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Equation of partial pressure of air

Equation of partial pressure of individual constituent gas (Pgas)

P total = PH2O + Sum of Pconstituent gases

Pgas = (Pbarometric - PH2O) x n gas

Pbarometric = atmospheric pressure


How is barometric pressure affected by higher altitude?

At higher altitude, barometric pressure decreases as air gets thinner which then decreases pp of other gases e.g. O2


What structures and mediums does gas exchange involving diffusion have to go through?

1. O2 enters the alveolar airspace from the atmosphere
2. O2 dissolves in alveolar lining fluid (ALF)
3. O2 diffuses through alveolar epithelium, basement membrane and capillary endothelial cells
4. O2 dissolves in blood plasma
5. O2 bind Hb molecule in RBC


What is gas exchange?

Gas Exchange = diffusion of gases between air & blood


Rate of diffusion equation

Rate of diffusion = Surface area/Distance2 x (PA - PC)

Surface area = alveolar surface area
Distance = Epithelial and endothelial cell thickness + basement membrane thickness + fluid layer depth
(PA - PC) = Partial pressure gradient between alveolar air and capillary blood

For maximum diffusion:
- Increase partial pressure gradient
- Increase surface area
- Decrease distance (barrier thickness)


Defects affecting increase in partial pressure gradient at gas exchange surfaces

- Hypoventilation (type II respiratory failure) - Decrease alveolar pressure
- Hypoperfusion (type I respiratory failure) - Decrease in capillary pressure


Defects affecting surface area at gas exchange surfaces

Emphysema which decreases surface area


Defects affecting distance (barrier thickness) at gas exchange surfaces

- Fibrosis which increases basement membrane thickness

- Pulmonary oedema (e.g. pneumonia) which increases thickness of fluid layer/oedema


What are the adaptations alveoli have to maximise the rate of gas exchange?

Alveoli have intricate structures

- Large surface area (lungs have high surface area-volume ratio due to 3D structure)
- The wall of alveoli are one cell layer thick and basement membrane is fused with blood vessel
- Richly innervated by capillaries (adequate blood supply)


How are pressure gradients between alveoli and blood maintained?

Adequate ventilation


Definition of PA, Pa, Pv

PA = Alveolar partial pressure
Pa= Arterial partial pressure
PV = Venous partial pressure


Describe PaO2, PaCO2, hypoventilation and hyperventilation

Hyperventilation = same PaO2

Hypoventilation = Decrease PaO2

Hyperventilation = Decrease PaCO2

Hypoventilation = Increase PaCO2


Describe Perfusion(Q ̇) and Ventilation (VA)

V/Q ratio

Perfusion (Q) blood flow through pulmonary cappilaries

Perfusion needs to be matched to alveolar ventilation to enable efficient oxygen saturation

- V ̇/Q ̇ ratio describes this relationship. 1L of blood can carry ≈ 200mL O2, 1L dry air ≈ 210ml O2
- V ̇/Q ̇ should = 1.
- At rest ventilation and perfusion both ≈V ̇/Q ̇ ≈ 1 - 0.8


What is the V/Q ratio during hypoperfusion?

>1 which is the dead space effect


What is the V/Q ratio during hypoventilation?

<1 Shunt


What maintains ventilation-perfusion coupling?

Ventilation-perfusion coupling is maintained by hypoxic vasoconstriction

- Homeostatic mechanism exist to reduce ventilation-perfusion mismatching
- Hypoxic vasoconstriction of capillaries diverts blood flow from poor to well ventilated alveoli

If many regions of the lung are hyperventilated on a chronic basis, it causes many areas to be hypoxic vasconstriction which causes the heart to work extremely hard to overcome the constrictions causing atrophy of the heart, increased blood pressure and can cause heart failure.


Describe ventilation-perfusion mismatch: physiological dead space (ventilation without perfusion)

Reduced perfusion of lung regions, causes an increase in V/Q ratio (factors):

Reduced perfusion of lung perfusion, causes an increase in V/Q ratio (factors):
- Heart failure (cardiac arrest)
- Blocked vessels (pulmonary embolism)
- Loss/Damage of capillaries (emphysema)

The affected alveoli will have physiological dead space as no/reduced gas exchange occurs


What is the dead space effect (VA. Q, V/Q , O2 therapy)

Physiological dead space is where no/reduced gas exchange occurs

VA = normal
Q= Decrease (0)
V/Q = Increase to infinity

It responds to O2 therapy


Describe ventilation-perfusion mismatch: shunt (perfusion without ventilation)

Reduced perfusion of lung regions causes an decrease in V/Q ratio:
- Asthma, COPD
- Pneumonia, fibrosis

Blood returns to the left heart from the right, without taking part in gas exchange (shunt).

VA= Decrease (0)
Q= Normal
V/Q= Decrease (0)

The response to O2 therapy is poor


Define hypoxaemia and hypoxia

Hypoxaemia= low levels of O2 within blood vessels

Hypoxia = Insufficient O2 supply to tissues


How can PAO2 be calculated?

- Sample gas directly from alveoli
- Using the equation

PAO2 = F1O2 x (PB - PH2O) - PaCO2/RER

F1O2= fraction of oxygen present in inspired air
PB= Barometric pressure
RER= Respiratory exchange ratio


Why is it useful to know alveolar oxygen pressure (PAO2)?

Clinical situations arise where it is useful to know alveolar oxygen pressure (PAO2), e.g. determining the cause of respiratory failure – is it due to hypoventilation or poor oxygenation


What is the respiratory exchange ratio (RER)?

The relationship between CO2 elimination & O2 consumption

RER= VCO2 produced/VO2 consumed


What is the main determinant of RER?

Main determinant of RER is the particular metabolic substrate being used (e.g. fat or carbohydrate
RER for modern diet should roughly be 0.8

Oxidation of Carbohydrates produces higher RER than oxidation of fatty acids


What 2 equations are used to investigate hypoxaemia?

Alveolar gas equation (alveolar O2 content) = O2 inspired - O2 consumed

Alveolar-arterial oxygen gradient

A-a O2 gradient is the difference between alveolar and arterial pressure:
= PAO2 – PaO2.
Normally ≈ <2kPa


What level of PaCO2 suggests hypoventilating?

PaCO2 > 5kPa


Explain how efficacy of O2 therapy in hypoxaemia depends on the nature of the hypoventilation

The response to oxygen depends on the individuals state of airway. 
 e.g. Oxygen may not get to a part of the lung due to obstruction no matter how much you increase the levels of oxygen