19) Blood gas transport Flashcards
1
Q
How is oxygen transported in the blood to the tissues?
A
- First air is inhaled and travels down the airway tract
- At the alveoli it dissolves in the blood plasma (aqueous portion of the blood)
- From here it diffuses into RBCs where it binds to Hb
- In circulation a vast amount of the O2 is found bound to Hb (98%) and only very little is dissolved in the plasma (2%)
- At the tissues they dissolve back into the plasma and then diffuse into respiring tissue
2
Q
How is carbon dioxide transported out of the body?
A
- CO2 is first produced by respiring tissue and dissolves into the blood plasma
- It is converted into a different form where it is either bound to Hb at a different binding site from O2 binding site or they can be transported as HCO3- (bicarbonate)
- In circulation very tiny amounts are found dissolved in the plasma (7%) whereas the majority are found bound to Hb (23%) or as HCO3- (70%)
- At the lungs they are dissolved into the plasma as CO2 molecules where they can diffuse into the lungs and be exchanged for O2
3
Q
What is plasma?
A
- The aqueous portion of the blood
4
Q
Why is oxygen in circulation mainly bound to Hb?
A
- Oxygen has a very low solubility in blood plasma.
- In order to supply the tissues with oxygen from plasma alone a very high amount of alveolar PO2
- Hb overcomes this problem as it increases the carrying capacity of oxygen in the blood causing it to be more concentrated
- This means more oxygen can be carried to gas exchange surfaces which can be released into respiring tissues
5
Q
What are the different ways of quantifying oxygen in the blood?
A
- O2 partial pressure (PaO2): How much blood there is in the plasma at equilibrium (in kPa)
- Total O2 content (CaO2): The volume of oxygen carried in each unit of blood including O2 in the plasma and bound to haemoglobin (expressed as mL of O2 per L of blood)
- O2 saturation: The % of total haemoglobin binding sites that are occupied by oxygen
6
Q
What is the Oxygen-Haemoglobin Dissociation Curve?
A
- A graph which shows the relationship between O2 conc. (as O2 content), partial pressure (in plasma) and saturation (as a %) in the blood
- In other words it shows the relationship of oxygen haemoglobin binding
- It has a sigmoidal (S) shape
- As PaO2 in arterial plasma increases there is a higher O2 content and a higher saturation of O2.
- This is because more oxygen is bound to the haemoglobin
7
Q
Why does an Oxygen-Haemoglobin dissociation curve have a sigmoidal shape?
A
- Initially there is a steep increase
- This is due to the cooperative binding of O2 to Hb
- This means that after the first O2 binds it becomes easier for the next O2 molecule to bind
- The reason for this is due to structural changes of Hb brought about by O2 binding
- Eventually the graph plateaus because we run out of Hb that is free to bind as saturation of O2 bound to Hb is high
- Hence it becomes harder to bind to free Hb binding sites
8
Q
What can affect the shape of an Oxygen-Haemoglobin Dissociation curve?
A
- The shape of the curve can change based on the affinity of Hb for oxygen
- If Hb has a stronger affinity for oxygen then the curve shifts to the left and hence less PaO2 is needed to get the same level of oxygen content/ saturation. At lungs we take in more oxygen and in respiring tissue we give off less oxygen
- If Hb affinity for oxygen decreases the curve shifts to the right so more PaO2 is needed to get the same level of oxygen content/saturation. At lungs we take in less oxygen and in respiring tissues we give off more oxygen.
9
Q
What are the different situations that cause the Oxygen-Haemoglobin Dissociation curve to shift to the left?
A
- Decrease in CO2
- Increase in pH (alkalosis)
- Decrease in 2,3-DPG
- Decrease in temperature
10
Q
What are the different situations that cause the Oxygen-Haemoglobin Dissociation curve to shift to the right?
A
- Increase in CO2
- Decrease in pH (acidosis)
- Increase in 2,3-DPG
- Increase in temperature
11
Q
What is 2,3-DPG?
A
- 2,3 Diphosphoglyceric Acid
- It is a product of glycolysis in anaerobic respiration
- So the more anaerobic respiration that occurs the more 2,3-DPG will be released
12
Q
What is the “Bohr effect”?
