1. H6 Gas exchange

2. Assessment Statements
H Define partial pressure.
H Explain the oxygen dissociation curves of adult hemoglobin, fetal hemoglobin and myoglobin.
H Describe how carbon dioxide is carried by the blood, including the action of carbonic anhydrase, the chloride shift and buffering by plasma proteins.
H Explain the role of the Bohr shift in the supply of oxygen to respiring tissues.
H Explain how and why ventilation rate varies with exercise.
H Outline the possible causes of asthma and its effects on the gas exchange system.
H Explain the problem of gas exchange at high altitudes and the way the body acclimatizes.

3. Define the term partial pressure
partial pressure is the pressure exerted by a given gas in a mixture
the symbol for partial pressure is P, and the partial pressure for a gas x is Px. So, PO2 denotes the partial pressure of oxygen
what is the partial pressure of the oxygen in the air around us?
At sea level, the atmospheric pressure is typically about kPa of which 21.0% is O2, So, PO2 is given by:
101.3 𝑥 = 21.3 kPa
Atmospheric air is a mixture of gases; nitrogen, oxygen, carbon dioxide, water vapour & inert gases.
At sea level, the atmospheric pressure is about kPa.
What proportion of atmospheric pressure is due to oxygen?

4. Role of haemoglobin
1 molecule of oxygen will combine with each haem group, meaning, each haemoglobin molecule is able to transport 4 molecules of oxygen:
oxyhaemoglobin is the form in which oxygen is transported from the lungs to the respiring body tissues
at respiring tissue cells, oxyhaemoglobin breaks down, releasing oxygen & haemoglobin
oxygen is used up by tissue cells while haemoglobin is returned to the lungs to pick up more oxygen
haemoglobin occurs in the red cells
haemoglobin molecule is built of four interlocking subunits
each subunit is composed of a large globular protein with a non-protein haem group attached, containing iron

5. Oxygen dissociation curve
the affinity of haemoglobin for oxygen is measured experimentally by finding the percentage saturation with oxygen of blood exposed to air mixtures containing different partial pressures of oxygen
the result is called an oxygen dissociation curve
oxygen dissociation curve is S-shaped, the amount of oxygen held by haemoglobin depends on the partial pressure of oxygen
in the body, too, the amount of oxygen held by haemoglobin depends on the partial pressure
in respiring tissues, the oxygen partial pressure is much lower than that in the lungs
at lower partial pressures, oxyhaemoglobin breaks down, releasing oxygen in solution and this rapidly diffuses into the surrounding tissues

6. Oxygen dissociation curve of adult haemoglobin
oxygen dissociation curve for oxyhaemoglobin is S/sigmoid-shaped
it shows how the saturation of haemoglobin with oxygen varies with partial pressure of oxygen
haemoglobin has an increasing affinity for oxygen, initial uptake of one oxygen molecule by haemoglobin facilitates the further uptake of oxygen molecules
low partial pressure of oxygen corresponds to the situation in the tissue, when partial pressure of oxygen is low, oxygen is released
low pH, increased carbon dioxide & increased lactic acid causes the curve to shifts the to the right and oxygen is more readily released to respiring tissues – this is known as the Bohr effect
high partial pressure of oxygen corresponds to the situation in the lungs, when partial pressure of oxygen is high, oxygen is taken up by haemoglobin
Shift to the right; (decreased affinity) low pH, increased CO2, increased lactic acid

7. Oxygen dissociation curve of fetal haemoglobin
like adult haemoglobin, fetal haemoglobin have S-shaped oxygen dissociation curves
fetal haemoglobin have a high affinity for oxygen at high partial pressure of oxygen
fetal haemoglobin always has a higher affinity for oxygen at corresponding partial pressures of oxygen than adult haemoglobin, thus fetal haemoglobin dissociation curve lies to the left of the adult dissociation curve
in the placenta where maternal and fetal blood come into close proximity there is a low oxygen partial pressure
fetal haemoglobin must have a greater affinity for oxygen otherwise the maternal oxy-haemoglobin would not dissociate
relationship between fetal and adult haemoglobin dissociation curves does NOT change at all partial pressures of oxygen
the difference in adult and fetal haemoglobin structures lead to differences in affinity
between foetal & adult haemoglobin, which one has a higher affinity for oxygen?
why it is advantageous that fetal haemoglobin higher affinity for oxygen than adult haemoglobin?

