19) Blood gas transport

Question 1

How is oxygen transported in the blood to the tissues?

Answer 1

• 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

Question 2

How is carbon dioxide transported out of the body?

Answer 2

• 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

Question 3

What is plasma?

Answer 3

• The aqueous portion of the blood

Question 4

Why is oxygen in circulation mainly bound to Hb?

Answer 4

• 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

Question 5

What are the different ways of quantifying oxygen in the blood?

Answer 5

• 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

Question 6

What is the Oxygen-Haemoglobin Dissociation Curve?

Answer 6

• 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

Question 7

Why does an Oxygen-Haemoglobin dissociation curve have a sigmoidal shape?

Answer 7

• 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

Question 8

What can affect the shape of an Oxygen-Haemoglobin Dissociation curve?

Answer 8

• 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.

Question 9

What are the different situations that cause the Oxygen-Haemoglobin Dissociation curve to shift to the left?

Answer 9

• Decrease in CO2
• Increase in pH (alkalosis)
• Decrease in 2,3-DPG
• Decrease in temperature

Question 10

What are the different situations that cause the Oxygen-Haemoglobin Dissociation curve to shift to the right?

Answer 10

• Increase in CO2
• Decrease in pH (acidosis)
• Increase in 2,3-DPG
• Increase in temperature

Question 11

What is 2,3-DPG?

Answer 11

• 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

Question 12

What is the “Bohr effect”?

Answer 12

• It is the effect of CO2 and pH on Hb-O2 affinity

Question 13

What is the purpose of changing the Hb-O2 affinities in the body?

Answer 13

• 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)

Question 14

How are the Hb-O2 affinities altered in the lungs to suit the local environment?

Answer 14

• 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

Question 15

How are the Hb-O2 affinities altered in resting tissues to suit the local environment?

Answer 15

• 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.

Question 16

How are the Hb-O2 affinities altered in hard working tissues to suit the local environment?

Answer 16

• 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

Question 17

What are the colours of the different types of blood?

Answer 17

• Oxyhaemoglobin (Hb-O2) is red
• Deoxyhaemoglobin (Hb) is blue
• The colour of blood is determined by the relative concentrations of the two

Question 18

What is cyanosis?

Answer 18

• The purple discolouration of the skin and tissue that occurs when the concentration of deoxyhaemoglobin in blood becomes excessive

Question 19

What are the different types of cyanosis?

Answer 19

• 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)

Question 20

Why is cyanosis harder to spot in patients with low RBC density?

Answer 20

• 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

Question 21

What is tissue hypoxia?

Answer 21

• When the blood is not able to supply tissues with adequate oxygen to meet demands.
• This can occur despite adequate ventilation and perfusion

Question 22

What are different clinical problems that can affect Hb-O2 transport?

Answer 22

• Anaemia

- CO poisoning

Question 23

What is anaemia?

Answer 23

• A condition in which there is an insufficient amount of RBCs/ haemoglobin

Question 24

What are the different causes of anemia?

Answer 24

• Iron deficiency (causing decreased production of RBCs)

- Haemorrhage (causing the increased loss of RBCs)

Question 25

What are typical symptoms of anaemia?

Answer 25

• Pale skin
• Tiredness
• Pale mucous membranes

Question 26

How do we spot CO poisoning?

Answer 26

• 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

Question 27

Why is CO poisoning a problem?

Answer 27

• 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)

Question 28

What are the symptoms of CO poisoning?

Answer 28

• Headaches
• Nausea
• Dizziness
• Breathlessness
• Collapse
• Loss of consciousness
• Death

Question 29

How does the transport of CO2 differ from O2?

Answer 29

• 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

Question 30

What is ‘The Haldane effect’?

Answer 30

• 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

Question 31

How is the build of CO2 in the tissues prevented?

Answer 31

• 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

Question 32

How does the Haldane effect cause a major increase in CO2 release in the blood?

Answer 32

• 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

Question 33

How does the Haldane effect cause acidaemia in tissue?

Answer 33

• 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

Question 34

What is the difference between the Haldane effect and the Bohr effect?

Answer 34

• 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)

Question 35

How does an increase in CO2 accumulation cause acidosis?

Answer 35

• 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

Question 36

How does excessive removal of CO2 cause alkalosis?

Answer 36

• 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

Question 37

How does the lungs control acid-base balance?

Answer 37

• 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