Gas exchange and gas transport Flashcards
What are the 2 functions of the lung?
- Ventilation – the movement of air in and out of the lungs
- Gas exchange – the exchange of oxygen and carbon dioxide between the airspace of the alveoli and the blood
What does the red line show?
Less efficient alveoli caused by fluid overload
What is the lung ventilated by and perfused by?
The lung is ventilated by air and perfused by blood
How does ventilation/perfusion (V/Q) matching work?
- Pulmonary circulation is a low pressure system (15 mmHg)
- Upright position barely enough pressure to perfuse the apices with overperfusion of the bases
THEREFORE
•Uneven distribution throughout the lung – lower zones more compliant
What are steady-state conditions?
The amount of CO2 produced by the body and the amount of O2 absorbed depends upon the metabolic activity of the body – the ‘Respiratory Quotient’ (RQ)
What is the Respiratory Quotient equation?
What is the RQ value for
Fat metabolism
Carbohydrate metabolism
- Fat metabolism 0.7
- Carbohydrate metabolism 1.0
What does the partial pressure of CO2 in alveolar air (PACO2) depend on?
If CO2 is produced at a constant rate by the body then the partial pressure of CO2 (PCO2) of alveolar air (PACO2) is dependant on alveolar ventilation
What is the relationship between alveolar ventilation and PACO2
- Increase in alveolar ventilation -> decrease in PACO2
- Decrease in alveolar ventilation -> increase in PACO2
What is PACO2?
The partial pressure of CO2 of alveolar air
What is PCO2?
Partial pressure of CO2
What is PAO2?
Partial pressure of O2 in alveolar air
What is the relationship between alveolar ventilation and PAO2?
- Increase in alveolar ventilation -> increase in PAO2
- A big enough increase in alveolar ventilation will allow the PAO2 to approach PO2
- Decrease in alveolar ventilation -> decrease in PAO2
How does gas get into the blood?
Diffusion through tissues is described by Fick’s Law
What is 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.
Alveoli have large surface area so gas transfer is quicker.
What does this graph show about the carriage of CO2 and O2 by blood?
Oxygen and CO2 have different dissociation curves.
For O2, maximum saturation is at 13.3 kPa.
Above this, any high partial pressures of O2 won’t add any oxygen to the haemoglobin because it is fully saturated.
What does a low V/Q result in?
Low V/Q - Decrease in ventilation relative to blood flow -> increase in 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?
High V/Q - increase in ventilation relative to blood flow -> 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)
What is the alveolar gas equation?
The relationship between PaCO2 and PaO2 is summarised by the alveolar gas equation
What is type 2 respiratory failure?
Pure underventilation leads to an increased PaCO2 and a proportionate fall in PaO2
What is type 1 respiratory failure?
Disturbance in V/Q matching leads to a fall in PaO2 with no change in PaCO2
What can the alveolar gas equation show?
Alveolar gas equation allows determination if an observed reduction in PaO2 is related to underventilation alone or weather there is an intrinsic lung issue
What is the Alveolar-Arterial (A-a) gradient?
- Unlike CO2 there is normally a difference between alveolar and arterial PO2
- This is known as the Alveolar-Arterial (A-a) gradient
What is the A-a gradient in healthy young adults?
Healthy young adults, breathing air, the A-a gradient is small (<2 kPa)
What does a high A-a gradient indicate?
- If A-a gradient is high the abnormality in the blood gas cannot be explained by a change in ventilation
- Therefore there must be an abnormality intrinsic to the lungs or its vasculature causing a V/Q mismatch
