Blood gas transport

Question 1

What is oxygen transported by?

Answer 1

98% by haemoglobin

2% dissolved in plasma

Question 2

What does oxygen have to do before binding to haemoglobin?

Answer 2

Dissolve in blood plasma

Question 3

Explain transport of oxygen

Answer 3

Oxygen exchange occurs in lungs and then dissolves in plasma. Most of the oxygen gets bound to haemoglobin (but it’s reversible so some oxygen is purely transported just by plasma). Then at tissues, oxygen leaves haemoglobin and dissolves in plasma; oxygen diffuses into tissues.

Question 4

Explain how carbon dioxide is transported

Answer 4

70% as hydrogen carbonate ion
23% bound to haemoglobin
7% dissolved in plasma
Carbon dioxide is more soluble than oxygen so more carbon dioxide is transported as dissolved in plasma.

The carbon dioxide transported as hydrogen carbonate ion or bound haemoglobin come out into blood to dissolve for exchange at lungs.
Carbon dioxide produced by tissues dissolve in blood then bind to haemoglobin or form hydrogen carbonate ion.

Question 5

Why do we need haemoglobin?

Answer 5

For the transport of oxygen. Oxygen has a very low solubility in plasma so haemoglobin is needed to concentrate oxygen within blood (as it increases carry capacity) at gas exchange surface.

Question 6

What is oxygen partial pressure (PaO2) and what are the units?

Answer 6

The partial pressure of oxygen within a gas phase that would yield this is much oxygen in the plasma at equilibrium = how much oxygen is dissolved in plasma. Units are kPa

Question 7

What is total oxygen content (CaO2) and what are the units?

Answer 7

It is the volume of oxygen being carried in each litre of blood including the oxygen dissolved in plasma and the oxygen bound to haemoglobin. Units are mL/L

Question 8

What is oxygen saturation? What are the units?

Answer 8

There are 2 methods of measuring how much haemoglobin out of the total haemoglobin in your body is being occupied by oxygen:
1. SaO2 = measured directly in arterial blood
2. SpO2 = estimated by pulse oximetry
Expressed in %.

Question 9

Why does oxygen haemoglobin dissociation curve have a sigmoid shape?

Answer 9

First part of graph (less steep) is due to difficulty for the first oxygen molecule to bind to a haem group in haemoglobin. Then we get cooperative binding due to the first oxygen molecule making it easier for the rest of the oxygen molecules to bind. We get plateau due to saturation of the haemoglobin.

Question 10

What does oxygen haemoglobin dissociation curve describe?

Answer 10

Affinity of haemoglobin to oxygen at different partial pressures of oxygen.

Question 11

Why is a high level of haemoglobin binding to oxygen and saturation achieved at a relatively low partial pressure of oxygen?

Answer 11

Because the structure of haemoglobin produces a high affinity for oxygen. Only very very low levels of oxygen will affect the curve.

Question 12

Describe the oxygen carrying capacity.

Answer 12

3mL/ L (plasma) + 197mL/L (hb) = 200mL/ L
It’s the concentration of haem groups and haemoglobin contained in the red blood cells that enable the high carrying capacity.

Question 13

Why can the oxygen dissociation curve be shifted?

Answer 13

Because the affinity can be affected by the environment.

Question 14

Describe the left shift of the oxygen dissociation curve

Answer 14

Haemoglobin has a higher affinity for oxygen at any partial pressure of oxygen. Becomes saturated a low partial pressures of oxygen. Low carbon dioxide levels, high pH (alkalosis), lower 2-3 DPG, lower temperature.
This is what happens in the lungs, whereby partial pressure of oxygen is high, partial pressure of carbon dioxide is low and pH is high. Leads to oxygen saturation.

Question 15

Describe the Bohr shift.

Answer 15

Shift to the right. Haemoglobin has a lower affinity for oxygen at any partial pressure Of oxygen. Occurs in hard working tissue. Partial pressure of oxygen is low, partial pressure of carbon dioxide is high due to increased anaerobic respiration which also produces lactic acid. Increased levels of 2,3-DPG.
Oxygen demands are high. Low pH. Lower haemoglobin and oxygen affinity + binding. Reduced oxygen saturation. Oxygen moves from haemoglobin to tissues.

