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Respiratory Physiology, Medicine and Critical Care > Gas Transport in the Blood > Flashcards

Gas Transport in the Blood Flashcards

1
Q

What is the alveolar PO2?

A

100 mmHg or 13.3 kPa

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2
Q

What is the alveolar PCO2?

A

40 mmHg or 5.3 kPa

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3
Q

How does hypoventilation affect the alveolar partial pressures of O2 and CO2?

A
  • PO2 decreases 30 mmHg,
  • PCO2 increases to approximately 100 mmHg
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4
Q

How does hyperventilation affect the alveolar partial pressures of O2 and CO2?

A
  • PO2 increases to approximately 120 mmHg
  • PCO2 decreases to approximately 20 mmHg
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9
Q

Why are there only small fluctuations in alveolar partial pressure during normal breathing, rather than large fluctuations?

A
  • Amount of oxygen that enters the alveoli with each breath is roughly equal to the amount of oxygen which enters the blood
  • Amount of air that enters the lung with each breath is only about 10% of the total lung volume at the end of inspiration
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10
Q

Why does the partial pressure of oxygen change from 160 mmHg (in air) to 100 mmHg in the lungs?

A
  • Dilution via water vapour - reduces the concentration of oxygen by displacing oxygen molecules per unit volume of air
  • Anatomical Dead Space
  • Residual Volume - Air that has experienced a certain level of gas exchange then mixes with fresh air, thus reducing (slightly) the partial pressure of O2.
  • Gas exchange at the alveoli
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11
Q

What is the arterial PO2 after oxygenation?

A

95 mmHg (can vary from 85-100 mmHg)

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12
Q

What is the arterial PCO2 after gas exchange in the lungs?

A

40 mmHg (can vary from 35-45 mmHg)

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13
Q

What is the PO2 in peripheral cells?

A

40 mmHg - facilitates partial pressure gradient

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14
Q

What is the PCO2 in peripheral cells?

A

46 mmHg - facilitates diffusion of CO2 out of the cells

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15
Q

What is the venous PO2?

A

46 mmHg

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16
Q

What is the venous PCO2?

A

46 mmHg

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17
Q

How is oxygen transported in the blood?

A

Oxygen dissolved in plasma (approx 3 ml/L) and oxygen bound to haemoglobin (approx 197 ml/L).

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18
Q

What determines the amount of oxygen that binds to haemoglobin?

A
  • PO2 of surrounding plasma
  • Number of Hb binding sites
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19
Q

What determines the binding site saturation on haemoglobin?

A

Plasma PO2 - the higher the PO2, the more oxygen binds to Hb

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20
Q

What is SpO2 when plasma PO2 is 100 mmHg?

A

Approximately 98% - haemoglobin can pick up maximum amount it can carry when the alveoli are properly ventilated

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21
Q

What does the oxygen dissociation curve demonstrate in terms of falling levels of PO2?

A

SpO2 is still maintained at high concentrations down to PO2 levels of 60 mmHg (still approximately 90%). After this, saturation begins to decline more sharply, meaning that small decreases in PO2 result in larger decrements in saturation.

At 40 mmHg, saturations are sitting at about 75%

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22
Q

What is the structure of haemoglobin?

A

Haemoglobin is a multi sub-unit globular protein, composed of four alpha-helical chains. Each chain is folded in a particular conformation due to hydrogen bonding within and between the chains.

Each protein subunit is closely associated with a non-protein haem group, which has a central iron ion held in a heterocyclic ring known as porphyrin. The iron ion is where oxygen reversibly binds to.

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25
Q

What volume of O2 does normal plasma hold?

A

3 ml/L

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27
Q

What happens to the haemoglobin molecule as oxygen binds to it?

A

It alters the conformation of the Hb subunit next to it, thus increasing its affinity to bind O2 (sequential binding; allosteric)

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28
Q

How is the partial pressure gradient between plasma and alveoli maintained?

A

Haemoglobin sequesters O2 from plasma

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29
Q

How much oxygen can blood hold in total?

A

Approximately 200 ml/L

30
Q

How many oxygen atoms can haemoglobin carry?

A

4

32
Q

What advantage does the structure of foetal haemoglobin give?

A

Enhances ability of foetal haemoglobin to bind oxygen at lower PO2 values. This is needed due to the placenta being a low oxygen environment, meaning that any oxygen released by maternal haemoglobin is picked up by foetal haemoglobin.

Higher affinity does mean that release of oxygen to foetal tissue is more difficult, but this is counteracted by a higher haematocrit, and exacerbated Bohr effect.

33
Q

Why is the affinity of myoglobin so high?

