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Flashcards in Gas Exchange Deck (21):

What factors affect diffusion rate of a gas in a liquid?

• Solubility
• Pressure difference
• Cross section area of the fluid
• Distance the molecules must diffuse
• Molecular weight of the gas
• Temperature of the fluid (as we’re in the human body this is negligible)


How do oxygen and carbon dioxide differ in terms of solubility?

CO2 is much more soluble than oxygen and so diffuses in a liquid 20 times faster than oxygen hence why the gradient is able to be smaller.

This difference in diffusion rates is cancelled out in the body by oxygen having a much larger difference in partial pressure. As a result, in a diseased lung oxygen is usually more impaired than CO2 because of oxygen’s slower diffusion rate.


What is the diffusion co-efficient?

The diffusion coefficient (mobility once dissolved) of a molecule is proportional to the Solubility/√Molecular weight.


What barriers are there in the way of oxygen and carbon dioxide diffusing between capillaries and the air in alveoli?

• 5 cell membrane to diffuse across (red blood cell membrane is the 5th)
• 3 layers of cytoplasm
• 2 layers of tissue fluid


What is the distance form the alveoli to the capillary lumen?

Total distance to the capillary lumen is only about 0.6um and even less when inhaling.


What is diffusion resistance related to?

Diffusion resistance depends upon the nature of the barrier and of the gas – Permeability = (Diffusion coefficient (D) x Solubility in lipid membrane)/Thickness of the membrane. Note that larger molecules will have smaller diffusion coefficients.


How does the saturation of water in air change as we breath in and what does this mean for the other gases?

Variable amount of water vapour – changed from 2% to 6%
Inspired air must be saturated with water for diffusion to happen effectively. Therefore, the other gases are diluted slightly.


Why are the partial pressure of oxygen in alveoli and of the air that is exhaled into the trachea not what we'd expect it to be?

In alveoli PO2 is much lower 13% due to the old air left in the alveoli and diluting effect of CO2 moving in but PO2 is still higher than capillaries so diffusion takes place. As we exhale it mixes again with high percentage of oxygen and so this increases again to 15% in the exhaled air of the trachea.


What are the percentages of gases in air: Inhaled, tracheal, alveolar and exhaled.

Inhaled - O2 21%, CO2 0.04% and water 0.5%

Tracheal - O2 19.6%, CO2 0.04% and water 6.2%

Alveolar - O2 13.3%, CO2 5.3% and water 6.2%

Exhaled - O2 15.7%, CO2 3.6% and water 6.2%

Nitrogen is just 100 minus all of these


What happens to the carbon dioxide in our blood?

Carbon dioxide reacts with water in the blood to form carbonic acid. This dissociates to bicarbonate ion and hydrogen ions.


What forms is carbon dioxide carried around in in the body?

This H+ is buffered by haemoglobin inside the RBC. About 5% of total body CO2 dissolves in the plasma, 5% is carried in carbamino compounds on proteins 90% is carried as bicarbonate ions in the plasma.


What partial pressures of oxygen and carbon dioxide are there in blood leaving the alveolar capillaries, arriving at the lungs and in ambient air?

PCO2 is higher in the capillaries feeding the alveoli than the air in the alveoli so CO2 diffuses out of the blood. However, blood leaving the alveolar is at equilibrium with alveolar air so has the same PO2 and PCO2 (13.3kPa and 5.1KPa respectively). In venous blood the typical values are PO2 = 5.3kPa and PCO2 = 6.1kPa finally in ambient air PCO2 is typically 0.04kPa and PO2 is typically 20.8kPa.


How quickly is the equilibrium in partial pressure reached between alveolar air and capillary blood?

When passing through the capillaries the PO2 in the blood usually equals about the same as in the alveoli by a 1/3 of its distance. This is important as when heart rate and cardiac output increases during exercise blood spends ½ as much time passing the alveoli. This is similar for the diffusion of CO2. As CO2 diffuses 20x faster than oxygen CO2 is never the limiting step.


What factors affect the rate of diffusion?

1. Thickness of membrane – this increases with oedema and fibrotic lung tissue
2. Surface area of the membrane – increases in emphysema due to combining of alveoli
3. Diffusion coefficient of the gas – never changes
4. Pressure difference of the gas – changes with altitude and humidity


What is Dead space?

Dead space is the parts of the respiratory system that do not partake in (effective) gas exchange and so the air in them is ‘wasted’ and not used. This is known as the serial or anatomical dead space and is usually about 0.15l.


What is Distributive dead space?

Distributive dead space is any air in dead or damaged alveoli or alveoli that aren’t properly perfused and equates to about 0.02l.


What is physiological dead space?

Distributive dead space + anatomical dead space


What is a normal tidal volume?



What is alveolar ventilation rate?

Alveolar ventilation rate is the pulmonary ventilation rate minus the dead space volume times by the respiration rate (14-16).


What is AVR (alveolar ventilation rate) normally?

AVR is about 350ml and with a RR of 14 this about 5l/min CO is also about 5l/min so the V/Q (ventilation/perfusion) ratio is about 1 ideally.


How does the V/Q ratio change as you change perfusion and ventilation?

V/Q changes from zero to infinity as you decrease perfusion from max and increase ventilation from nil.