A
- It is the effect of CO2 and pH on Hb-O2 affinity
13
Q
What is the purpose of changing the Hb-O2 affinities in the body?
A
- Hb-O2 affinity changes depending on the local environment which allows O2 delivery to be coupled to demand
- This means that Hb will give off more oxygen where oxygen demand is higher (e.g. at respiring tissue)
14
Q
How are the Hb-O2 affinities altered in the lungs to suit the local environment?
A
- At the lungs the blood needs to take in oxygen.
- Here we find high levels of PO2
- We also find low levels of PCO2 which means there is a high pH
- These conditions cause the curve to shift to the left.
- Hence Hb-O2 affinity increases and so we end with a higher level of saturation for the same PO2
15
Q
How are the Hb-O2 affinities altered in resting tissues to suit the local environment?
A
- In resting tissues there is a low PO2, as they are still respiring but not as much as hard working tissue.
- This means they have a smaller demand for oxygen compared to hard working tissue.
- They have medium/normal levels of PCO2 (so a normal pH).
- This means the curve hasn’t shifted and so affinity is not affected.
- As a result there is a slight decrease in O2 saturation as Hb gives off O2 to meet the smaller demands.
16
Q
How are the Hb-O2 affinities altered in hard working tissues to suit the local environment?
A
- In working tissues there is major decrease in PO2 leading as a lot of anaerobic respiration takes place
- This anaerobic respiration produces lactic acid (decreasing pH), CO2 and 2,3-DPG.
- Hence there is a high oxygen demand due to the hypoxia and so the curve shifts to the right due to the conditions
- This means there is a lower Hb-O2 affinity and so a lower saturation of O2 as more oxygen is given off to tissue from the Hb
17
Q
What are the colours of the different types of blood?
A
- Oxyhaemoglobin (Hb-O2) is red
- Deoxyhaemoglobin (Hb) is blue
- The colour of blood is determined by the relative concentrations of the two
18
Q
What is cyanosis?
A
- The purple discolouration of the skin and tissue that occurs when the concentration of deoxyhaemoglobin in blood becomes excessive
19
Q
What are the different types of cyanosis?
A
- Central cyanosis: The bluish discolouration of core regions of the body, mucous membranes and extremities. It is caused by an overall inadequate oxygenation of blood (e.g. during V/Q mismatch)
- Peripheral cyanosis: Bluish discolouration confined to extremities (e.g. fingers). This is caused by an inadequate supply of O2 to these extremities (e.g. small vessel circulation issues)
20
Q
Why is cyanosis harder to spot in patients with low RBC density?
A
- This is because overall the concentration of deoxyhaemoglobin will be low
- This means that discolouration will be less visible and hence is harder to spot
21
Q
What is tissue hypoxia?
A
- When the blood is not able to supply tissues with adequate oxygen to meet demands.
- This can occur despite adequate ventilation and perfusion
22
Q
What are different clinical problems that can affect Hb-O2 transport?
A
- Anaemia
- CO poisoning
23
Q
What is anaemia?
A
- A condition in which there is an insufficient amount of RBCs/ haemoglobin
24
Q
What are the different causes of anemia?
A
- Iron deficiency (causing decreased production of RBCs)
- Haemorrhage (causing the increased loss of RBCs)
25
What are typical symptoms of anaemia?
- Pale skin
- Tiredness
- Pale mucous membranes
26
How do we spot CO poisoning?
- When CO binds to Hb Carboxyhaemoglobin forms which has a cherry red pigmentation
- Hence we see skin go bright cherry red when excessive carboxyhaemoglobin is in circulation
- CO poisoning can lead to hypoxia in the absence of cyanosis
27
Why is CO poisoning a problem?
- Haemoglobin has a much higher affinity for CO than O2
- Furthermore they compete for the same binding site
- Therefore at certain levels the CO will displace O2 from the Hb binding sites and so less O2 is transported around the body (decreased O2 capacity)
28
What are the symptoms of CO poisoning?
- Headaches
- Nausea
- Dizziness
- Breathlessness
- Collapse
- Loss of consciousness
- Death
29
How does the transport of CO2 differ from O2?
