The carriage of respiratory gases Slide 0 The carriage of respiratory gases Click mouse button or use and keys esc to end
Feb 02, 2016
The carriage of respiratory gases Slide 0
The carriage of respiratory
gases
The carriage of respiratory
gases
Click mouse button or use and keysesc to end
The carriage of respiratory gases Slide 1
Basics 1. The composition of air
(%) N2 O2 CO2
Atmosphere 79 21 0.03Alveoli 79 14 6
Exhaled air is therefore a mixture of these values, typically 79 16 4but varies according to physiological state.
The carriage of respiratory gases Slide 2
Basics 2. Atmospheric and partial pressures
Partial Pressure is the pressure exerted by one gas in a mixture of gases.
Atmospheric pressure (at sea level) is about 100 kPa.
One fifth of the air is oxygen. So one fifth of atmospheric pressure is due to oxygen.
Therefore the PARTIAL PRESSURE OF OXYGEN is about 20 kPa.
(Partial pressure of oxygen can be written as ppO2)
The carriage of respiratory gases Slide 3
Basics 3. Partial pressures of oxygen
Some key values for ppO2 are:
Atmospheric 20 kPaAlveolar 13 kPa
And a typical value for blood flowingthrough tissues of the body: 5kPa
The carriage of respiratory gases Slide 4
4. Oxygen transport
Oxygen is picked up by the blood in the lungs and transported in the circulation.
As the blood passes through the capillary beds of the tissues, a proportion of the oxygen is lost from the blood to the tissues.
Oxygen is hardly soluble in plasma; it is carried by HAEMOGLOBIN in the red blood cells.
Thus haemoglobin loads oxygen in the lung capillaries and unloads it in the tissue capillaries.
The carriage of respiratory gases Slide 5
5. Oxygen loading
So to function efficiently, haemoglobin must attract oxygen under certain conditions, but lose it under other conditions.
Loading/unloading is a reversible reaction, in which one haemoglobin (Hb) can attract up to 4 oxygen molecules:
Hb + 4O2 HbO8loading
unloading
The carriage of respiratory gases Slide 6
6. A reminder of the structure of a haemoglobin molecule
Haemoglobin is a conjugated quaternary protein comprising:
• two alpha-globulins• two beta-globulins• four haem groups, each with an iron atom at its core
It is the iron atoms which bind oxygen.
The carriage of respiratory gases Slide 7
7. Haemoglobin structure
The carriage of respiratory gases Slide 8
8. Loading and saturation
Hb loads/unloads one O2 at a time, so can exist as Hb, HbO2, HbO4, HbO6, or HbO8.
There are billions of Hb molecules in the blood. The term %saturation refers to the overall degree of loading of Hb with oxygen.
The carriage of respiratory gases Slide 9
9. So what factors affect the loading/unloading equilibrium?
Hb + 4O2 HbO8 loading
unloading
• oxygen levels, i.e. ppO2
• carbon dioxide levels, i.e. ppCO2
• blood pH
• temperature
The carriage of respiratory gases Slide 10
10. Loading
Hb + 4O2 HbO8 loading
unloading
•Loading occurs at the alveolar exchange surface, where:
•ppO2 is
•ppCO2 is
•blood pH is therefore
•and temperature is
relatively high (13 kPa)
relatively low
relatively high
relatively low
WHY?
The carriage of respiratory gases Slide 11
11. Unloading
Hb + 4O2 HbO8 loading
unloading
•Unloading occurs in the tissue capillaries, where:
•ppO2 is
•ppCO2 is
•blood pH is therefore
•and temperature is
relatively low (5 kPa)
relatively high
relatively lower
relatively higher
WHY?
The carriage of respiratory gases Slide 12
12. Saturation and oxygen dissociation
Hb + 4O2 HbO8 In fact, blood passing through the tissues never totally unloads all its oxygen. Here, blood oxygen saturation may fall to around 50%, in contrast to 100% saturation in the lung capillaries.
OXYGEN DISSOCIATION CURVES are graphs which show the relationship between the ppO2 and the degree of saturation.
loading
unloading
The carriage of respiratory gases Slide 13
13. Oxygen dissociation curve
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20
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partial pressure of oxygen (kPa)
What happens in the lungs and in the tissues?
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O2
(%)
The carriage of respiratory gases Slide 14
14. Oxygen dissociation curve explained
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partial pressure of oxygen (kPa)
In the upper part of the graph Hb is loaded with oxygen.
In the lower part Hb is unloading its oxygen.
lungs
tissues
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The carriage of respiratory gases Slide 14a
14a. Effect of changes in ppO2
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partial pressure of oxygen (kPa)
What happens to %saturation when there is a small change in ppO2
a) in the lungs?b) in the tissues?
See how actively respiring tissues promote more O2 unloading.
lungstissues
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The carriage of respiratory gases Slide 15
15. Oxygen dissociation curve
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partial pressure of oxygen (kPa)
How do actively respiring tissues affect other factors:•ppCO2
•pH•temp?
