Gas Exchange

Question 1

Describe the factors that affect the rate of gas exchange in the lungs

Answer 1

• Alveolar surface area
• Partial pressure gradient between Alveolar air and capillary blood
• Epithelial and endothelial cell thickness, basement membrane thickness and fluid layer depth

Question 2

What is required for maximum diffusion?

Answer 2

Increase in partial pressure gradient
Increase in surface area
Decrease in distance (barrier thickness)

Question 3

How do gas exchange factors change in specific disease states?

Answer 3

Hypoventilation (type II resp failure) = decrease in partial pressure gradient
Emphysema = decrease in surface area
Fibrosis = increase in basement membrane thickness
Pulmonary oedema = increase in thickness of the fluid layer

Question 4

How is the rate of diffusion determined

Answer 4

Rate of diffusion=Surface area/Distance2 x(Pa-Pc)

Question 5

What are the adaptations of the alveoli

Answer 5

Large surface area
Thin outer structure
Richly innervated by capillaries

Question 5

What are the adaptations of the alveoli

Answer 5

Large surface area
Thin outer structure
Richly innervated by capillaries

Question 6

How does ventilation determine the level of gas exchange

Answer 6

It changes according to the metabolic demands of the body e.g. due to exercise, injury or infection which require varying levels of gas exchange to supply oxygen and remove carbon dioxide.
This is achieved by changing the rate of alveolar ventilation, in order to modulate the partial pressure gradients between the alveoli and blood.

Question 7

Why are ventilation and perfusion coupled to facilitate efficient gas exchange

Answer 7

There must be sufficient blood, specifically haemoglobin binding sites, to absorb the quantity of oxygen arriving in the alveoli.

Question 8

What is the V/Q ration?

Answer 8

The degree to which the level of ventilation (supply of oxygen) and perfusion (supply of blood)are matched.
For efficient gas exchange, the V/Q ratio should be as close to 1.

Question 9

What does a significant V/Q mismatch result in?

Answer 9

Results in reduced gas exchange and decreased oxygenation of the blood, leading to hypoxaemia.

Question 10

likely cause of V/Q > 1

Answer 10

Hypoperfusion (deadspace effect)

Question 11

likely cause of V/Q < 1

Answer 11

Hypoventilation (shunt)

Question 12

What is the dead-space effect?

Answer 12

It is ventilation without perfusion.
If perfusion is reduced relative to ventilation, the inspired oxygen will in effect be “wasted” and not participate in gas exchange.
Affected regions are called “physiological dead-space”

Question 13

How can the dead-space effect occur?

Answer 13

Can occur due to reduced supply of blood to a specific regions of the lung e.g. pulmonary embolism, damage/blockade of blood vessels.
Heart failure
Blocked vessels (pulmonary embolism)
Loss/damage to capillaries (emphysema)

Question 14

Effect of dead-space effect on V/Q

Answer 14

V(alveolar) - normal
Perfusion - decreased (0)
V/Q - increased
Response to O2 therapy - helps

Question 15

What is a pulmonary shunt and what does it cause

Answer 15

Reduced ventilation of alveoli or limits to diffusion.
Causes decrease in V/Q ratio.
Deoxygenated blood returns to the left side of the heart from the right without taking part in gas exchange.

Question 16

What causes pulmonary shunts

Answer 16

Pneumonia
Pulmonary oedema
Acute Respiratory Distress Syndrome (ARDS)
Alveolar collapse

Question 17

Effect of pulmonary shunt on V/Q

Answer 17

V(alveolar) - decreased
Q - normal
V/Q - decreased
Response to O2 therapy is poor

Question 18

Why does hypoxaemia caused by a shunt have a much more limited response to oxygen therapy

Answer 18

Regardless of the degree of oxygenation occurring in the blood perfusing well-ventilated alveoli, the blood will eventually mix with deoxygenated blood returning from areas affected by shunt thus reducing overall Pa02.

Question 19

What is hypoxic vasoconstriction?

Answer 19

It occurs when ventilation to an alveoli is reduced. The decrease in ventilation results in rising CO2 levels and decreasing O2 levels which cause constriction of the vascular smooth muscle within nearby capillaries.
The constriction of these blood vessels diverts blood to other capillaries that innervate better-ventilated alveoli.

Question 20

Why is hypoxic vasoconstriction pathological in certain lung diseases?

Answer 20

In COPD, chronic hypoventilation occurs withing large sections of the lungs leading to prolonged and widespread pulmonary vasoconstriction. This increases resistance within the pulmonary vasculature, resulting in pulmonary hypertension this can lead to right heart hypertrophy and eventually right heart failure