Blood gases

Question 1

How do partial pressures change?

Answer 1

There is little change in PO2 of 13kPa and PCO2 of 5kPa as they go through the systemic arteries - these are conjugate arteries for transport.
There is gas exchange in the tissues down a partial pressure gradient.
O2 then falls to 5kPa in venous blood.
CO2 then increases to 6kPa in venous blood.

Question 2

How is metabolism measured?

Answer 2

In a cell, how much ATP is used per minute.
For a cell to use the ATP, it must generate ATP.
ATP is generated by oxidative phosphorylation, which uses oxygen, so metabolism can be measured by VO2 - O2 consumption in ml/min, and VO2 production.

Question 3

How is metabolism supported?

Answer 3

A certain amount of O2 must be delivered per minute, and a certain amount of CO2 transported out per minute.
The diffusion distance must be short.
CO2 that is produced in the TCA cycle needs to easily diffuse through the membrane into the capillaries.

Question 4

What are the determinants of how much gas is transported?

Answer 4

How much is contained in a litre of blood - both dissolved and bound to carrier molecules.
Modulation of gas content in blood - using the Bohr and Haldane effects.
How much blood is transported, using cardiac output.

Question 5

How is partial pressure affected by gas transport?

Answer 5

Gas that is transported in solution - dissolved - contributes to partial pressure.
When the gas is bound to carrier molecules such as haemoglobin or albumin, it does not contribute to the partial pressure.

Question 6

What is the proportionality constant?

Answer 6

The solubility of gas.
The amount of gas dissolved in solution is proportional to its partial pressure.
The amount dissolved = partial pressure x solubility coefficient.

Question 7

What is the solubility of gases?

Answer 7

CO2 solubility is 20x more than O2.
For PO2 of 13kPa, the amount dissolved is 3ml.
If there was half of PO2, there would be half dissolved - it is a linear relationship.
The PCO2 is less than PO2, but it is more soluble, so there is more ml/L - 25ml/L.

Question 8

What does the solubility of O2 look like on a graph?

Answer 8

See picture.

Question 9

What does the solubility of O2 show about O2 transport?

Answer 9

Resting O2 consumption is 250ml/min.
If there is 3ml/L O2 dissolved, and cardiac output of 5L/min, then only 15ml /min of O2 is delivered.
150ml of O2 is needed to support metabolism at rest, so there are other ways of transporting O2 other than dissolved in solution.

Question 10

What is the structure of haemoglobin?

Answer 10

4 haem chains, each with a peptide globin chain associated.
Each moiety will bind an O2 molecule.
There are 2 a chains and 2 b chains.

Question 11

What are the globin chains?

Answer 11

A and b chains will bind CO2 and H+.
B chains also bind 2,3-DPG.
The shape of haemoglobin gives it its functional property.
When CO2 and H+ bind, it changes the shape and therefore property.

Question 12

Why is haemoglobin isolated from the plasma?

Answer 12

Haemoglobin is packaged densely within the RBCs.
It is not in the plasma as the viscosity would be too high, and place too much load on the heart.
Protects the haemoglobin from enzymatic breakdown.
As haemoglobin is broken down, it would be excreted by the kidney, and the half-life be too short, so being in the RBCs extends its half-life.

Question 13

How is haemoglobin saturation a continuous scale?

Answer 13

Each haemoglobin can bind up to 4 O2 molecules, and the amount bound is determined by the partial pressure of O2.
But not every haemoglobin has the same amount bound, so there is a continuous variable of saturation.
If they all had the same amount bound, it would be discrete, 0, 25, 50, 75, or 100% saturated.
This means there is a continuous curve of saturation.

Question 14

What is the O2 haemoglobin saturation curve?

Answer 14

It is a cooperative curve, because of allosteric changes.
When the first O2 binds, it causes a shape change in haemoglobin.
This makes it easier for the second O2 to bind, which causes a shape change, and makes it easier for the third O2 to bind.
It is then harder for the fourth O2 to bind, because there is less space and availability.

Question 15

What does the haemoglobin saturation curve look like?

Answer 15

See picture.

Question 16

What is P50?

Answer 16

The partial pressure of O2 that gives 50% saturation of haemoglobin.
This is a measure of affinity of haemoglobin for O2.

Question 17

What are the saturation levels of haemoglobin in the body?

Answer 17

In the lungs, there is high PO2, so the haemoglobin is 100% saturated, all the binding sites are occupied.
In venous blood, haemoglobin is still 75% saturated.

