Theme 4: Lecture 3 - Gas exchange and gas transport Flashcards
What are the 2 functions of the lungs
- Ventilation
- Gas exchange
What is ventilation
Movement of air in and out of the lungs
What is gas exchange
The exchange of oxygen and carbon dioxide between the airspace of the alveoli and the blood
Ventilation/Perfusion (V/Q) matching
- Uneven distribution of air is inspired into the lungs throughout the lung – lower zones more compliant so have more air
- Pulmonary circulation is a low pressure system (15 mmHg). The upright position has barely enough pressure to perfuse the apices and there’s overperfusion of the bases
- Therefore areas with more air inspired are also better perfused
What does the amount of CO2 produced by the body and the amount of O2 absorbed depend on
The metabolic activity of the body
Respiratory quotient equation
RQ = CO2 produced/ O2 absorbed
What is the value of the respiratory quotient at rest
0.8
What does a respiratory quotient greater than 1 mean
That someone is in the anaerobic process of exercise
What is the respiratory quotient in fat metabolism
0.7
What is the respiratory quotient in carbohydrate metabolism
1.0
What does increased alveolar ventilation cause in terms of CO2
Decreased partial pressure of CO2 in the alveoli
What does decreased alveolar ventilation cause in terms of CO2
Increased partial pressure of CO2 in the alveoli
What does increased alveolar ventilation cause in terms of O2
- Increased partial pressure of O2 in the alveoli
- A large enough increase in alveolar ventilation will allow the partial pressure of O2 in the alveoli reach the partial pressure of O2 in the atmosphere
What does decreased alveolar ventilation cause in terms of O2
Decreased partial pressure of O2 in the alveoli
How does gas get into the blood
- Diffusion through tissues is described by Fick’s Law
- The rate of transfer of a gas through a tissue is proportional to the tissue area and the difference in gas partial pressure between the two sides, and inversely proportional to the tissue thickness
What does a low V/Q result in
- Decreased ventilation relative to blood flow
- This leads to increased arterial PCO2 and a decrease in PO2 (Dissociation curve produces a rise in arterial CO2 content and a reduction in arterial O2 content)
What does a high V/Q result in
- Increased ventilation relative to blood flow
- This leads to a decrease in arterial PCO2 and an increase in PO2 (Dissociation curve produces a fall in arterial CO2 content but no increase in arterial O2 content)
Partial pressure
the pressure that would be exerted by one of the gases in a mixture if it occupied the same volume on its own.
What summarises the relationship between the partial pressure of arterial CO2 and the partial pressure of arterial O2
The alveolar gas equation
Type 2 respiratory failure
Pure underventilation leads to an increased partial pressure of arterial CO2 and a proportionate fall in partial pressure of arterial O2
Type 1 respiratory failure
Disturbance in V/Q matching leads to a fall in the partial pressure of arterial O2 with no change in the partial pressure of arterial CO2
Can type 1 and type 2 respiratory failure occur at the same time
Yes
What does the alveolar gas equation allow
It allows determination if an observed reduction in the partial pressure of arterial O2 is related to underventilation alone or weather there is an intrinsic lung issue
State the alveolar gas equation
Partial pressure of alveoli O2 = Partial pressure of inspired O2 - (partial pressure of arterial CO2/0.8)
