Gas Exchange

Question 1

Describe typical gas exchange in the lungs.

Answer 1

• Lungs consist of branching system of tubes and large area of alveoli
• Air enters lungs and pressure in alveoli falls as intrathoracic pressure falls. Air draw into airways and into alveoli which expand.
• Beyond 11th generation of airways, gas transfer by diffusion, not bulk flow

Question 2

Define anatomical dead space.

Answer 2

• For each breath, some of the air remains in airways and no gas transfer
• ALVEOLAR VENTILATION = (TIDAL VOLUME - DEAD SPACE) X FREQUENCY OF BREATHING

Question 3

Give the reason for why the physiological dead space exists.

Answer 3

• Not all areas of lung are equally perfused or ventilated
• Greatest changes in intrathoracic pressure at base of lung so where most blood flow and ventilation occur
• OPPOSITE FOR apices of lungs - less blood as relatively low pulmonary artery pressure
• Dead space - due to disturbed balance of ventilatio/perfusion in disease

Question 4

What does the physiological dead space describe and how can it be calculated?

Answer 4

• Area of lungs not involved in gas exchange
• Calculated using the Bohr equation

Question 5

What are the causes of increased physiological dead space?

Answer 5

• REDUCED VENTILATION - Collapsed lung , pneumonia, abnormal alveolar structure (thickned alveolar walls in fibrosis/alveolar destruction in emphysema)
• REDUCED PERFUSION - Shunts, obstructed pulmonary arteries in pulmonary emboli

Question 6

Describe V/Q mismatch

Answer 6

• Pneumonia - alveoli are perfused but not ventilated
• COVID/ARDS - falling ventilation and local tissue hypoxia lead to decreased perfusion by hypoxic vasoconstriction
• Pulmonary emboli - reduced perfusion, ventilation of alveoli may still be normal

Question 7

Describe gas transfer for CO2

Answer 7

• Carried in blood dissolved, as bicarbonate, as carbaminohaemoglobin
• Mostly as bicarbonate
• CO2 content of air = 0.04 kPa. Arterial PCO2 = 5-6.5 kPa
• Diffuses readily across alveolar membrane and expired

Question 8

Describe the effects of ventilation on carbon dioxide transfer.

Answer 8

• INCREASED VENTILATION - greater elimination of CO2
• DECREASED VENTILATION - CO2 retention
• Arterial PCO2 rises in high V/Q mismatch and compromised ventilation

Question 9

Describe gas transfer of oxygen in alveoli.

Answer 9

• Oxygen level in alveoli lower than in air
• Due to mixing of oxygen with some of expired air which has low PO2
• ALVEOLI - O2 passes across alveolar membrane, bound in pulmonary capillaries to Hb to be delivered to tissues
• ALVEOLAR DISEASE - reduced O2 carriage

Question 10

What factors does oxygen transfer from alveoli into the blood depend on?

Answer 10

• Pressure difference between oxygen in alveoli and oxygen in pulmonary capillary
• Permeability of membrane

Question 11

Define partial pressure of a gas.

Answer 11

• Pressure that one gas in mixture of gases would exert if it were the only gas present in whole volume occupied by mixture at given temperature
• Determines the pressure gradient which gases use to move across membranes

Question 12

What is Dalton’s Law?

Answer 12

• Total pressure exerted by gaseous mixture = sum of partial pressures of each individual gas

Question 13

What is the alveolar gas equation?

Answer 13

PAO2 = PiO2 - [PaCO2/0.8]
- PAO2 - partial pressure of O2 in alveolar air
- PiO2 - partial pressure of O2 in inspired air
- PaCO2 - CO2 partial pressure in arterial blood
- 0.8 is RER (Volume of CO2 produced/O2 consumed)

Question 14

Describe the gradient between alveolar and arterial PO2.

Answer 14

• Small gradient is normal
• Big gradient - indicate problems with gas exchange in lungs or right to left shunt in lungs

Question 15

What is the formula to measure partial pressure oxygen in alveoli?

Answer 15

FiO2 (Patm- PH2O) – PaCO2/RER
- FiO2 - inspired oxygen concentration
- Patm - atmospheric/barometric pressure
- PH2O - pressure due to water vapour

Question 16

What is a simple way of calculating the gradient between the alveoli and blood vessels?

Answer 16

FiO2 - PaO2

Question 17

Describe the factors that determine oxygen transfer into the capillaries across the alveolar membrane.

Answer 17

• Pressure and concentration of oxygen
• Determined by Henry’s Law - amount of dissolved gas in liquid proportional to gas pressure above liquid
• CONCENTRATION = pressure of gas x Henry’s constant

Question 18

Describe the alveoli.

Answer 18

• Inflatable sacs
• Walls consist of single layer of flattened type I pneumocytes
• Surrounded by pulmonary capillaries
• Narrow interstitial space

Question 19

Describe the effect of surface area and membrane thickness on gas diffusion, with reference to the lungs and alveoli.

Answer 19

• Amount of gas moving across sheet of tissue per unit time proportional to area of sheet and inversely proportional to thickness
• Lungs - large SA with thin membranes
• Repeated division of airways
• Smaller airways form alveoli
• ALL OF THIS INCREASES SA for gas exchange

Question 20

How can gas transfer be measured?

Answer 20

• TLCO
• Per litre of lung volume, KCO

Question 21

What does it mean for decreased/increased TLCO?

Answer 21

• DECREASED - Anaemia, V/Q mismatch and decreased perfusion/ventilation
• INCREASED - Increased cardiac output, polycythaemia

Question 22

What does the Fick principle state?

Answer 22

Volume of gas diffusing across tissue sheet per unit time
- Proportional to area of sheet and pressure difference
- Inverself proportional to thickness
- Dependent on permeability coefficient of gas

Question 23

What is the equation for the Fick principle?

Answer 23

Rate = (Area/thickness) x pressure gradient x diffusion constant)

Question 24

What is diffusion constant dependent on?

Answer 24

• Solubility (more soluble, easier to pass)
• Molecular weight of gas

Question 25

Outline Graham’s Law

Answer 25

• Rate of diffusion inversely proportional to square root of molecular weight
• BASICALLY - big objects diffuse slower

Question 26

Using Fick’s Law, when may gas exchanged be reduced in disease?

Answer 26

• Reduced SA - lobectomy, reduced ventilation from airway obstruction with emphysema
• Increased thickness of membrane - pulmonary fibrosis, acute lung injury
• Reduced O2 concentration at high altitudes
• Inadequate time for gas transfer on exercise in lung disease

Question 27

How is gas transfer measured using carbon monoxide? PART 1

Answer 27

• Use gas which passes through alveolar membrane and not return i.e CO which binds firmly to Hb
• Measure amount of CO in gas mixture breathed in
• Hold breath for 10 seconds

Question 28

How is gas transfer measured using carbon monoxide? PART 2

Answer 28

• Measure CO concentration in end expiratory gas
• Compare against alveolar volume calculated - look at helium concentration in inspired and expired gas samples

Question 29

How would gas transfer be investigated using helium?

Answer 29

• Helium does not pass alveolar membrane at normal pressure
• Alveolar volume = HeI/HeE x size of breath taken
• HeI - helium concentration in inspired gas
• HeE - helium concentration in expired gas

Question 30

Describe gas transfer for nitrogen.

Answer 30

• Nitrogen more insoluble than O2 and CO2 - doesn’t pass into body in normal conditions