb compared to O2?
CO binds to Hb 250 times more strongly than oxygen
What happens when CO binds to Hb instead of oxygen?
The Hb carrying CO is no longer available to carry oxygen
What are the main sources of carbon monoxide?
Smoking and house fires
What is a buffer?
A solution that can minimise changes in the free H+ concentration and therefore in pH
How is pH calculated?
pH = -log [H+]
What does a buffer consist of?
A weak acid and its base in equilibrium
What is the general buffer equation and the equation for carbonic acid?
acid H+ + base
H2CO3 H+ + HCO3
What happens if H+ ions are added and removed to the buffer system?
When H+ is added, the equation shifts to the left to remove some of the added ions
When H+ is removed, the equation shifts to the right as some acid dissociates to increase H+ concentration
What is the role of a buffer?
It minimises changes in pH
It does NOT correct the pH and make it normal
What are the 4 buffer systems in extracellular fluid?
What is their capacity (mmol H+ per L)?
bicarbonate - 18
plasma protein - 1.7
haemoglobin - 8
phosphate - 0.3
What is the total buffering capacity of the blood?
28 mmol H+ per L
How do proteins act as buffers?
They have carboxyl and amino groups at the end of a chain, which act as buffers
They also have side chains which contain buffers
What is the problem with proteins as buffers?
Protein molecules are large which means there are not many protein molecules in the blood
How does Hb act as an effective buffer?
It has more side chains with buffering ability than any other protein
What is the main buffer of intracellular fluid?
phosphate
What are the limits of blood pH and what H+ concentration does this correspond to?
Blood pH is tightly controlled between 7.35 and 7.45
[H+] of 35 - 45 mmol/L
Why must blood pH be tightly controlled?
Most biological functions depend on the activity of proteins
The structure of proteins is very pH dependent
What are the 3 ways in which CO2 can be carried in the blood?
- dissolved in the blood
- carbamino compounds
- carbonic acid/bicarbonate
What determines how much CO2 can dissolve directly in the blood?
It is dependent on the pCO2 of the blood and how soluble CO2 is at 37oC
At 37oC, how much CO2 is dissolved in the blood?
3 ml CO2 dissolved per 100 ml of blood
This is 10 x the amount of oxygen dissolved
How does a carbamino compound form?
The carbon dioxide molecule is attached to an amino group on an amino acid
Where are amino groups found within proteins?
The N-terminal group of all proteins and the side chains of lysine and arginine
How does Hb form a carbamino compound?
Hb has lots of amino acids with NH2 side chains where CO2 can bind
How much CO2 is carried in the blood in the form of carbamino compounds?
4 ml of CO2 per decilitre of blood
How much carbon dioxide is carried in the blood in the form of bicarbonate ions?
45 ml of CO2 per decilitre of blood
