3. Gas Exchange And Diffusion

Question 1

What is the definition of partial pressure?

Answer 1

Partial pressure of a gas is the pressure exerted by an individual gas in a mixture of gases

Question 2

What is the atmospheric pressure at sea level?

Answer 2

760mmHg/101kPa

Question 3

What is the percentage of oxygen in the air?

Answer 3

20.9%

Question 4

What is the saturated vapour pressure?

Answer 4

6.28kPa at body temperature

Question 5

What is the partial pressure of oxygen in upper respiratory tract?

Answer 5

19.8kPa

Question 6

What is the alveolar partial pressure of oxygen?

Answer 6

13.3kPa

Question 7

Why is alveolar partial pressure of oxygen lower than that in the upper respiratory tract?

Answer 7

Oxygen from alveolar air constantly diffusing into pulmonary circulation
Alveolar air only partially replaced with each breath

Question 8

What is the average tidal volume?

Answer 8

450ml

Question 9

How much of normal tidal volume fills anatomical dead space?

Answer 9

30% (~150ml)

Question 10

What is the average alveolar ventilation?

Answer 10

300ml

450-150

Question 11

What happens when inspired gases come into contact with body fluids?

Answer 11

Gas molecules will enter fluid and dissolve n the liquid

The amount of gas that dissolves is directly proportional to the partial pressure of that gas

Question 12

How do you work out the dissolved amount of gas in a liquid as a concentration?

Answer 12

Partial pressure of that gas x gas solubility coefficient

Question 13

What is solubility coefficient of a gas?

Answer 13

Volume of gas that can be dissolved in a fixed volume of solvent at a given temperature

Question 14

What is the solubility coefficient of O2 in plasma?

Answer 14

0.01mmol/L/kPa

Question 15

What 2 factors can affect diffusion of gases?

Answer 15

Solubility

Molecular weight

Question 16

Why does CO2 diffuse faster than O2?

Answer 16

CO2 is more soluble so the effect of solubility is much greater

Question 17

How does O2 compensate for slower diffusion?

Answer 17

Larger difference in partial pressure

Question 18

What does gas diffusing from alveolar air to RBC in capillary have to cross?

Answer 18

Fluid film lining alveolus
Epithelial cell of alveolus
Interstitial space
Endothelial cell of capillary
Plasma
Red cell membrane

Question 19

What factors affect rate of gas diffusion in disease?

Answer 19

Thickness of membrane - thicker, impaired diffusion
Surface area of membrane - decreased, impaired diffusion
Diffusion coefficient of gas - low pO2

Question 20

What are examples of diseases with diffusion impairment?

Answer 20

Fibrotic lung disease
Emphysema
Pulmonary oedema

Question 21

What is the V/Q ratio?

Answer 21

Ventilation to perfusion ratio
1=normal
<1 less ventilation
>1 more ventilation

Question 22

What is V/Q mismatch?

Answer 22

When perfusion exceeds ventilation <1, low V/Q

Arterial blood will have low oxygen levels

Question 23

What happens when alveolar PaO2 is low?

Answer 23

Indicative of impaired ventilation, hypoxic vasoconstriction of pulmonary arterioles occurs
Diverts blood to better ventilated alveoli

Question 24

What is V/Q in normal lungs?

Answer 24

0.8-0.9

Question 25

Why do we have V/Q mismatch in normal lungs?

Answer 25

Reserve - V/Q match approaches 1 when we exercise

Question 26

What is V/Q mismatch the most common cause of?

Answer 26

Systemic arterial hypoxaemia in people with cardio-pulmonary disease

Question 27

What is hypoxaemia?

Answer 27

Low partial pressure of oxygen in arterial blood

Question 28

What diseases/conditions cause V/Q mismatch?

Answer 28

``` Asthma Early stages of COPD Pneumonia Respiratory distress syndrome in the newborn Pulmonary oedema Pulmonary embolism ```

Question 29

Why can’t over-ventilated alveoli compensate for under-ventilated alveoli?

Answer 29

Going from a partial pressure of 13kPa to 15kPa only increases dissolved oxygen by 0.02 mmol/L Haemoglobin is already carrying all that is can carry - it is saturated So very little additional oxygen for loading onto haemoglobin from poorly perfused alveoli

Question 30

What happens to alveoli in pulmonary embolism?

Answer 30

There are alveoli that receive no blood, though they are still ventilated, this s known as dead space V/Q = 1/0 = infinity If ventilation to other alveoli cant increase commensurate with the increased blood flow then V/Q mismatch