Question 18

What is the physiological significance of the O2 haemoglobin dissociation curve?

Answer 18

In the lungs - the association stage, even if ventilation drops below metabolism, and PO2 drops to 9kPa, the plateau of the curve means haemoglobin saturation will not change.
In venous blood, the dissociation stage, this is the steep part of the curve.
A small drop in PO2 will cause lots of O2 to be dissociated.
So there is resilience in lungs to not affect saturation, sensitivity in tissues to PO2 to offload O2.

Question 19

What is the Bohr effect?

Answer 19

There is decreased haemoglobin affinity with increased PCO2, H+, 2,3-DPG, temperature, which all occur in a higher metabolic rate.
This causes the curve to shift right, P50 is increased, and affinity is decreased so O2 is not held onto.
If these factors decrease, the curve shifts left, P50 decreases, and O2 affinity increases so more binds.

Question 20

What does the Bohr effect look like?

Answer 20

See picture.

Question 21

What is the physiological significance of the Bohr effect?

Answer 21

It allows local matching of O2 delivery to tissue metabolic needs.
During exercise, there would be increased CO2 in the legs, which would desaturate haemoglobin of O2 to support local metabolism.
But in the gut, the haemoglobin saturation stays the same.

Question 22

What is 2,3-DPG?

Answer 22

RBCs have no mitochondria, so respire anaerobically by glycolysis.
2,3-DPG is formed as a side reaction.
It binds to the B chains, more favourably to deoxyhaemoglobin than oxyhaemoglobin.

Question 23

What is the advantage of 2,3-DPG?

Answer 23

During chronic hypoxia, e.g. at high altitude, and in blood alkalosis, it shifts the saturation curve right, which counteracts the left shift of hypocapnia (low CO2) and respiratory alkalosis.
This helps to maintain O2 delivery to the tissues.

Question 24

What is the disadvantage of 2,3-DPG?

Answer 24

It decreases under storage e.g. in blood banks, or in chronic acidosis.
This shifts the curve left and decreases the O2 availability in transfused blood.
Transfusing fresh blood is better.

Question 25

What is O2 capacity?

Answer 25

The capacity of haemoglobin to bind to O2, if it is 100% saturated. Haematocrit - the measure of red cell volume. Capacity = [Hb] x 1.39ml O2/g Hb. Capacity = 145g/L x 1.39 O2/g Hb = 202ml O2/L blood.

Question 26

What is the rate of O2 consumption?

Answer 26

The rate of oxygen consumed by the periphery = the difference between the oxygen content in arterial blood leaving the heart and in mixed venous blood returning to the heart. VO2 = (CAO2 - CVO2) x cardiac output.

Question 27

What are the values of arterial O2 content?

Answer 27

If haemoglobin is 97% saturated: [Hb] x capacity/g x SaO2 = 145g/L x 1.39ml/g x 0.97 = 195.5 ml O2/L. CAO2 (arterial blood to tissues) = 195.5 + 2.8ml O2 dissolved = 198.3ml O2/L. X by cardiac output of 5L/min = 991.5ml O2/min.

Question 28

What are the values of venous O2 content?

Answer 28

If haemoglobin 73% saturated: 145g/L x 1.39ml/g x 0.73 = 147.1 O2 ml/L. 147.1 + 1.1 O2 dissolved = 148.2 O2 ml/L. 148.2 x 5 = 735.7 O2 mil/min.

Question 29

What is the value of VO2?

Answer 29

991.1 - 735.7 = 255.4 O2 ml/min. So the metabolic need is satisfied, and the venous blood is still 75% saturated.

Question 30

How can VO2 be increased to meet the metabolic rate?

Answer 30

Cannot change the arterial content. But can take more O2 out of the blood so less is in the venous blood - the haemoglobin will be less saturated. Can also change blood flow - L/min.

Question 31

What are the mechanisms for CO2 transport?

Answer 31

Dissolved. Gas carrier molecules. CO2 + H2O <--> H2CO3 <--> H+ + HCO3- This reaction is too slow in plasma to contribute, but in RBCs carbonic anhydrase can speed up the reaction. Carbonic anhydrase would have too short a half-life in the plasma due to enzymatic breakdown.

Question 32

How is oxygen transported into tissues?

