Gas exchange and gas transport

Question 1

What are the 2 functions of the lung?

Answer 1

1. Ventilation – the movement of air in and out of the lungs
2. Gas exchange – the exchange of oxygen and carbon dioxide between the airspace of the alveoli and the blood

Question 2

What does the red line show?

Answer 2

Less efficient alveoli caused by fluid overload

Question 3

What is the lung ventilated by and perfused by?

Answer 3

The lung is ventilated by air and perfused by blood

Question 4

How does ventilation/perfusion (V/Q) matching work?

Answer 4

• 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

Question 5

What are steady-state conditions?

Answer 5

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)

Question 6

What is the Respiratory Quotient equation?

Answer 6

Question 7

What is the RQ value for

Fat metabolism

Carbohydrate metabolism

Answer 7

• Fat metabolism 0.7
• Carbohydrate metabolism 1.0

Question 8

What does the partial pressure of CO2 in alveolar air (P_ACO2) depend on?

Answer 8

If CO2 is produced at a constant rate by the body then the partial pressure of CO2 (PCO2) of alveolar air (P_ACO2) is dependant on alveolar ventilation

Question 9

What is the relationship between alveolar ventilation and P_ACO2

Answer 9

• Increase in alveolar ventilation -> decrease in P_ACO2
• Decrease in alveolar ventilation -> increase in P_ACO2

Question 10

What is P_ACO2?

Answer 10

The partial pressure of CO2 of alveolar air

Question 11

What is PCO2?

Answer 11

Partial pressure of CO2

Question 12

What is PAO2?

Answer 12

Partial pressure of O2 in alveolar air

Question 13

What is the relationship between alveolar ventilation and P_AO2?

Answer 13

• Increase in alveolar ventilation -> increase in P_AO2
• A big enough increase in alveolar ventilation will allow the P_AO2 to approach PO2
• Decrease in alveolar ventilation -> decrease in P_AO2

Question 14

How does gas get into the blood?

Answer 14

Diffusion through tissues is described by Fick’s Law

Question 15

What is Fick’s Law?

Answer 15

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.

Question 16

What does this graph show about the carriage of CO2 and O2 by blood?

Answer 16

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.

Question 17

What does a low V/Q result in?

Answer 17

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)

Question 18

What does a high V/Q result in?

Answer 18

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)

Question 19

What is the alveolar gas equation?

Answer 19

The relationship between PaCO2 and PaO2 is summarised by the alveolar gas equation

Question 20

What is type 2 respiratory failure?

Answer 20

Pure underventilation leads to an increased PaCO2 and a proportionate fall in PaO2

Question 21

What is type 1 respiratory failure?

Answer 21

Disturbance in V/Q matching leads to a fall in PaO2 with no change in PaCO2

Question 22

What can the alveolar gas equation show?

Answer 22

Alveolar gas equation allows determination if an observed reduction in PaO2 is related to underventilation alone or weather there is an intrinsic lung issue

Question 23

What is the Alveolar-Arterial (A-a) gradient?

Answer 23

• Unlike CO2 there is normally a difference between alveolar and arterial PO2
• This is known as the Alveolar-Arterial (A-a) gradient

Question 24

What is the A-a gradient in healthy young adults?

Answer 24

Healthy young adults, breathing air, the A-a gradient is small (<2 kPa)

Question 25

What does a high A-a gradient indicate?

Answer 25

• 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