S3: gas exchange & oxygen transport Flashcards
Explain the concept of the ‘partial pressure’ of an individual gas in a gas mixture
Dalton’s law = in a mixture of gases, each component gas exerts a ‘partial pressure’ in proportion to its volume percentage in the mixture
The sum of the partial pressures of all the gases equals the total pressure
List the partial pressure of constituent gases in atmospheric air
Total atmospheric pressure at sea level = 101 kPa
Partial pressure of O2 = 21.1 kPa
Partial pressure of N2 = 78.7 kPa
Partial pressure of CO2 = 0.03 kPa
Explain the effects of altitude on partial pressure in atmospheric air
Pressure exerted by the weight of the air above the earth in the atmosphere
Therefore, at high altitudes, atmospheric pressure is lower (weight of air pressing down is less)
Partial pressure of oxygen is lower
Explain the effect of saturated water vapour pressure on partial pressure of inhaled gases such as oxygen
When air enters our upper respiratory tract it is humidified – water molecules are added to air
Water molecules entering the air exert vapour pressure; only affected by temperature
Saturated vapour pressure (SVP) = 6.28kPa at body temp.
-SVP displaces 6.28kPa of the atmospheric gas pressure, total gas pressure becomes 94.72kPa
Reduces the partial pressure of inhaled gases
Explain what is meant by “partial pressure of oxygen” in blood, and how it is different from the “content” of oxygen in the blood
Total amount of oxygen in the blood is determined by the oxygen bound to Hb & the amount of oxygen dissolved in the blood
Arterial partial pressure of oxygen in the blood is a function of the amount of dissolved oxygen, not a reflection of the amount of oxygen carried on Hb
Oxygen bound to Hb does not contribute to arterial partial pressure as it is no longer a free gas but rather is chemically bound
Explain the different partial pressures of O2 and CO2 observed in inspired air and alveolar air
When we breathe, we do not completely replace all air in lungs
Healthy lungs – 30% of normal tidal volumes fills anatomical dead space
70% reaches respiratory portion of lung = alveolar ventilation (affected by rate and depth of breathing)
Fresh air is diluted by older air in lung
Older air has had O2 continually extracted & CO2 constantly being added
Percentage O2 less in alveolar air, hence PAO2 is lower than in URT and PACO2 is higher
State the normal PO2 and PCO2 in alveolar air, arterial blood and mixed venous blood
PvO2 = 6 kPa PvCO2 = 6.1 kPa PAO2 = 13.3 kPa PACO2 = 5.3 kPa PaO2 = 13.3 kPa PaCO2 = 5.3 kPa
List the layers making up the diffusion barrier at the air-blood interphase
Alveolar epithelium Tissue fluid Capillary endothelium Plasma Red cell membrane
Describe factors affecting the rate of diffusion across the air blood interphase
Membrane thickness
Membrane surface area
Pressure difference across the membrane
Diffusion coefficient of the gas
Describe diseases that cause diffusion defects
1) Interstitial lung disease – characterised by excessive deposition of collagen in the interstitial space -> lengthens diffusion pathway
2) Pulmonary oedema – fluid in the interstitium & alveolus increases the length of the diffusion pathway
3) Emphysema – destruction of alveolar walls results in large airspaces, rather than in multiple small airspaces -> reduces total surface area (DIFFUSION DISTANCE REMAINS UNCHANGED)
Explain why gas exchange depends on the partial pressure gradient across the diffusion barrier
In the body gases diffuse down their partial pressure gradient
From an area of high partial pressure to an area of low partial pressure
After Hb is fully saturated, O2 continues to move down its partial pressure gradient until equilibrium is reached (equilibrium is when pO2 of plasma = pO2 of alveolar air)
State and explain the difference in the diffusion rates of O2 and CO2
CO2 is much more soluble than O2 – so diffuses faster than O2
Molecular weight of CO2 > O2 molecules weight (slows down CO2)
However, effect of solubility is greater & CO2 diffuses 20 times faster than O2
Explain why lung disease causing a diffusion defect affects the diffusion of O2 more than the diffusion of CO2
In diseases with impaired diffusion:
-CO2 always diffuses faster than O2
-diffusion of O2 affected -> pO2 is low
-diffusion of CO2 not affected -> pCO2 is normal
Lower alveolar O2 partial pressure means there is less partial pressure gradient for O2 to diffuse (O2 diffuses slower & in healthy lungs there is a larger difference in partial pressure which compensates for this)
List the properties of the haemoglobin molecule which facilitate the transport of oxygen in the blood
Low affinity for oxygen in the T state (tense) – difficult for oxygen to bind
High affinity for oxygen in R state (relaxed) – easier for oxygen to bind
When pO2 is low Hb is tense:
-hard for the first O2 molecule to bind
-as each O2 binds the molecule becomes more relaxed and binding of the next O2 molecule is easier = POSITIVE COOPERATIVITY
Describe the ventilation-perfusion mismatch
If we perfuse a poorly ventilated alveolus then there will be some gas exchange but it will be limited – V:Q < 1
If we ventilate a poorly perfused alveolus then there will be some gas exchange, but it will again be limited – V:Q > 1
Gas exchange is determined by both ventilation and perfusion – optimal when ventilation is matched with perfusion
Global V/Q ratio for healthy resting lung is approx. 0.9
