Alveolar gases and diffusion Flashcards
What are the features of convection?
Mass transfer by bulk motion, down a pressure gradient.
Flux = velocity x concentration.
Found in larger tubes - conducting airways and larger blood vessels.
Used to move O2/CO2 long distances around the body.
What are the features of diffusion?
Fick’s first law of diffusion.
Moves down concentration gradient.
At sites of gas exchange - alveoli and capillaries.
Used to move O2/CO2 short distances around the body.
How does branching affect cross-sectional area in the respiratory system?
The airways become more and smaller through the system.
Small tubes running in parallel, sum of cross sectional area sums to a larger total cross-sectional area.
Why is cross-sectional area important?
Total flow = velocity x total area.
As total area increases, velocity decreases, so total flow remains constant.
As velocity decreases, transport changes from convection to diffusion.
What is the velocity of the trachea?
Have small cross-sectional area, so have high velocity.
Transport is by convection, and relies on pressure gradient.
What is the velocity of the respiratory bronchioles?
Large cross-sectional area, so velocity is low.
Transport is by diffusion and relies on concentration gradient/partial pressure gradient.
Alveoli appear in the walls of bronchioles, due to transport being diffusion.
What is Fick’s first law of diffusion?
The rate of diffusion increases with concentration gradient, diffusivity - ease of gases to diffuse.
Diffusion rate decreases with distance/ thickness of membrane.
Diffusion = concentration gradient x diffusivity / thickness
Total diffusion flux = concentration gradient x diffusivity x surface area / thickness.
What is the surface area for diffusion?
The larger the surface area, the more gas exchange.
There are 300 million alveoli which are folded so very big surface area.
There are many capillaries surrounding each alveoli.
What is the thickness of the diffusion barrier?
Very thin, because the alveolar epithelium and capillary endothelium are very thin.
The distance between them is very short, so diffusion is rapid.
In pulmonary oedema, the gap will largen.
What is the diffusivity of O2 and CO2?
Dependent on molecular weight (larger = slower) and solubility.
CO2 is 20x more diffusible than O2.
In health complete equilibration in pulmonary capillary.
In disease e.g. pulmonary oedema, problems are seen first with O2.
What are the partial pressure gradients of O2 and CO2?
Surface area and thickness of diffusion barrier and diffusibility of O2/CO2 are constant in health.
The partial pressure gradient for O2 and CO2 is therefore crucial to drive diffusion.
What is the diffusion of oxygen in the lungs?
As blood is pumped through the capillaries, there is a certain time where gas exchange occurs.
The RBCs take around 0.75s to go from venous to arterial blood.
Under normal conditions, it takes 0.25s for equilibrium of PO2 13kPa in the alveoli and the blood to occur, about 1/3 across.
What is the diffusion reserve?
The spare capacity after equilibrium is reached, before the RBC passes by the alveolus.
This is used if the need for diffusion goes above the normal levels, and can be impacted by disease.
What happens to diffusion reserve during exercise?
Cardiac output increases, and the RBCs flow faster through the capillaries, and are in the capillary for less time.
Equilibrium of oxygen still takes 0.25s, but as blood flows faster, it won’t occur until further down the capillary, so the diffusion reserve is decreased in exercise.
Equilibrium is still reached.
Why do we become breathless during exercise?
The cardiovascular system cannot move blood fast enough to the tissues to oxygenate them, so they increase the breathing rate.
The lungs have the capacity for gas exchange and breathing rate, so this is not the cause.
What is the effect of a thickened diffusion barrier at rest?
The rate of diffusion is decreased.
It takes longer to reach equilibrium, so the diffusion reserve is decreased.
Equilibrium is still reached at rest though.
What is the effect of a thickened diffusion barrier during exercise?
During exercise, there is increased blood flow, so the RBC is in the capillary for less time.
Full equilibration is not achieved, and there is no diffusion reserve.
Arterial PO2 is reduced as the pressure gradient is less.
What is the effect of high altitude on diffusion?
There is a decreased partial pressure gradient, because there is a lower barometric PO2.
This decreases the rate of diffusion, so it takes longer to reach equilibrium.
A low PIO2 results in a lower PAO2.
What is the useful ventilation for gas exchange?
Conducting zone is anatomic dead space, always present in every breath, it has no gas exchange.
Air available to go into alveolar space is the gas available for gas exchange - about 350ml - alveolar ventilation.
Only useful ventilation can support metabolism.
What is the effect of water on pressure?
As air is inspired, the nasal cavity warms and humidifies the air, so the membranes of the alveoli are moist, not cracked.
Adding this water vapour takes up around 6kPa of pressure, so only 94kPa is available for remaining gases.
So PO2 is about 19.7kPa.
What are the advantages of anatomic dead space?
Carbon dioxide is retained, resulting in bicarbonate-buffered blood and interstitium.
Inspired air is raised or lowered to body temperature, increasing the affinity of haemoglobin for O2, and improving O2 uptake.
Particulate matter is trapped in the mucus that lines the conducting airways, allowing it to be removed by mucociliary transport.
Inspired air is humidified, so improves the quality of airway mucous.
How is PAO2 kept constant?
PAO2 is 13kPa.
Functional residual capacity (FRC) is about 2.5-3L, which is half full.
So when inspiring can add 350ml, which tops up PAO2, which has been lost from gas exchange.
How does gas composition change in the alveoli?
The conservation of mass means that O2 and CO2 do not just disappear, but the composition of gas in the alveoli can change.
Venous blood brings in a level of O2 and CO2.
Arterial blood takes away a different level of O2 and CO2.
Alveolar ventilation brings in a level of O2 and takes away CO2.
How does O2 alveolar pressure change?
As we inspire air, ventilation adds O2 to the alveoli.
Gas exchange occurs, then perfusion removes O2 from the alveoli.
To keep PAO2 constant, ventilation and perfusion/metabolism need to be balanced.
How does CO2 alveolar pressure change?
Metabolism/perfusion brings in CO2.
Ventilation then removes CO2.
So if ventilation and metabolism are balanced then PACO2 remains constant.
What happens when CO2 metabolism and ventilation are not constant?
If ventilation decreases, and metabolism is constant, then PCO2 increases. Hypoventilation occurs - ventilation below the metabolic need.
If ventilation increases above the metabolic need, and metabolism is constant, then PCO2 decreases, and hyperventilation occurs.
What is the relationship of PAO2 and PACO2?
PAO2 and PACO2 are inversely related.
If ventilation is constant, and metabolism increases, then O2 decreases, and the curve shifts down.
CO2 will increase, and the PACO2 curve shifts up.
See picture.