Question 16

What is myoglobin?

Answer 16

A type of oxygen binding protein that has a very high affinity for oxygen at any partial pressures. It is a reservoir of oxygen for muscles and only releases oxygen at very low partial pressures of oxygen. Shift to the left.

Question 17

Explain foetal haemoglobin

Answer 17

Shift to the left. Haemoglobin has a higher affinity for oxygen at any partial pressures and becomes fully saturated at lower partial pressures. This is important for foetus because it has to be able to absorb oxygen from the mother.

Question 18

What colour does deoxygenated haemoglobin and oxygenated haemoglobin occur?

Answer 18

Deoxygenated haemoglobin = blue which is why venous blood appears more purple
Oxygenated haemoglobin = red which is why arterial blood is red

Question 19

What is cyanosis?

Answer 19

It is the purple discolouration of the skin and tissue that occurs due to excessive deoxyhaemoglobin (haemoglobin not bound to oxygen).

Question 20

Why is cyanosis often less obvious in patients with low red blood cell density (e.g anaemia)?

Answer 20

Patients with anaemia already have low haemoglobin levels, so the amount of oxygenated haemoglobin and deoxygenated haemoglobin decreases. So because the deoxygenated haemoglobin drops (due to lack of haemoglobin in the blood), it takes much lower oxygen levels to cause deoxygenated haemoglobin to rise and become excessive. This is because they already have low haemoglobin levels so an even lower amount of oxygen is needed to cause excessive deoxygenated haemoglobin.

Question 21

Describe central cyanosis.

Answer 21

Blue-ish discolouration of core, mucous membrane, lips, mouth and tongue. Due to inadequate oxygenation of blood by e.g hypoventilation or V/Q mismatch.

Question 22

Describe peripheral cyanosis

Answer 22

Blue-ish colouration of hands and feet. It is due to inadequate oxygen supply to extremities. So there’s not an actual problem with ventilation or perfusion in alveoli, it’s due to problems in circulation itself e.g restricted blood flow to feet and hands.

Question 23

Why can hypoxia occur even though ventilation and perfusion is fine?

Answer 23

Because the blood isn’t able to carry enough oxygen to meet tissue demands. This occurs in anaemia due to lack of red blood cells or insufficient haemoglobin.
Haemorrhage can cause anaemia. Iron deficiency can cause anaemia due to less haemoglobin production. So not enough oxygen can be carried to tissues.

Question 24

Symptoms of anaemia

Answer 24

Fatigue, pale conjunctiva, low haematocrit

Question 25

Why is the transport of oxygen and carbon dioxide different?

Answer 25

1.Because carbon dioxide has a greater solubility in water than oxygen.
Concentration = partial pressure x solubility
2.Carbon dioxide binds to different sites than oxygen (so not at haem groups, at the amino terminus of the protein to form carbamino haemoglobin). This has a lower affinity so less carbon dioxide is transported bound to haemoglobin.
3. Carbon dioxide reacts with water to form carbonic acid to form hydrogen carbonate ion- this is how most of carbon dioxide is transported.

Question 26

Why does venous blood carry more carbon dioxide than arterial blood? What is it called?

Answer 26

Venous blood carries less oxygen. So there’s more deoxygenated haemoglobin than oxygenated haemoglobin. Deoxygenated haemoglobin has a higher affinity for carbon dioxide and hydrogen ions.
Key point: As oxy-hb increases, the amount of carbon dioxide carried decreases. It’s like the opposite of Bohr’s shift in terms of that this time, it’s the effect of oxygen on carbon dioxide transport.
It’s called the Haldane effect.

Question 27

How does the Haldane effect work?

Answer 27

When oxygen is present, the oxygen will cause the hydrogen ions bound to haemoglobin to be released and bind with hydrogen carbonate ion to form carbonic acid to then form carbon dioxide. So less carbon dioxide becomes dissolved in red blood cells, and more carbon dioxide becomes dissolved in plasma. If we have excess carbon dioxide and oxygenation occurs, the carbon dioxide can’t be transported so it will accumulate causing acidosis. (Oxygen also causes carbon dioxide bound to haemoglobin to be released into plasma).