And how do these affect loading / unloading?
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O2
(%)
The carriage of respiratory gases Slide 16
16. REMEMBER:
•Unloading occurs in the tissue capillaries, where:
•ppO2 is relatively low (5 kPa)
•ppCO2 is relatively high
•blood pH is therefore relatively lower
•and temperature is relatively higher
The carriage of respiratory gases Slide 17
17. Shift to the right - the Bohr effect of higher ppCO2
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partial pressure of oxygen (kPa)
Increased ppCO2, lower pH and increased tempall have the effect of pushing the curve to the right.
See how this decreases Hb’s affinity for O2, so more is unloaded.
higher CO2normal CO2
Higher affinity
lower affinitySat
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O2
(%)
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partial pressure of oxygen (kPa)
The carriage of respiratory gases Slide 18
18. Shift to the left
What are the consequences of a shift to the left?
Is Hb’s affinity for oxygen more or less?
Under what circumstances might this graph apply?
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The carriage of respiratory gases Slide 19
19. Questions
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partial pressure of oxygen (kPa)
If the black curve is ‘normal’, which curve represents the effect of elevated levels of carbon dioxide?
Sat
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O2
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The carriage of respiratory gases Slide 20
20. Questions
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partial pressure of oxygen (kPa)
If the black curve is human haemoglobin, which curve represents bird Hb and which lugworm Hb?
Sat
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The carriage of respiratory gases Slide 21
21. Questions
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partial pressure of oxygen (kPa)
If the black curve is human maternal haemoglobin, which curve represents foetal Hb?
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The carriage of respiratory gases Slide 22
22. The carriage of CO2
CO2 is rather more soluble than O2. It is transported from tissues to lungs in three ways:
• dissolved directly in plasma (about 5%)• combined with the polypeptides of haemoglobin (forming carbamino-haemoglobin) (about 10%)• as HCO3- in the plasma, following a series of reactions within the red blood cells
These reactions will explain why there is a direct relationship between the level of CO2 and the degree unloading of O2 by haemoglobin:
The carriage of respiratory gases Slide 23
23. Reactions inside the RBC - CO2 reacts with water
CO2 CO2
CO2 from respiring tissues enters the red blood cell and combines with water, forming carbonic acid. The reaction is accelerated by CARBONIC ANHYDRASE.
carbonicanhydrase
H2CO3+ H2O
The carriage of respiratory gases Slide 24
24. Reactions inside the RBC - dissociation of carbonic acid
CO2 CO2 + H2O H2CO3
Carbonic acid dissociates and HCO3- is transported out of the
RBC, in exchange for Cl-. (This is the CHLORIDE SHIFT)
carbonicanhydrase
HCO3-H+
Cl-
The carriage of respiratory gases Slide 25
25. Reactions inside the RBC - unloading of oxygen
CO2 CO2 + H2O H2CO3
H+ displaces O2 from haemoglobin, forming HHb - reduced haemoglobin. The O2 is liberated to the tissues.
carbonicanhydrase
HCO3-H+
Cl-
HbO2
HHb
O2
The carriage of respiratory gases Slide 26
26. Reactions inside the RBC - explanation of the Bohr effect
CO2 CO2 + H2O H2CO3
The more CO2 , the greater the displacement of O2 from Hb. CO2 reduces the affinity of Hb for O2 . This explains the Bohr effect.
carbonicanhydrase
HCO3-H+
Cl-
HbO2
HHb
O2
The carriage of respiratory gases Slide 27
27. Reactions inside the RBC - reversal in the lungs.
CO2 CO2 + H2O H2CO3
All these reactions are reversible. In the lung capillaries ppO2 is higher and ppCO2 is lower and so this happens:
carbonicanhydrase
HCO3-H+
Cl-
HbO2
HHb
O2
The carriage of respiratory gases Slide 28
28. Reactions inside the RBC - release of CO2.
CO2 CO2 + H2O
In the lung capillaries ppO2 is higher and ppCO2 is lower, so now O2 binds to Hb and this results in the release of CO2
carbonicanhydrase
HCO3-H+
Cl-
HbO2
HHb
O2
H2CO3
Don’t worry - you don’t have to learn these reactions!
The carriage of respiratory gases Slide 29
29. Summary
• Most oxygen is transported bound to haemoglobin, as
oxyhaemoglobin• The oxygen saturation of haemoglobin is affected by:
• ppO2
• ppCO2
• pH• temperature
• CO2 is transported in three ways:• dissolved in plasma• bound to haemoglobin as carbamino-haemoglobin• converted to hydrogencarbonate ions in the red cells
• High levels of CO2 facilitate O2 unloading from haemoglobin
through the formation of hydrogen ions. The effect of increased
CO2 / decreased pH on O2 unloading is called the Bohr effect.• High levels of O2 facilitate CO2 unloading from the blood.
• • • end • • •