Gas exchange in the lungs

Question 1

Why is pressure the unit of measurement for quantifying gas?

Answer 1

Because gases react, dissolve, and diffuse more in accordance with their pressure than concentration.

Question 2

What is meant by total pressure?

Answer 2

The sum of the partial pressures of the individual gases within a mixture

Question 3

What is meant by partial pressure?

Answer 3

The hypothetical pressure of the gas if it alone occupied the total volume of the mixture

Question 4

How is partial pressure of a gas that is dissolved in a liquid (e.g. blood) measured?

Answer 4

The partial pressure of the specific gas is proportional to the local amount that is present in the liquid.

Question 5

How is the concentration of a dissolved gas calculated?

Answer 5

Concentration = Partial pressure x Solubility

Question 6

What are the structures of the alveoli that are specific for gas exchange?

Answer 6

They have a large surface area (both individually and cumulatively), have a thin outer structure (typically one cell thick), and are richly innervated by capillaries, all of which help to maximise the rate of diffusion.

Question 7

Outline the structures and fluid layers through which oxygen must pass/dissolve in order to travel from alveolar air to capillary blood. (5 steps)

Answer 7

1. O2 enters the aveolar air space from the atmosphere
2. O2 dissolves in the alveolar lining fluid
3. O2 diffuses through the alveolar epithelium, basement membrane and capillary endothelial cells
4. O2 dissolves in blood plasma
5. O2 binds to Hb molecule.

Question 8

What are the factors that affect the rate of diffusion?

Answer 8

Partial pressure gradient between 2 areas
The size of the diffusion distance
Surface area

Question 9

What are the pathological effects on the partial pressure gradient?

Answer 9

Hypo-perfusion (type I respiratory pressure) causes a decrease in partial pressure in capillary blood
Hypo-ventilation (type II respiratory failure) causes a decrease in partial pressure in the alveolar air

Question 10

what area the pathological effects on the surface area?

Answer 10

Emphysema decreases surface area

Question 11

What are the pathological effects on the diffusion distance?

Answer 11

Fibrosis causes an increase in the basement membrane thickness
Pulmonary oedema (e.g. pneumonia) causes an increase in thickness if fluid layer/oedema

Question 12

What is meant by V/Q?

Answer 12

This is the ventilation (supply of oxygen)/perfusion (supply of blood) ratio. This ratio should be close to 1 to ensure there is sufficient blood (specifically haemoglobin binding sites) to absorb the quantity of oxygen arriving at the alveoli.

Question 13

What is ventilation-perfusion inequality/mismatch?

Answer 13

Where ventilation and perfusion to individual alveolar units are not matched causing gas exchange to be reduced.

Question 14

What is the dead-space effect?

Answer 14

This is ventilation with little to no perfusion. V/Q ratio will increase (V/Q >1) and the inspired oxygen will in effect be ‘wasted’ and not participate in gas exchange. This can occur due to reduced blood supply to specific regions of the lung.

Question 15

What is pulmonary embolism?

Answer 15

A blockage of an artery in the lungs

Question 16

What is meant by pulmonary shunt?

Answer 16

Reduced ventilation of alveoli or limits to diffusion will result in a 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 17

What is hypoxic vasoconstriction?

Answer 17

Hypoxic vasoconstriction occurs when ventilation to an alveoli is reduced; the decreased ventilation results in rising CO2 and falling O2 levels. The constriction of these blood vessels diverts blood to other capillaries that innervate better-ventilated alveoli.
This is a homeostatic function.