Question 28

Explain hydrogen carbonate formation

Answer 28

1. The carbon dioxide produced by respiring cells dissolves in the plasma and enters the red blood cells.
2. Carbonic anhydrase catalyses the reaction between carbon dioxide and water to produce carbonic acid. This reduces the concentration of carbon dioxide in the red blood cells, allowing more carbon dioxide to diffuse into them to transport carbon dioxide.
3. Carbonic acid dissociates to produce hydrogen ions and hydrogen carbonate ions. The red blood cell membrane is impermeable to hydrogen ions so hydrogen ions can’t leave.
4. Oxygen moves out of tissues, increasing the concentration of deoxygenated haemoglobin which enables more carbon dioxide to be transported. The deoxygenated haemoglobin binds to hydrogen ions preventing termination of the hydrogen carbonate ion production reaction.
5. This increases the level of deoxygenated haemoglobin which enables more carbon dioxide transport.
6. The increase in hydrogen carbonate ion concentration creates a concentration gradient for hydrogen carbonate ion to leave the cell. It is exchanged for chloride ions to maintain electrochemical neutrality.

Question 29

Explain the hydrogen carbonate ion at lungs. (Haldane’s effect)

Answer 29

1. Low alveolar partial pressure of carbon dioxide creates a diffusion gradient for carbon dioxide to diffuse out of blood and into the alveoli.
2. Increased alveolar partial pressure of oxygen leads to oxygen binding to haemoglobin. Oxygenated haemoglobin binds less hydrogen ions than deoxygenated haemoglobin, increasing the concentration of free hydrogen ions.
3. Increased free hydrogen ions causes an increase in carbonic acid hence increased carbon dioxide which contributes to carbon dioxide plasma saturation. This carbon dioxide is then removed at lungs.
4. The production of carbon dioxide in step 3 leads to a decreased concentration of hydrogen carbonate ion which creates s diffusion gradient that allows hydrogen carbonate ions to enter the red blood cells in exchange for chloride ions. This continuously allows the same thing to happen to the hydrogen carbonate ions so that carbon dioxide can be removed at lungs.

The overall effect:
Deoxygenated blood carries more carbon dioxide
Oxygenation of blood causes the carbon dioxide to leave

Question 30

What does binding of oxygen do the structure of haemoglobin?

Answer 30

It changes structure of haemoglobin slightly lowering haemoglobin’s affinity for hydrogen ions and carbon dioxide so less hydrogen ions and carbon dioxide can bind.

Question 31

Why does venous blood carry more carbon dioxide than arterial blood?

Answer 31

Because venous blood has less oxygen - Haldane’s effect.

Question 32

How does anaemia limit blood oxygen transport?

Answer 32

Because it reduces the total number of oxygen haemoglobin binding sites.

Question 33

Why is the SpO2 normal in anaemia?

Answer 33

Because in anaemia, there is less haemoglobin. But the RATIO of haemoglobin saturated will be the same as the amount of haemoglobin that would be saturated if haemoglobin levels were normal. ( So even though there’s less haemoglobin, the same percentage will be saturated).
So it’s due to the fact that it’s about percentage/ ratio.

If the SpO2 did decrease, this would be due to the fact that the haemoglobin isn’t saturated due to a defect so it can’t accept oxygen. Or it could be because oxygen levels are very low so less oxygen binds to haemoglobin so less saturation so this affects ratio.

Question 34

What is erythropoietin and what does it do?

Answer 34

It is a hormone secreted by the kidneys in response to hypoxia which increases production of red blood cells in the bone marrow. As oxygen transport from the lungs to tissues is dependent on haemoglobin concentration and PaO2 (which is itself dependent on PAO2), a method of compensating for the presence of
chronic hypoxia is to increase haemoglobin concentration; although Hb saturation will decrease due to reduced PaO2, this is somewhat compensated for by a greater overall number of haemoglobin.
bin.
Similarly, cardiac output will increase via increased heart rate in response to hypoxia to increase overall oxygen transport (the same number of Hb molecules are cycled from the lungs to tissue and back again more rapidly, increasing the total volume of oxygen transported per unit of time.