Gas Exchange In The Lung Flashcards
Gas exchange involves diffusion of blood gases through multiple structures and mediums.
Describe how oxygen gets from the lungs to the blood. PART 1
- O2 enters the alveolar airspace from the atmosphere.
- O2 dissolves into the alveolar fluid lining.
- O2 diffuses through the alveolar epithelium, the basement membrane and the capillary endothelial cells.
Gas exchange involves diffusion of blood gases through multiple structures and mediums.
Describe how oxygen gets from the lungs to the blood. PART 2
- O2 dissolves in blood plasma.
- O2 binds to the haemoglobin molecule.
What is the rate of diffusion determined by?
- Partial pressure gradient between alveolar air and capillary blood
- Diffusion distance e.g epithelial/endothelial thickness or basement membrane thickness
- Alveolar surface area.
List some diseases that affect the rate of diffusion, and how they affect it.
- hypoventilation (type II respiratory failure) = lowers partial pressure gradient
- hypoperfusion (type I respiratory failure) = lowers Pc
- emphysema = lowers surface area
- fibrosis = increase basement membrane thickness
- pulmonary oedema (eg. pneumonia) = increases thickness of fluid layer/ oedema
Alveoli have intricate structures and multiple adaptations to maximise the rate of gas exchange.
List a few of them.
- Large surface area (lungs have a high surface-area to volume ratio due to their 3D structure)
- Walls are only one cell thick, and the basement membrane is fused with the blood vessel, which decreases the distance for diffusion
- Innervated by capillaries (given an adequate blood supply to maintain the gradient)
How are the pressure gradients between the alveoli and the blood maintained?
Adequate ventilation and perfusion.
What are hypoventilation and hyperventilation defined by (in terms of CO2 levels)?
- Hyperventilation is defined by excessive levels of CO2 within arterial blood
- Hypoventilation is defined by reduced levels of CO2 within arterial blood
What is the effect of increasing rate of ventilation?
- Increases alveolar oxygen partial pressure and decreases alveolar carbon dioxide partial pressure
- Leads to decreased oxygenation of blood flowing through innervating capillaries
What is the effect of decreasing rate of ventilation?
- Decreases alveolar oxygen partial pressure and increases alveolar carbon dioxide partial pressure
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.
What relationship does V/Q describe?
V/Q ratio describes the relationship between pulmonary perfusion (Q) and alveolar ventilation (V).
How is ventilation-perfusion coupling maintained?
Hypoxic vasoconstriction - diverts blood flow from poor to well-ventilated alveoli
Outline the mechanism by which hypoxic vasoconstriction maintains ventilation-perfusion coupling. PART 1
- Under normal conditions, blood flow and ventilation are matched.
- If ventilation of alveoli decreases, PACO2 will rise and PAO2 will fall.
Outline the mechanism by which hypoxic vasoconstriction maintains ventilation-perfusion coupling. PART 2
- Therefore, decreased oxygenation of blood flowing through the innervating capillaries.
- The decreased PaO2 induces vasoconstriction, 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)?
- Increase in V/Q ratio.
CAN LEAD TO:
- heart failure (cardiac arrest)
- blocked vessels (pulmonary embolism)
- loss/ damage to capillaries (emphysema)
- reduced gas exchange
What happens when there is a ventilation-perfusion mismatch (with reduced ventilation)?
- Decrease in V/Q ratio.
CAN LEAD TO:
- asthma (COPD)
- pneumonia (fibrosis)
- Blood returns to the left part of the heart from the right without taking part in gas exchange.
What is the formula for partial pressure of oxygen?
PAO2 = FIO2 x (P B - P H2O) - PaCO2/RER
FIO2 is the fraction of oxygen present in inspired gas, P B is the barometric pressure, P H2O is the water vapour pressure, PaCO2 is the arterial CO2 pressure and RER is the respiratory exchange ratio.
What is the RER (respiratory exchange ratio)?
RER = VCO2 produced/ VO2 consumed
- Describes relationship between the CO2 elimination and O2 consumption.
- Measures difference between O2 and CO2 in inspired and expired air.
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
How is partial pressure calculated?
Multiplying total pressure by mole fraction
What is concentration of a gas dissolved within a liquid determined by?
Partial pressure and solubility of a gas
What does partial pressure of a gas dissolved in a liquid mean?
Volume of gas that would dissolve at equilibrium if the liquid was placed in contact with a gas phase of equivalent partial pressure
What features do you want for maximum diffusion?
High partial pressure gradient
High surface area
Low distance
What happens if the VQ ratio is more than 1?
Hypoperfusion (dead space effect)
What happens if the VQ ratio is less than 1?
Hypoventilation (shunt)
What happens when an embolism blocks the pulmonary artery supplying a region of the lung?
Perfusion to the other alveoli increases as cardiac output is diverted
Unless ventilation of alveoli is increased, hypoxia and hypercapnia will occur
What causes reduced perfusion to lungs?
Heart failure
Blocked vessels
Loss/damage to capillaries
What is a shunt in the lungs?
Perfusion without ventilation
What causes shunt in the lungs?
Pneumonia, acute lung injury, respiratory distress syndrome
What does reduced ventilation do to the VQ ratio?
Decrease it
What is the equation for concentration of dissolved gas in a liquid?
Concentration = Partial pressure x Solubility
Why does efficient gas exchange need V/Q coupling?
- For effective oxygenation to take place ensure any blood that flows through pulmonary capillaries are perfusing an alveoli which is ventilated properly
What causes VQ to go above 1?
Perfusion (Q) decreases in a particular part of the alveoli but ventilation (V) stays the same
What causes VQ go below 1?
Ventilation (V) decreases but perfusion (Q) stays the same
Why does PaCO2 stay the same during VQ mismatch?
As partial pressure of CO2 increases hyperventilation induced within the lung which clears the excess CO2
How does a pulmonary embolism lead to V/Q mismatch? PART 1
- Occludes blood vessels - blood is unable to flow to the alveoli (called physiological dead space)
- As a result the alveoli has an increase in the V/Q (as perfusion has decreased).
- Since cardiac output remains the same the blood will now flow through another alveoli
How does a pulmonary embolism lead to V/Q mismatch? PART 2
- In this secondary alveoli V/Q decreases (as perfusion increases)
- Excessive perfusion for the amount of ventilation
- Blood flows through the alveoli which are not fully oxygenated before it returns back to the heart
What is a physiological dead space?
- Alveoli within the lung that do not carry out gas exchange due to reduced perfusion
- There is ventilation without perfusion and hence occurs when there is an increase in V/Q
- Responds well to oxygen theraphy
What is a pulmonary shunt effect?
- Movement of blood across the alveoli without taking part in gas exchange
- Occurs when there is a decrease in ventilation but perfusion stays the same and hence V/Q will decrease
- Poor response to oxygen therapy
Why does oxygen therapy not produce a good response during a shunt? PART 1
- Upon perfusion of blood past the inactive alveoli they will return with the same partial pressure of oxygen they entered
- Upon pumping oxygen, oxygen will flow into a secondary alveoli and improve the ventilation here
- As a result it will cause a higher increase in partial pressure in the blood
Why does oxygen therapy not produce a good response during a shunt? PART 2
- Finite amount of haemoglobin in the blood
- Only a finite amount of oxygen can be carried and so the extra oxygen that has been taken in cannot compensate for the oxygen lost by the non-functional alveoli
For gases, why is pressure used rather than concentration?
Gases react, dissolve, and diffuse in accordance with their pressure
