gas exchnage in the lungs

Primary factor determining gas behaviour in the lung

Partial pressure

What does barometric pressure refer too, and if it decreases, what effect does it have on the PO2

• barometric pressure refers to atmospheric pressure

• If barometric pressure decreases, the PO2 decreases

Explain the steps of gas exchange from the air into the capillary

1. O2 enters the alveolar airspace from the atmosphere

2. o2 dissolves into the Alevolar lining fluid

3. O2 diffuses through the alveolar epithelium, basement membrane & capillary endothelial cell

4. O2 dissolves into the blood plasma

5. O2 binds to Hb molecule

What will happen to the rate of diffusion if

• pulmonary oedema occurs and what does it affect

• Thickness to any membrane

• pulmonary oedema will increase the alveolar lining fluid, increasing the distance and decreasing rate of diffusion

• Thickness to any membrane increases the distance, slows the rate of diffusion

What does this graph show

¼ and 1/8 shows reduced oxygenation due to thickening of the blood gas barrier and slower rate of diffusion

What is rate of diffusion proportional to?

Surface area/(Distance)2 x (alveolar air (PA) - Capillary blood (PC)

Increases SA, Smaller the distance = increase the rate of diffusion

Increasing ventilation will cause PA to get bigger, increasing rate of diffusion

What factors contribute to maximum diffusion

• Increases partial pressure gradient

• Increases SA

• Decreases in distance

What is the pathological cause of basement membrane thickness

Pulmonary fibrosis

Pathological cause of decreases alveolar SA

Emphysema

How are alveoli specialised for their function

• large SA

• One cell thick - basement membrane fused with blood vessel

• Richly innervated by capillaries (adequate blood supply)

Definition of ventilation-perfusion ratio V/Q

The ratio of alveolar ventilation (Va) to pulmonary perfusion (Q)

what is the ideal V/Q ratio in a healthy resting lung

1

Ventilation (Va) and perfusion (Q) need to match to enable efficient gas exchange

Definition of physiological dead space effect

• what is the VQ

Ventilation without perfusion = meaning ventilated alveoli do not gas exchange

• VQ more than 1

Definition of Pulmonary shunt

• what is the VQ

Perfusion without ventilation

• less than one

What is the mechanism maintaining ventilation-perfusion coupling

Hypoxic pulmonary vasoconstriction

Explain hypoxic pulmonary vasoconstriction

When ventilation of a specific alveoli decreases, PACO2 will rise, PAO2 decreases.

The reduction in PaO2 induces vasoconstriction will decreases blood flow, causing the blood to divert to alveoli with good ventilation.

In ventilation-perfusion mismatch, what reflex does it induce

• what does it do to PaO2 and PaCO2

• Affect on gas exchange

Affects both O2 and CO2, the increase if PaCO2 will induce hyperventilation to clear the excess Co2

There is NO increase in PaO2

Rate of gas exchange decrease due to a number of alveoli not being ventilated = hypoxaemia

Ventilation-perfusion mismatch: Pulmonary embolism

Explain the mechanism

A blood clot stops blood from reaching alveoli

VQ more than 1, as the alveoli is still being ventilated, but no perfusion is occurring = physiologic dead space

Perfusion to the working alveoli increases as cardiac output is diverted

Ventilation must increase to match perfusion, otherwise hypothermia and hypercapina will occur.

Effect of supplemental oxygen therapy on physiological dead space

Positive effect as it increases oxygen delivery to functional areas

What is a Shunt

• What happens to the VQ ratio

Perfusion without ventilation

• VQ ratio decreases

Effects of oxygen therapy on pulmonary shunt

O2 therapy does not help

Functional alveoli cannot compensate for poorly ventilated alveoli.

On the healthy side, maximum oxygen gas exchange is occurring