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
Q
What are typical symptoms of anaemia?
A
- Pale skin
- Tiredness
- Pale mucous membranes
26
Q
How do we spot CO poisoning?
A
- 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
Q
Why is CO poisoning a problem?
A
- 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
Q
What are the symptoms of CO poisoning?
A
- Headaches
- Nausea
- Dizziness
- Breathlessness
- Collapse
- Loss of consciousness
- Death
29
Q
How does the transport of CO2 differ from O2?
A
- 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
Q
What is ‘The Haldane effect’?
A
- 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
Q
How is the build of CO2 in the tissues prevented?
A
- 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
Q
How does the Haldane effect cause a major increase in CO2 release in the blood?
A
- 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
Q
How does the Haldane effect cause acidaemia in tissue?
A
- 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
Q
What is the difference between the Haldane effect and the Bohr effect?
A
- 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
Q
How does an increase in CO2 accumulation cause acidosis?
A
- 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
Q
How does excessive removal of CO2 cause alkalosis?
A
- 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
Q
How does the lungs control acid-base balance?
A
- 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