Answer 32

CO2 diffuses across the membrane into RBCs, until it reaches equilibrium. Some CO2 is dissolved. Some CO2 binds to haemoglobin, and deoxy-Hb-NHCOO- is better at transporting CO2 than O2. As O2 is delivered there is more deoxy-Hb. Some CO2 is transported using carbonic anhydrase and converting to HCO3-.

Question 33

How does the HCO3- reaction transport CO2?

Answer 33

If the HCO3- reaction happens fast, there would be a build-up of HCO3- and H+, which would reach equilibrium and stop CO2 being converted to H2CO3. So to stop the build-up, HCO3- is transported from the RBC into the plasma. There is a negative charge leaving, so to balance electric charge, Cl- is exchanged - the chloride shift. This means there is a limitless way to carry CO2.

Question 34

What are the relative contributions of CO2 transport?

Answer 34

85% carried as HCO3-. 65% out in plasma, from formed inside RBC and pumped out, not in plasma as too slow. Dissolved is 10%. 5% is bound to proteins - largest component is binding to haemoglobin.

Question 35

How important are the different types of CO2 transport?

Answer 35

The largest CO2 store is HCO3-. Haemoglobin is important for CO2 flux - the difference in partial pressure between venous and arterial blood.

Question 36

What is the Haldane curve?

Answer 36

The CO2 content in the blood against the PCO2. At normal arterial PCO2 of 5kPa, there is 480ml/L of blood. In the physiological range, the pressure and content is a straight line. See picture.

Question 37

What is the Haldane effect of hypoxia on CO2 content?

Answer 37

In hypoxia (low O2) there is more deoxyhaemoglobin, which allows it to bind CO2 and H+ better. So for the same partial pressure of CO2, there will be more CO2 content, which helps to transport it to the lungs. The curve shifts upwards. See picture.

Question 38

What is the Haldane effect on CO2 content in venous blood?

Answer 38

In venous blood, the PCO2 is higher, and the curve shifts upwards due to more deoxyhaemoglobin. So the CO2 content increases to 520ml/L. As oxygen is lost from the haemoglobin into the tissue, the haemoglobin changes shape to bind and transport CO2 better. See picture.

Question 39

What is the significance of the Haldane effect?

Answer 39

As O2 is delivered to the tissue, there is more deoxyhaemoglobin, so more CO2 diffuses into the RBC. More HCO3- is formed, so there is more Cl- shift to prevent build up. Deoxyhaemoglobin also removes H+ to buffer pH, which allows more HCO3- to form, so more CO2 is transported. Deoxyhaemoglobin is a stronger base than oxyhaemoglobin to accept H+.

Question 40

How is O2 and CO2 transport a reciprocal arrangement in the lungs?

Answer 40

CO2 diffuses from the blood into the alveoli, so the HCO3- reaction goes backwards. Blood picks up O2 in the alveoli, so there is more oxyhaemoglobin, which keeps the reaction going backwards to generate and release more CO2. So the picking up of O2 helps to get rid of CO2 in the lungs.

Question 41

How is O2 and CO2 transport a reciprocal arrangement in the tissue?

Answer 41

As O2 is delivered, more diffuses, so there is more deoxyhaemoglobin, and more CO2 can bins. CO2 can form HCO3- which then prevents H+ build up and stopping the reaction, as well as buffers pH.

Question 42

What are the arterial-venous differences of O2?

Answer 42

50ml/L difference. Cardiac output is 5L/min, so there is an A-V transport difference of 250ml/min of O2, which is VO2.

Question 43

What are the arterial venous differences of CO2?

Answer 43

40ml/L difference. Cardiac output is 5L/min, so there is an A-V transport difference of 200ml/min of CO2, which is VCO2.

Question 44

The diagram overleaf shows the oxygen dissociation curve for human blood at a pH of 7.40, a PCO2 of 5 kPa and a temperature of 37oC. Which of the points (A to F) provides the best description of the following situations? Briefly give the reasons for your choices. left atrial blood in a subject undergoing exercise. right atrial blood in a resting subject. inferior vena cava blood during severe exercise. arterial blood in a subject with only half the normal concentration of haemoglobin.

Answer 44

Left atrial blood in a subject undergoing exercise is E because the blood comes from the lungs so should be well oxygenated. Right atrial blood in a resting subject is B because it comes from the tissues, so has a higher PCO2 and lower O2. The curve shifts right. Inferior vena cava during severe exercise is D because there is less O2 and more CO2, so dissociation curve shifts right, and the veins are only 75% saturated anyway. Arterial blood with half haemoglobin is E.