3. Gas Exchange And Diffusion Flashcards
What is the definition of partial pressure?
Partial pressure of a gas is the pressure exerted by an individual gas in a mixture of gases
What is the atmospheric pressure at sea level?
760mmHg/101kPa
What is the percentage of oxygen in the air?
20.9%
What is the saturated vapour pressure?
6.28kPa at body temperature
What is the partial pressure of oxygen in upper respiratory tract?
19.8kPa
What is the alveolar partial pressure of oxygen?
13.3kPa
Why is alveolar partial pressure of oxygen lower than that in the upper respiratory tract?
Oxygen from alveolar air constantly diffusing into pulmonary circulation
Alveolar air only partially replaced with each breath
What is the average tidal volume?
450ml
How much of normal tidal volume fills anatomical dead space?
30% (~150ml)
What is the average alveolar ventilation?
300ml
450-150
What happens when inspired gases come into contact with body fluids?
Gas molecules will enter fluid and dissolve n the liquid
The amount of gas that dissolves is directly proportional to the partial pressure of that gas
How do you work out the dissolved amount of gas in a liquid as a concentration?
Partial pressure of that gas x gas solubility coefficient
What is solubility coefficient of a gas?
Volume of gas that can be dissolved in a fixed volume of solvent at a given temperature
What is the solubility coefficient of O2 in plasma?
0.01mmol/L/kPa
What 2 factors can affect diffusion of gases?
Solubility
Molecular weight
Why does CO2 diffuse faster than O2?
CO2 is more soluble so the effect of solubility is much greater
How does O2 compensate for slower diffusion?
Larger difference in partial pressure
What does gas diffusing from alveolar air to RBC in capillary have to cross?
Fluid film lining alveolus Epithelial cell of alveolus Interstitial space Endothelial cell of capillary Plasma Red cell membrane
What factors affect rate of gas diffusion in disease?
Thickness of membrane - thicker, impaired diffusion
Surface area of membrane - decreased, impaired diffusion
Diffusion coefficient of gas - low pO2
What are examples of diseases with diffusion impairment?
Fibrotic lung disease
Emphysema
Pulmonary oedema
What is the V/Q ratio?
Ventilation to perfusion ratio
1=normal
<1 less ventilation
>1 more ventilation
What is V/Q mismatch?
When perfusion exceeds ventilation <1, low V/Q
Arterial blood will have low oxygen levels
What happens when alveolar PaO2 is low?
Indicative of impaired ventilation, hypoxic vasoconstriction of pulmonary arterioles occurs
Diverts blood to better ventilated alveoli
What is V/Q in normal lungs?
0.8-0.9
Why do we have V/Q mismatch in normal lungs?
Reserve - V/Q match approaches 1 when we exercise
What is V/Q mismatch the most common cause of?
Systemic arterial hypoxaemia in people with cardio-pulmonary disease
What is hypoxaemia?
Low partial pressure of oxygen in arterial blood
What diseases/conditions cause V/Q mismatch?
Asthma Early stages of COPD Pneumonia Respiratory distress syndrome in the newborn Pulmonary oedema Pulmonary embolism
Why can’t over-ventilated alveoli compensate for under-ventilated alveoli?
Going from a partial pressure of 13kPa to 15kPa only increases dissolved oxygen by 0.02 mmol/L
Haemoglobin is already carrying all that is can carry - it is saturated
So very little additional oxygen for loading onto haemoglobin from poorly perfused alveoli
What happens to alveoli in pulmonary embolism?
There are alveoli that receive no blood, though they are still ventilated, this s known as dead space
V/Q = 1/0 = infinity
If ventilation to other alveoli cant increase commensurate with the increased blood flow then V/Q mismatch
