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Flashcards in Gas Exchange in the Lungs Deck (19)
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What is the difference between Pa and PA?

Pa = Arteriole pressure.

PA = Alveolar pressure.


What is the condition where there is too much CO2 in the blood called?

Hypercapnia -

  • It is a condition arising from too much carbon dioxide in the blood.
  • It is often caused by hypoventilation or disordered breathing where not enough oxygen enters the lungs and not enough carbon dioxide is emitted.


Describe how oxygen gets from the lungs to the blood.

  1. O2 enters the alveolar airspace from the atmosphere.
  2. O2 dissolves into the alveolar fluid lining.
  3. O2 diffuses through the alveolar epithelium, the basement membrane and the capillary endothelial cells.
  4. O2 dissolves in blood plasma.
  5. O2 binds to the haemoglobin molecule.


What could lead to the abnormal (reduced) diffusion of oxygen into the blood?

The thickening of blood-gas barrier can lead to the abnormal (reduced) diffusion of oxygen into the blood.


What is the rate of diffusion determined by?

Rate of diffusion ∝ ( Surface area / distance) X ( PA - PC )

  • The surface area is the alveolar surface area.
  • The distance is the epithelial and endothelial cell thickness + bsement membrane thickness + fluid layer depth.
  • The partial pressure gradient is the gradient between alveolar air (PA) and capillary blood (PC).


What are the conditions for maximal diffusion? What diseases can affect these conditions and decrease the rate of gas exchange?

  1. A high partial pressure gradient - 
    1. Hypoventilation (type II respiratory failure) = lowers PA. 
    2. Hypoperfusion (type I respiratory failure) = lowers Pc.
  2. A high surface area - 
    1. Emphysema = lowers surface area.
  3. A low distance (barrier thickness) - 
    1. Fibrosis = increase basement membrane thickness.
    2. Pulmonary oedema (eg. pneumonia) = increases thickness of fluid layer / oedema.

Hypoperfusion meaning - It is the decreased blood flow through an organ.

Emphysema meaning - In this condition the breathing tubes are narrowed and the air sacs are damaged.


How are the pressure gradients between the alveoli and the blood maintained?

  • They are maintained by adequate ventilation and adequate perfusion.

Perfusion meaning - the passage of blood, a blood substitute, or other fluid through the blood vessels or other natural channels in an organ or tissue.


What adaptations do alveoli have to maximise the rate of gas exchange?

  • They have a large surface area (lungs have a high surface-area to volume ratio due to their 3D structure).
  • Their walls are only one cell thick, and the basement membrane is fused with the blood vessel, which decreased the distance for diffusion.
  • They are richly supplied by capillaries (given an adequate blood supply to maintain the gradient).


What are hypoventilation and hyperventilation defined by (in terms of CO2 levels)?

  1. Hyperventilation -
    1. It is defined by reduced levels of CO2 within arterial blood (PaCO2 < 4.9 kPa). [Same O2 levels]
    2. It can lead to hypocapnia.
  2. Hypoventilation - 
    1. It is defined by increased levels of CO2 within arterial blood (PaCO2 > 4.9 kPa). [Low O2 levels]
    2. Causes respiratory acidosis.
    3. It can lead to hypercapnia.
    4. It is the precursor to hypoxia (reduced O2 levels in blood).
  • Increasing the rate of ventilation increases alveolar oxygen partial pressure (PAO2), and decreases alveolar carbon dioxide partial pressure (PACO2). Decreasing the rate of ventilation has the opposite effects.

Hypocapnia meaning - is a state of reduced carbon dioxidein the blood.

Respiratory acidosis meaning -  When decreased ventilation (hypoventilation) increases the concentration of carbon dioxide in the blood and decreases the blood's pH.


Explain how adequate perfusion is required for maintaining the pressure gradients for diffusion.

  • Blood flow through the pulmonary capillaries (perfusion, Q) needs to be matched to alveolar ventilation (VA) to enable efficient gas exchange, as there is a maximum amount of O2 each unit of blood can carry.
  • This relationship is described by the V/Q ratio.


What relationship does V/Q ratio describe?

  • The V/Q ratio describes the relationship between pulmonary perfusion (Q) and alveolar ventilation (V).
  • 1L of blood can carry ~ 200 ml of O2, and 1L of dry air contains about 200 ml of O2.
  • Therefore, ideally, the V/Q value should be = 1.
  • At rest, ventilation and perfusion are both around 5L/min, so the V/Q should be ~ 1 - 0.8.


How is ventilation-perfusion coupling maintained?

Ventilation-perfusion coupling is maintained by hypoxic vasoconstriction. Homeostatic mechanisms exist to reduce ventilation-perfusion mismatching. Hypoxic vasoconstriction of capillaries diverts blood flow from poor to well-ventilated alveoli.