A

Primarily due to the oxygen requirements of exercising muscle. If affinity was lower, less oxygen would be transported to the muscles.

Muscle cells have even lower PO2 than normal cells, meaning that the increased affinity isn’t a problem during oxygen release.

34
Q

What is the main subtype of haemoglobin molecule?

A

HbA (92%)

35
Q

How long does it take for haemoglobin to become saturated?

A

0.25s

38
Q

How many grams of haemoglobin are there per Litre of plasma?

A

150g/L

39
Q

What is the structural difference between adult and foetal haemoglobin?

A

Foetal haemoglobin has two gamma protein chains in place of the two beta chains found in adult haemoglobin

40
Q

What factors favour CO2 unloading?

A
  • Alveolar PCO2
  • High O2 levels
41
Q

What is the haldane effect?

A

Describes how oxygen concentrations determine hemoglobin’s affinity for carbon dioxide. For example, high oxygen concentrations enhance the unloading of carbon dioxide. The converse is also true: low oxygen concentations promote loading of carbon dioxide onto hemoglobin. In both situations, it is oxygen that causes the change in carbon dioxide levels.

42
Q

What are the forms of CO2 transport in the blood?

A
  • Dissolved in plasma
  • Carbaminohaemoglobin
  • Bicarbonate ions
43
Q

Which is more soluble; CO2 or O2?

A

CO2

44
Q

What percentage of total CO2 carried in blood is carried in plasma?

A

7%

45
Q

What is carbaminohaemoglobin?

A

Compound of hemoglobin and carbon dioxide, and is one of the forms in which carbon dioxide exists in the blood. CO2 binds to exposed amino groups left by oxygen

46
Q

What percentage of CO2 carried in the blood is carried in the form of carbaminohaemoglobin?

A

23%

47
Q

What is the function of carbonic anhydrase in the blood?

A

Facilitates bicarbonate formation by combining CO2 with H2O to form carbonic acid

48
Q

What happens to carbonic acid in RBC’s after it has been formed from H2O and CO2?

A

It dissociates to H+ and HCO3-. This is a reaction that continues until equilibrium is reached

49
Q

How do red blood cells prevent equilibrium (of H+ and HCO3-) being reached in the following reaction?

A
  • Chlorine shift - HCO3 is removed in echange for cholride ions via an antiport protein. HCO3- acts as an extracellular buffer
  • H+ binds to haemoglobin - sequestered
52
Q

What happens when the capacity of haemoglobin to bind H+ occurs when PCO2 is elevated?

A

Respiratory acidosis

53
Q

What factors affect gas exchange in the lungs and tissues?

A
  • Gradient magnitude
  • Gas solubility
  • Surface area
  • Membrane thickness
  • Distance
54
Q

What is the difference between partial pressure and gas content?

A

Gas content represents the amount of gas that is in solution

Partial pressure the the pressure of a particular gas in a mixture of gases

55
Q

What are factors which determine the arterial PO2 of haemoglobin?

A
  • pH
  • PCO2
  • 2-3 BPG
  • Temperature
  • Total number of binding sites
56
Q

How does increased pH influence binding affinity of oxygen to haemoglobin?

A

Increases binding affinity

57
Q

How does decreased pH influence binding affinity of oxygen to haemoglobin?

A

Decreased binding affinity - good example is in muscles.

58
Q

How does increased CO2 influence oxygen binding affinity to haemoglobin?

A

Decreased binding affinity

59
Q

How does decreased CO2 influence oxygen binding affinity to haemoglobin?

A

Increased binding affinity

60
Q

How does increased temperature influence oxygen binding affinity to haemoglobin?

A

Decreased binding affinity

61
Q

How does decreased temperature influence oxygen binding affinity to haemoglobin?

A

Increased binding affinity

63
Q

What happens to oxygen binding affinity when 2-3 BPG increases?

A

Decreased binding affinity - due to it binding to deoxygenated haemoglobin instead of oxygen

64
Q

What happens to oxygen binding affinity when 2-3BPG decreases?

A

Binding affinity increases - less 2-3 BPG available to bind to deoxygenated haemoglobin

65
Q

What factors determine the arterial PO2 of oxygen dissolved in plasma?

A
  • Alveolar PO2 (ventilation)
  • Composition of air inspired
  • Oxygen diffusion between alveoli and blood
  • Alveolar Perfusion
72
Q

What is 1,3-bisphosphoglycerate?

A

Intermediate in the glycolysis in Red blood cells

73
Q

What is the bohr effect?

A

Increases in the carbon dioxide partial pressure of blood or decreases in blood pH result in a lower affinity of hemoglobin for oxygen.

Respiratory Physiology, Medicine and Critical Care (42 decks)

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