- CO2 binds to Hb at different sites from O2 and with decreased affinity so a lower percentage is transported in this manner
- CO2 has a higher solubility in water (i.e. in plasma) than O2 therefore a greater percentage of CO2 is transported whilst dissolved in the plasma
- CO2 reacts with H2O to form carbonic acid HCO3- which accounts for majority of the CO2 transported
30
What is 'The Haldane effect'?
- Deoxygenated (Venous) blood carries more CO2 than oxygenated (arterial) blood
- This occurs because deoxyhaemoglobin has a higher affinity to CO2 and H+ than oxyhaemoglobin
- As we oxygenate the Hb the affinity for CO2 starts to decrease and so less CO2 is carried
31
How is the build of CO2 in the tissues prevented?
- In the blood CO2 react with H2O to form H2CO3
- This H2CO3 will go on further to form bicarbonate and H+.
- This takes CO2 out of circulation and causes it to form a new product which is not bound to the Hb
- As a result the Hb can now bind to more CO2 from respiring tissue, taking them away and preventing build up
32
How does the Haldane effect cause a major increase in CO2 release in the blood?
- In circulation the conversion between the different transport methods of CO2 is in equilibrium.
- So changing the concentration of one form (e.g. CO2 dissolved in the plasma) will cause a change in the other forms (e.g. CO2 dissolved in RBCs)
- However at the lungs the deoxygenated blood comes into contact with oxygen and as a result the affinity of Hb for H+ and for CO2 decreases
- This causes CO2 disassociate and results in an increase in conc. of CO2 dissolved in RBCs
- H+ also dissociates which increases the amount of H+ present
- The increased H+ will react with HCO3- to form H2CO3 which in turn will be broken down into CO2 and H2O.
- This increases the amount of CO2 present to maintain equilibrium
- As a result there is a major increase in CO2 conc and so the plasma will force CO2 out into the lungs
33
How does the Haldane effect cause acidaemia in tissue?
- If a person has respiratory problems the CO2 from blood cannot be expelled by the lungs at a sufficient rate as there is a decreased ventilation
- However upon binding to oxygen the CO2 will be dissociated from the Hb
- This causes chronic hypercapnia as the CO2 will build up in the blood
- This means the blood is unable to take on more CO2 from the tissues as it remains saturated
- Hence CO2 builds up in the tissues (as they are not taken up) causing acidosis leading to acidaemia
34
What is the difference between the Haldane effect and the Bohr effect?
- Bohr effect: Describes the impact of CO2 on O2 transport. Binding of CO2 and H+ causes a structure change which reduces the affinity of Hb to O2. So Hb releases more O2 when CO2 and/or H+ levels are high
- Haldane effect: Describes the impact of O2 on CO2 transport. Binding of O2 to Hb causes a structural change which reduces the affinity of Hb to CO2 and H+. So deoxygenated blood carries more CO2 and is released more easily at the lungs (where O2 is highest)
35
How does an increase in CO2 accumulation cause acidosis?
- CO2+ H20 <=> H2CO3 <=> H+ + HCO3-
- If CO2 were to accumulate (e.g. hypoventilation) more H2CO3 would be formed.
- An increase in H2CO3 would mean an increased production of carbonic acid.
- This means concentration of H+ increases so pH also decrease
- This decrease in pH causes acidosis
36
How does excessive removal of CO2 cause alkalosis?
- CO2+ H20 <=> H2CO3 <=> H+ + HCO3-
- If excess removal of CO2 took place (e.g. hyperventilation) H2CO3 would be converted into CO2 to replace the CO2 that is lost.
- This means more H2CO3 would be formed from the H+ ions to replace the lost H2CO3.
- So less H+ ions will be found in the blood which means pH decreases leading to alkalosis
37
How does the lungs control acid-base balance?
- CO2+ H20 <=> H2CO3 <=> H+ + HCO3-
- An increase in CO2 causes a decrease in pH
- The lungs play a key role in regulating CO2 levels and therefore contribute to the acid-base balance
- Signs of respiratory and metabolic distress can be diagnosed and interpreted from analysis of Arterial Blood Gas (ABG) and pH
