4 - O2/CO2 In Blood

Question 1

With regards to gas exchange, what is the primary aim of blood?

Answer 1

Blood with oxygen to the tissues. Blood with carbon dioxide away from the tissues.

Question 2

What is the normal pO2 in alveolar air? What is the solubility of oxygen at this pressure?

Answer 2

13.3kPa… 0.13mmol/L - not very soluble (body needs 12mmol/L).

Question 3

What are a few respiratory pigments that bind oxygen?

Answer 3

Haemoglobin - reversible. Myoglobin - irreversible. These process of combination are known as oxygenation.

Question 4

What are dissociation curves?

Answer 4

Plot of amount percentage of amount of oxygen bound (independent of pigment conc.) vs pO2 (kPa). It is a measure of the reversibility of oxygen binding. Total O2 content: O2 bound and dissolved.

Question 5

Is oxygen binding saturable?

Answer 5

Yes - chemical binding saturates about a given pO2.

Question 6

How do dissociation curves help us?

Answer 6

Tells you how much oxygen will be bound or given up when there are changes in pO2.

Question 7

Describe the molecular properties of haemoglobin.

Answer 7

It is a protein with a quaternary structure. It is a tetramer - 2 alpha & 2 beta subunits - each subunit having one haem and globin.

Question 8

What are the two states of haemoglobin?

Answer 8

Relaxed state - high affinity. Tense state - low affinity.

Question 9

Describe oxygen binding (to haemoglobin). What pO2s will give fully saturated, unsaturated and 50% saturated Hb?

Answer 9

It is tense at the start. Hb become more relaxed as the first molecule of O2 binds to it. (… first pint is difficult, gets easier as you carry on). It is a highly reversible reaction. [Hb: fully saturated - pO2 = 8.5kPa; unsaturated pO2 - ~1kPa; 50% saturation = 3.5-4kPa].

Question 10

What is the normal alveolar pO2? Will blood leavling the lungs be well saturated?

Answer 10

13.3kPa - therefore blood should be 100% saturated with O2 when leaving the lungs.

Question 11

What is the concentration of O2 in blood?

Answer 11

Bound: 2.2mmol/L of Hb, 4 O2 bound to 1 Hb, therefore O2 = 8.8mmol/L

Question 12

What is the pO2 and therefore concentration of O2 in the tissues?

Answer 12

~5kPa = ~65% saturation, therefore 0.35 * 8.8 has dissociated (~3mmol/L). 8.8 - 3 = 5.8mmol/L.

Question 13

Why doesn’t tissue pO2 fall further?

Answer 13

It must be high enough to still drive oxygen out to cells - cannot fall below 3kPa in most tissues. The higher the capillary density (contrast skeletal muscle and cardiac muscle) the lower it can fall.

Question 14

What is the Bohr Shift?

Answer 14

pH influences dissociation. The more acidic the more tense it is, (Lower the pH, the more to the right it goes). The more alkaline the more relaxed it is.

Question 15

What are the benefits of the pH in the tissues being lower? Does temperature affect dissociation?

Answer 15

Bohr Shift - more acidic, more tense (dissociation curve to the right) and reduced affinity. Therefore more likely to dissociate and more oxygen is given up. Higher temperature mimics the effect of the lower pH (more acidic).

Question 16

So then in tissues where pO2 can fall very low, what will the conditions be like there?

Answer 16

Acidic and warm. Up to 70% of oxygen can be given up.

Question 17

How much O2 is given up by arterial blood to tissues normally? Can this change?

Answer 17

27% but will increase upon exercise, where an ‘oxygen reserve’ can be used.

Question 18

How does CO2 differ from O2?

Answer 18

It is far more soluble and also more reactive. This means there is 3x more CO2 in the blood than O2 (22mmol/L in arterial blood).

Question 19

Why is there far more CO2 than O2 in the blood?

Answer 19

To maintain the acid base balance.

Question 20

What is the normal pCO2 in alveolar air? What is the solubility of carbon dioxide at this pressure?

Answer 20

5.3kPa - 1.2mmol/L (almost 10x more than O2).

Question 21

In plasma, how does CO2 react with water?

Answer 21

CO2 + H2O H+ + HCO3- Catalysed by carbonic anhydrase, rate of reaction depends on concentration of reactants and products N.B. pH = -log[H+]

Question 22

What is [HCO3-]? Where does it come from?

Answer 22

25mmol/L. NaHCO3 NOT CO2.

Question 23

Considering the dissolved [CO2] and [HCO3-], which way will the equilibrium tend towards: CO2 + H2O H+ + HCO3-? How will this affect the pH?

