Gas Exchange Flashcards
Describe typical gas exchange in the lungs.
- 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
Define anatomical dead space.
- For each breath, some of the air remains in airways and no gas transfer
- ALVEOLAR VENTILATION = (TIDAL VOLUME - DEAD SPACE) X FREQUENCY OF BREATHING
Give the reason for why the physiological dead space exists.
- 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
What does the physiological dead space describe and how can it be calculated?
- Area of lungs not involved in gas exchange
- Calculated using the Bohr equation
What are the causes of increased physiological dead space?
- 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
Describe V/Q mismatch
- 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
Describe gas transfer for CO2
- 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
Describe the effects of ventilation on carbon dioxide transfer.
- INCREASED VENTILATION - greater elimination of CO2
- DECREASED VENTILATION - CO2 retention
- Arterial PCO2 rises in high V/Q mismatch and compromised ventilation
Describe gas transfer of oxygen in alveoli.
- 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
What factors does oxygen transfer from alveoli into the blood depend on?
- Pressure difference between oxygen in alveoli and oxygen in pulmonary capillary
- Permeability of membrane
Define partial pressure of a gas.
- 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
What is Dalton’s Law?
- Total pressure exerted by gaseous mixture = sum of partial pressures of each individual gas
What is the alveolar gas equation?
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)
Describe the gradient between alveolar and arterial PO2.
- Small gradient is normal
- Big gradient - indicate problems with gas exchange in lungs or right to left shunt in lungs
What is the formula to measure partial pressure oxygen in alveoli?
FiO2 (Patm- PH2O) – PaCO2/RER
- FiO2 - inspired oxygen concentration
- Patm - atmospheric/barometric pressure
- PH2O - pressure due to water vapour
What is a simple way of calculating the gradient between the alveoli and blood vessels?
FiO2 - PaO2
Describe the factors that determine oxygen transfer into the capillaries across the alveolar membrane.
- 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
Describe the alveoli.
- Inflatable sacs
- Walls consist of single layer of flattened type I pneumocytes
- Surrounded by pulmonary capillaries
- Narrow interstitial space
Describe the effect of surface area and membrane thickness on gas diffusion, with reference to the lungs and alveoli.
- 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
How can gas transfer be measured?
- TLCO
- Per litre of lung volume, KCO
What does it mean for decreased/increased TLCO?
- DECREASED - Anaemia, V/Q mismatch and decreased perfusion/ventilation
- INCREASED - Increased cardiac output, polycythaemia
What does the Fick principle state?
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
What is the equation for the Fick principle?
Rate = (Area/thickness) x pressure gradient x diffusion constant)
What is diffusion constant dependent on?
- Solubility (more soluble, easier to pass)
- Molecular weight of gas
Outline Graham’s Law
- Rate of diffusion inversely proportional to square root of molecular weight
- BASICALLY - big objects diffuse slower
Using Fick’s Law, when may gas exchanged be reduced in disease?
- 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
How is gas transfer measured using carbon monoxide? PART 1
- 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
How is gas transfer measured using carbon monoxide? PART 2
- Measure CO concentration in end expiratory gas
- Compare against alveolar volume calculated - look at helium concentration in inspired and expired gas samples
How would gas transfer be investigated using helium?
- 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
Describe gas transfer for nitrogen.
- Nitrogen more insoluble than O2 and CO2 - doesn’t pass into body in normal conditions