  1. Under normal conditions, blood flow and ventilation are matched.
  2. If ventilation of specific alveoli decreases, PACO2 will rise and PAO2 will fall. Therefore, there is decreased oxygenation of blood flowing through the innervating capillaries.
  3. The decreased PaO2 induces vasoconstriction (it causes the smooth muscle in the blood vessels to constrict), which decreases blood flow. The blood flow is then diverted to alveoli with increased ventilation.


What happens when there is a ventilation-perfusion mismatch (with reduced perfusion)?

  • The reduced perfusion of lung regions causes an increase in V/Q ratio.
  • This could lead to: -
    1. Heart failure (cardiac arrest).
    2. Blocked vessels (pulmonary embolism).
    3. Loss / damage to capillaries (emphysema).
  • The affected alveoli become physiological dead-space, as there is no / reduced gas exchange.

In the case for pulmonary embolism you can compensate for the reduced V/Q ratio by increasing the ventilation to the alveoli that is receiving more blood.

Pulmonary embolism meaning - It is a blockage of an artery in the lungs by a substance that has moved from elsewhere in the body through the bloodstream (embolism).


What happens when there is a ventilation-perfusion mismatch (with reduced ventilation)?

  • Reduced ventilation of alveoli (or limits to diffusion) causes a decrease in V/Q ratio.
  • This could lead to: -
    1. Pneumonia.
    2. Acute lung injury.
    3. Respiratory distress syndrome.
    4. Atelectasis.
  • Blood returns to the left part of the heart from the right without taking part in gas exchange (shunt).

Shunt-induced hypoxaemia responds poorly to supplemental oxygen therapy.

Steps in shunt:-

  1. In a alveoli where there is shunt there is a decrease in gas exchange.
  2. However the blood is still being perfused by the capillary that innovated it.
  3. The blood arrives at 60% saturation to the alveoli and leaves the alveoli at 60% saturation (as they are not being oxygenated properly).
  4. The blood that goes through the capillary to the working parts of the lung is still being oxygenated (it goes from 60% to 90%).
  5. When the improperly saturated (60%) and peoperly saturated (90%) blood mix together its much worse than before (75%).
  6. Since the saturation of oxygen is tightly regulated a small change can cause a lot of bad effects.

Atelectasis meaning - It is the collapse or closure of a lung resulting in reduced or absent gas exchange. It may affect part or all of a lung.


Why does shunt induced hypoxaemia respond poorly to supplemental oxygen therapy?

  1. The oxygen can only be pumped into the working part of the lung. The other part of the lung does not get the oxygen as it is not being ventilated properly (so it cant get any more oxygen).
  2. This increases the gas exchange in the working part of the lung leading to 100% saturation coming from the working part of the lung.
  3. The blood from the non working part of the lung is only 60% saturated.
  4. When the blood from both the parts mix it produces a blood saturation level of 80% (this is still lower than the 90% normal).
  5. You will never get a normal healthy saturation.


What happens during ventilation - perfusion in a normal healthy lung?

  1. The blood from both the sides of the lungs are oxygenated from 60% to 90%.
  2. It then mixes together to give 90% saturation of the blood returning to the systemic circulation.


How can we determine the cause of hypoxaemia?

  • Clinical situations arise where it is useful to know alveolar oxygen pressure (PAO2), for example, when determining the cause of respiratory failure.
  • We need to determine if it is due to hypoventilation or poor oxygenation.
  • It is not practical in such cases to sample gas directly from the alveoli; however, PAO2 can easily be calculated from other measurements using the alveolar gas equation:
  • PAO2 = FIO2 x (PB - PH2O) - PaCO2/RER
    • FIO2 is the fraction of oxygen present in inspired gas. 
    • PB is the barometric pressure.
    • PH2O is the water vapour pressure.
    • PaCO2 is the arterial CO2 pressure.
    • RER is the respiratory exchange ratio.
  • The normal value of PaO2 - PAO2 (A - a gradient) ~ <2kPa.
  • If the value is higher than 2 then it indicates that there are gas exchange or oxygenation problems.


What is the RER (respiratory exchange ratio)?

  • RER = VCO2 produced / VO2 consumed.
  • The RER describes the relationship between the CO2 elimination and O2 consumption.
  • It measures the difference between O2 and CO2 in inspired and expired air.
  • The main determinant of RER is the particular metabolic substrate being used (eg. for fat / carb, the RER for the modern diet is ~ 0.8).


What is the RER for the oxidation of carbohydrates, and for the oxidation of fatty acids?

  • Oxidation of CARBOHYDRATES = 1
  • Oxidation of FATTY ACIDS = 0.7

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