18) Gas exchange in the lungs

Question 1

What is air?

Answer 1

• Air is a mixture of gases which behave according to their partial pressures rather than their concentrations

Question 2

What is partial pressure?

Answer 2

• The amount of pressure a particular gas contributes to the total pressure of a mixture of gases

Question 3

What is the equation for partial pressure of a gas?

Answer 3

• Pressure of individual 𝑔𝑎𝑠=(atmospheric pressure − pressure of water vapour)× mole fraction of gas
• The mole fraction is shows what % of the total moles the individual mole makes up

Question 4

What is the equation for concentration of dissolved gas in a liquid?

Answer 4

• Concentration = Partial pressure x Solubility
• The partial pressure of gas dissolved in a liquid is the amount of gas that would dissolve in a liquid, at equilibrium, if the liquid was placed in contact with a gas phase of equivalent partial pressure
• By doubling the pressure of surrounding gas we will double the concentration of dissolved gases in a liquid

Question 5

How does oxygen get from the atmosphere to the blood?

Answer 5

• Oxygen enters the alveolar air space from the atmosphere and dissolves into the Alveolar Lining Fluid (ALF)
• The oxygen diffuses through the alveolar epithelium, basement membrane, and capillary endothelial cells
• The oxygen dissolves into the blood plasma where it binds to haemoglobin in the blood

Question 6

How long must oxygenation take?

Answer 6

• It must occur during the time it takes for a red blood cell to flow through the pulmonary capillaries
• In some respiratory illnesses there may be abnormal/reduced gas exchange (i.e. through the thickening of diffusion surfaces) which causes oxygenation to take longer or full oxygenation does not take place

Question 7

What is gas exchange?

Answer 7

• The diffusion of gasses between air and blood

Question 8

What is the equation for rate of diffusion/gas exchange?

Answer 8

• Rate of diffusion ∝ (surface area/distance^2) x (Pa-Pc)
• Surface area: Alveolar surface area
• Distance: Thickness of epithelial and endothelial cells, basement membrane and fluid layer depth
• Pa-Pc: Partial pressure gradient between alveolar air and capillary blood

Question 9

How do we obtain maximum diffusion?

Answer 9

• Steeper partial pressure gradient (Hypoventilation can decreases partial pressure gradient)
• Increased surface area (Emphysema can decrease surface area as the lungs degrade)
• Decreased distance/barrier thickness (Fibrosis can increase the basement membrane thickness so increases distance)

Question 10

How are alveoli adapted for efficient gas exchange?

Answer 10

• Large surface area as they have a large surface area to volume ratio because of their 3D structure
• The distance is kept short as their wall is one cell thick and the basement membrane is fused with blood vessels
• They have a consistently steep partial pressure as they are richly perfused by capillaries

Question 11

What is V/Q coupling?

Answer 11

• The ratio of blood flow (perfusion, Q) to volume of oxygen (ventilation, V) in the alveoli
• The healthy ideal V/Q ratio is 1 or anything close

Question 12

Why does efficient gas exchange need V/Q coupling?

Answer 12

• There is a maximum amount of oxygen each unit of blood can carry as there is only a finite amount of haemoglobin in a volume of blood.
• This means in order for effective oxygenation to take place we must ensure any blood that flows through pulmonary capillaries are perfusing an alveoli which is ventilated properly (i.e. the perfusion is matched with the ventilation)
• If blood travels to alveoli that is not ventilated properly, less gas exchange will take place and so blood will go back to systemic circulation less oxygenated and which will dilute the overall level of oxygen being circulated

Question 13

What can occur when the levels of perfusion and ventilation are changed?

Answer 13

• If perfusion (Q) decreases in a particular part of the alveoli but ventilation (V) stays the same it will cause VQ to increase above 1 causing hyperperfusion leading to the ‘dead space effect’
• If ventilation (V) decreases but perfusion (Q) stays the same the V/Q will decrease below 1 causing hypoventilation leading to a ‘pulmonary shunt’

Question 14

How is V/Q coupling maintained?

Answer 14

• Homeostatic mechanisms exist to reduce V/Q mismatch.
• Hypoxic vasoconstriction of capillaries diverts blood from poorly ventilated alveoli to well ventilated alveoli
• This is done through receptors found in the endothelium of the alveoli which detect/respond to levels of oxygen.
• If the receptor detects hypoxic conditions it will cause the smooth muscle to vasoconstrict (squeeze) which makes it harder for the blood to flow through (by increasing the resistance)
• This causes blood to flow to another vessel which is open (has less resistance)
• Hence this helps match ventilation to perfusion

Question 15

What happens when V/Q mismatch occurs?

Answer 15

• Gas exchange will be reduced as a result
• V/Q mismatch does not take into account the overall level of ventilation or perfusion of the lung but rather takes into account each individual alveoli
• V/Q mismatch can affect oxygen and carbon dioxide exchange.
• In most cases partial pressure of CO2 will stay the same. This is because as partial pressure of CO2 increases it will induce a hyperventilation within the lung which clears the excess CO2
• However it doesn’t affect partial pressure of oxygen

Question 16

How does a pulmonary embolism lead to V/Q mismatch?

Answer 16

• When a pulmonary embolism occurs it will occlude blood vessels that supplies a part of the lung. In this case 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).
• However since cardiac output remains the same the blood will now flow through another alveoli as it cannot flow through the alveoli
• In this secondary alveoli V/Q decreases (as perfusion increases)
• This means there is excessive perfusion for the amount of ventilation and hence there will be blood flowing through the alveoli which are not fully oxygenated before it returns back to the heart
• In order to treat this V/Q mismatch we can increase ventilation and so hypoxemia is treated

Question 17

What is a physiological dead space?

Answer 17

• Alveoli within the lung that do not carry out gas exchange due to reduced perfusion are known as physiological dead space
• There is ventilation without perfusion and hence occurs when there is an increase in V/Q
• Responds well to oxygen theraphy

Question 18

What is a pulmonary shunt effect?

Answer 18

• The movement of blood across the alveoli without taking part in gas exchange is known as a pulmonary shunt.
• It occurs when there is a decrease in ventilation but perfusion stays the same and hence V/Q will decrease
• Poor response to oxygen therapy

Question 19

Why does oxygen therapy not produce a good response during a shunt?

Answer 19

• In a shunt one of the alveoli has completely stopped working (e.g. oedema)
• Upon perfusion of blood past this alveoli they will return with the same partial pressure of oxygen they entered
• If we pump oxygen into the lungs they 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 compared to if oxygen was not supplied
• However since there is only a 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
• Hence hypoxemia still occurs