Gaseous diffusion 1 Flashcards
What is FO2?
Fractional concentration of oxygen
What is PIO2?
Partial pressure of inspired oxygen
What is PAO2?
Partial pressure of alveolar oxygen
What is PaO2?
Partial pressure of arterial oxygen
What is Dalton’s Law?
In a mixture of non-reacting gases, total pressure exerted is sum of individual partial pressures
How can you calculate PO2?
FO2 x PB
What is Henry’s Law?
C = kP
As the partial pressure of the gas increases, more of the gas dissolves in the liquid
Which is more soluble - oxygen or nitrogen?
Oxygen is twice as soluble as nitrogen (higher Henry’s Law constant)
Which is more soluble - carbon dioxide or oxygen?
Carbon dioxide is 20x more soluble than oxygen (much higher Henry’s Law constant)
What is the bends?
Nitrogen less likely to dissolve in plasma than oxygen
Pressure surrounding you is greater during diving
More nitrogen goes into solution
When you ascend, if you go to quickly, nitrogen will rapidly come out of solution and form bubbles in muscles/SC
What 2 factors does the water vapour pressure (PH20) depend on?
Temperature
Saturation
What is the water vapour pressure in the lungs?
6.3kPa
Is the water vapour pressure in the lungs constant or does it change?
Constant at 6.3kPa
100% saturated
What is the alveolar gas equation?
PAO2 = PIO2 - PACO2/R
What is the respiratory exchange ratio, R?
CO2 production/O2 consumption
What is the PO2 of atmospheric air?
21kPa
What is the PO2 in alveolar gas?
13.5kPa
What is the PCO2 in alveolar gas?
5.3kPa
What is the PH20 in alveolar gas?
6.3kPa
What is the PO2 in mixed venous blood?
5.3kPa
What drives O2 movement across alveoli into capillary?
Partial pressure gradient
What determines the rate of gas transfer?
Partial pressure gradient NOT concentration gradient
What limits diffusion of gases in the lung?
Respiratory disease - disrupts the alveolar/capillary unit
Which factors affect rate of diffusion across a membrane?
Pressure gradient
Surface area
Thickness
What is the equation for rate of transfer of a gas?
DLg (PA - PC)
Where DLg = transfer factor
PA = alveolar pp
PC = capillary pp
Why can’t we use O2 to measure transfer factor?
Because mean capillary PO2 is unmeasurable
What gas is used to measure transfer factor and why?
Carbon monoxide - has a higher affinity for Hb than oxygen (240x higher)
What is the equation for gas transfer factor of CO?
DLCO = VCO/ PACO VCO = rate of CO uptake PACO = conc of CO in alveolar air
Which factors reduce the surface area of the lungs?
Emphysema
Lung resection
Decreased venous return
Which factors increase the thickness and reduce gas transfer factor in the lungs?
Fibrosis
Collagen vascular diseases
Congestive heart failure
What 3 factors decrease DLCO?
Reduction in alveolar-capillary area
Increased thickness of alveolar-capillary membrane
Anaemia
Which 2 factors increase DLCO?
Increased pulmonary blood volume (exercise)
Polycythaemia (increased no of RBCs)
What is the normal concentration of Hb in the blood?
150g/L
What is the oxygen capacity of normal blood?
200ml/L
What is the oxygen saturation of pulmonary venous blood?
Nearly 100%
What is the oxygen saturation of mixed venous blood returning to the heart?
75%
What is the Bohr shift?
Shift of oxygen-Hb dissociation curve to the right due to decrease in pH etc - easier to offload oxygen, e.g. in respiring tissues
What happens to the oxygen capacity, content and saturation at high altitudes?
Capacity - normal
Content - reduced
Saturation - reduced
What happens to the oxygen capacity, content and saturation at high altitudes?
Capacity - normal
Content - reduced
Saturation - reduced
What happens to the oxygen capacity, content and saturation in anaemia?
Capacity - reduced
Content - reduced
Saturation - normal
What is the PO2 in mixed expired air?
16kPa
What is the PCO2 in mixed expired air?
3.5kPa
What is the PH20 in mixed expired air?
Variable
