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Respiratory > 3. Gas Exchange And Diffusion > Flashcards

3. Gas Exchange And Diffusion Flashcards

1
Q

What is the definition of partial pressure?

A

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

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2
Q

What is the atmospheric pressure at sea level?

A

760mmHg/101kPa

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3
Q

What is the percentage of oxygen in the air?

A

20.9%

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4
Q

What is the saturated vapour pressure?

A

6.28kPa at body temperature

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5
Q

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

A

19.8kPa

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6
Q

What is the alveolar partial pressure of oxygen?

A

13.3kPa

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7
Q

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

A

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

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8
Q

What is the average tidal volume?

A

450ml

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9
Q

How much of normal tidal volume fills anatomical dead space?

A

30% (~150ml)

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10
Q

What is the average alveolar ventilation?

A

300ml

450-150

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11
Q

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

A

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

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12
Q

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

A

Partial pressure of that gas x gas solubility coefficient

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13
Q

What is solubility coefficient of a gas?

A

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

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14
Q

What is the solubility coefficient of O2 in plasma?

A

0.01mmol/L/kPa

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15
Q

What 2 factors can affect diffusion of gases?

A

Solubility

Molecular weight

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16
Q

Why does CO2 diffuse faster than O2?

A

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

17
Q

How does O2 compensate for slower diffusion?

A

Larger difference in partial pressure

18
Q

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

A 
Fluid film lining alveolus
Epithelial cell of alveolus
Interstitial space
Endothelial cell of capillary
Plasma
Red cell membrane
19
Q

What factors affect rate of gas diffusion in disease?

A

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

20
Q

What are examples of diseases with diffusion impairment?

A

Fibrotic lung disease
Emphysema
Pulmonary oedema

21
Q

What is the V/Q ratio?

A

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

22
Q

What is V/Q mismatch?

A

When perfusion exceeds ventilation <1, low V/Q

Arterial blood will have low oxygen levels

23
Q

What happens when alveolar PaO2 is low?

A

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

24
Q

What is V/Q in normal lungs?

A

0.8-0.9

25
Q

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

A

Reserve - V/Q match approaches 1 when we exercise

26
Q

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

A

Systemic arterial hypoxaemia in people with cardio-pulmonary disease

27
Q

What is hypoxaemia?

A

Low partial pressure of oxygen in arterial blood

28
Q

What diseases/conditions cause V/Q mismatch?

A 
Asthma
Early stages of COPD
Pneumonia
Respiratory distress syndrome in the newborn
Pulmonary oedema
Pulmonary embolism
29
Q

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

A

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

30
Q

What happens to alveoli in pulmonary embolism?

A

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

Respiratory (23 decks)

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