Answer 23

[CO2] = 1.2mmol/L; [HCO3-] = 25mmol/L therefore equilibrium tends towards the right. This will mean the pH of the plasma is slightly alkaline (~7.4).

Question 24

What does dissolved CO2 depend on?

Answer 24

pCO2 - if it rises pH will fall, due to the equilibrium tending towards no net movement (more H+) and vice-versa.

Question 25

What is the Henderson Hasselbalch equation?

Answer 25

pH = pK + log ([HCO3-] / (pCO2 x 0.23)) pK = 6.1; pCO2 x 0.23 ~ [CO2]

Question 26

What is the normal ratio between [HCO3-] and [CO2]? What would this make pH?

Answer 26

20x more HCO3- than CO2. Therefore pH = pK + log 20, 6.1 + 1.3 = 7.4

Question 27

Where does all this HCO3- come from?

Answer 27

In RBCs H+ may bind to Hb-. This means we are affecting the concentration of one of the products of the reaction (CO2 + H2O H+ + HCO3-). Therefore dissolved CO2 and H20 will increase their rate of reaction so that more H+ is produced. HCO3- will also be produced in the process. These reactions take place in RBCs.

Question 28

Having been produced in RBCs, how does HCO3- enter the plasma?

Answer 28

It leaves in exchange for inward movement of Cl-.

Question 29

What is the ratio of [HCO3-] and [CO2] dependent on?

Answer 29

Reaction of CO2 in RBC (producing HCO3-) Reaction of CO2 in plasma

Question 30

Does plasma [HCO3-] change much with pCO2?

Answer 30

No - it is more dependent on how much H+ binds to Hb-. Also the kidneys control excretion/reabsorption of Na+ - much more important.

Question 31

How is excess production of H+ managed? Why might excess H+ arise?

Answer 31

HCO3- acts as a buffer, reacting with it producing CO2 which is breathed out. Lactic acid, ketoacidosis, sulphuric acid?!

Question 32

What is the relationship between alveolar, arterial and dissolved concentrations of CO2? How is pH related?

Answer 32

Alveolar pCO2 determines Arterial pCO2 determines dissolved CO2 determines pH.

Question 33

What does buffering of H+ by Hb depend on? What will happen in venous blood? N.B. CO2 + H2O H+ + HCO3-

Answer 33

Oxygenation of Hb - when Hb has lost O2 it binds H+ with a greater affinity (more space for H+) = more HCO3- which is exported to the plasma.

Question 34

pCO2 is higher in venous blood, most of this will dissolve in the plasma. True or False? Will pH change? N.B. CO2 + H2O H+ + HCO3-

Answer 34

False - pCO2 is higher in venous blood so more CO2 SHOULD dissolve BUT more H+ is taken up by Hb- shifting equilibrium of CO2 + H2O H+ + HCO3- to the right. A lot of the CO2 is used to produce H+ and HCO3- so only a little more CO2 dissolves. pH will not change much because HCO3- and pCO2 increase ~proportionally.

Question 35

What will happen to Hb, O2 and CO2 when venous blood reaches the lungs?

Answer 35

O2 is taken up and protons are given up (reduced affinity / ‘space’). H+ will react with HCO3- forming H2O and CO2 - which can be breathed out.

Question 36

What are carbamino compounds? Where are they formed?

Answer 36

Where CO2 binds directly to proteins (which contain the NH2 group). This will contribute to CO2 transport but not acid base balance. Formed in the blood, slightly more in venous blood as there is a higher pCO2.

Question 37

How do we calculate the [CO2] in arterial blood?

Answer 37

Plasma: CO2 dissolves 0.7mmol/L; HCO3- 15.2mmol/L of blood Cells: CO2 dissolves 0.3mmol/L; HC03- 4.3mmol/L of blood Carbaminos: 1mmol/L in arterial blood Therefore 21.5mmol/L total CO2 in blood.

Question 38

How do we calculate the [CO2] in venous blood?

Answer 38

Plasma: CO2 dissolves 0.8mmol/L; HCO3- 16.3mmol/L of blood Cells: CO2 dissolves 0.4mmol/L; HC03- 4.8mmol/L of blood Carbaminos: 1.2mmol/L in arterial blood Therefore 23.5mmol/L total CO2 in blood.

Question 39

What is the concentration of transported CO2? What happens to this CO2?

Answer 39

23.5 - 21.5 = 2mmol/L. It is breathed out as waste.

Question 40

Of the 2mmol/L of transported CO2 in what forms does it travel?

Answer 40

80% HCO3-, 11% Carbamino compounds; 9% dissolved CO2.