8. Oxygen dissociation curve of myoglobin
myoglobin is specialized for oxygen storage
myoglobin has a higher affinity for oxygen than haemoglobin, its dissociation curve is to the left of that for haemoglobin
in normal conditions, at rest myoglobin is saturated with oxygen
myoglobin is used during intense muscle contraction when the oxygen supply is insufficient i.e. when muscle is very active its oxygen concentration may fall below 0.5 kPa
when this happens myoglobin releases oxygen to muscle cells
myoglobin oxygen dissociation curve is not sigmoid shaped, it has a steep rise below 5 kPa with no lag & has slower rise approaching 100 % above 5 kPa
myoglobin is a respiratory pigment built of a single haem–globin unit, similar to the four units in haemoglobin
myoglobin is only found in skeletal muscle cells, where it acts as a reserve of oxygen

9. Oxygen dissociation curves of adult haemoglobin, fetal haemoglobin and myoglobin
rapid saturation of oxygen in the lungs
rapid dissociation of oxygen as the oxygen concentration decreases
oxygen released in the tissues where it is needed
fetal haemoglobin:
fetal haemoglobin curve to the left of adult haemoglobin
higher affinity for oxygen than adult haemoglobin
oxygen moves from adult haemoglobin to fetal haemoglobin
myoglobin:
myoglobin to the left of fetal haemoglobin
only releases oxygen at very low oxygen concentrations in tissues
acts as oxygen reserve in muscle cells

10. How carbon dioxide is carried by the blood
carbon dioxide is carried in three forms in the blood:
carbon dioxide can be dissolved in the blood plasma forming carbonic acid (5 %)
carbon dioxide can be carried as dissociated carbonic acid i.e. H H CO3 − in red blood cells (85 %)
carbon dioxide can be carried as carbaminohemoglobin when it is bound to haemoglobin (10 %)
carbonic anhydrase is an enzyme found in red blood cells (erythrocytes)
carbonic anhydrase speeds up production of hydrogen carbonate (H CO3 −)
chloride shift i.e. movement of chloride ions into red blood cell, occurs to balance movement of hydrogen carbonate ion out

11. Role of the Bohr shift in the supply of oxygen to respiring tissues
hemoglobin carries up to four oxygen molecules
Bohr shift promotes the release of oxygen in respiring heart muscle
active respiration releases CO2 causing the partial pressure of CO2 increases
release of CO2 increases acidity i.e. lowers the pH due to formation of hydrogen ions (H+)
hydrogen ions bind to hemoglobin decreasing hemoglobin’s affinity for O2 so O2 is released from the oxyhemoglobin
this occurs due allosteric effect i.e. conformational change in hemoglobin which releases O2 more readily

12. Bohr shift

13. How and why ventilation rate varies with exercise
during exercise the rate of tissue respiration increases i.e. more carbon dioxide produced
carbon dioxide production in the tissues exceeds the rate of breathing it out
increase in carbonic acid (H2CO3), increase in H+ ions , pH drops in the blood plasma
lactic acid produced during strenuous exercise reduces pH
chemoreceptors, located in the carotid & aortic bodies, detect change in pH, increase in carbon dioxide & decrease in oxygen
Increased CO2 in the blood & lower pH are also detected by chemoreceptors in medulla
nerve impulses sent to the breathing Centre in the medulla of the brain from the chemoreceptors
nerve impulses are then sent to diaphragm & intercostal muscles from the breathing Centre in medulla to increase the rate & the depth of breathing
ventilation rate is controlled through negative feedback mechanism

15. Possible causes of asthma and its effects on the gas exchange system
asthma is a chronic inflammatory disease of the airway
it is caused by allergic reaction to allergens such as; dust, mites droppings, pollen, toxins, pets hairs, fungi etc.
immune responses releases histamine which causes:
constriction of muscles of wall of bronchioles
more mucus is produced
these restricts air flow thus ventilation is hard & gas exchange is reduced

16. Problem of gas exchange at high altitudes
at high altitudes partial pressure of oxygen is lower, at 7000m PO2 is 8.1 kPa
as air is exchanged in lungs hemoglobin does not become fully saturated with O2
oxygen deprivation of tissues occurs causing fatigue i.e. Monge disease
mountain sickness (increased pulse rate, nausea, headaches, sore throat, muscular weakness, dizziness ) may develop
ventilation rate & depth increases

17. How the body acclimatizes to high altitudes
ventilation rate increases
red blood cell (erythrocyte) concentration in blood increases
myoglobin concentration in muscles increases
capillary networks in the muscles develop greater density
lung working volume, vital capacity, increases
people living permanently at high altitude develops greater lung surface area

18. Revision Questions
Explain why ventilation rate varies with exercise [6]
Explain how and why ventilation rate varies with exercise [6]
Outline one possible cause of asthma and its effect on the gas exchange system [3]
Outline how the body acclimatizes to high altitudes. [3]
Explain the problem of gas exchange at high altitudes and the way the body acclimatizes [6]
Define the term partial pressure [1]
Explain the oxygen dissociation curves of adult haemoglobin, fetal haemoglobin and myoglobin. [6]
Describe how carbon dioxide is carried by the blood [4]
Explain, with the use of a diagram, the role of the Bohr shift in the supply of oxygen to respiring heart muscle. [6]
Explain the Bohr shift of an oxygen dissociation curve during gas exchange [6]

19. The oxygen dissociation curve is a graph that shows the percentage saturation of haemoglobin at various partial pressures of oxygen. Curve A shows the dissociation at a pH of 7 and curve B shows the dissociation at a different pH.
(i) State the possible cause of the curve shifting from A to B [1]
(ii) On the graph, draw the curve for myoglobin [2]

20. Explain the oxygen dissociation of myoglobin, completing the graph below to support your answer. Po2 is the partial pressure of oxygen.