Gas Transfer in the Lung and Lung function testing

Question 1

what is Flicks principle

Answer 1

This says that: “The volume of gas which diffuses per unit time across a tissue sheet is

• proportional to the area of the sheet,
• inversely proportional to the thickness of the sheet
• Proportional to the difference in partial pressure of the gas on the two sides
• Dependant upon the permeability coefficient* for that gas in the tissue
• this means that we maximise gas transport in the lungs by having a large exchange area, a thin diffusion membrane, a high partial pressure difference and a high permeability coefficient

Question 2

describe the permeability coefficient in terms of oxygen and carbon dioxide in the lungs

Answer 2

• For oxygen and CO2 in the lungs, the permeability coefficient is proportional to the solubility of these gases in the alveolar and capillary cell membranes.
• Both oxygen and CO2 are highly soluble in the lipids of these membranes; so both gases pass relatively easily through them.

Question 3

when is gas transfer from the alveoli to capillaries is reduced

Answer 3

1. reduced surface area, eg in pneumonectomy, lobectomy or reduced ventilation from airway obstruction or reduced effective area with emphysema (alveoli form large bags) or increased dead space, if alveoli was filled with fluid
2. increased thickness of alveolar membrane with pulmonary fibrosis, alveolar proteinosis, and acute lung injury scarring inflammation
3. reduced oxygen concentration as in high altitude (reduced partial pressure)
4. inadequate time for gas transfer if there is lung disease – takes time for gases to diffuse across the membrane

Question 4

what is the average partial pressure of oxygen in alveolar gas

Answer 4

100mmHg

Question 5

describe the partial pressure of oxygen in the lung apices compared to the lung bases

Answer 5

• due to ventilation perfusion mismatch the gas in the alveoli in the lung apices has a higher partial pressure of around 135mmHg in comparison the gas in the alveoli of the lung bases has a lower partial pressure of oxygen

Question 6

what is the average partial pressure of oxygen in alveolar gas in the apical part of the lung

Answer 6

135 mm Hg

Question 7

what is the average partial pressure of oxygen in alveolar gas in the base part of the lung

Answer 7

92 mm Hg

Question 8

what happens to the blood passing through the apices and bases

Answer 8

Because of the difference in oxygen levels in the apical and basal alveoli, the blood passing through the apices will have a partial pressure of about 130 mm Hg, whereas that passing through the bases may be as low as 88 mm Hg.

Question 9

describe how the partial pressure of oxygen changes in the pulmonary capillaries

Answer 9

• oxygen transporter is efficient between alveoli and blood oxygen dissolves easily in the alveolar membrane
• oxygen partial pressure is about 45mmHg, so the partial pressure difference driving oxygen into the first parts of the pulmonary capillaries is about 55mmHg
• because of the efficient transfer by the time the blood has reached the end of pulmonary capilalries the partial pressure has risen to 95mmhG

Question 10

why won’t changing breathing rate not normally effect the arterial oxygen saturation

Answer 10

Haemoglobin will be effectively saturated with oxygen even if the partial pressure in the pulmonary capillary blood is as low as 80 mm Hg

Question 11

what will change the breathing rate effect

Answer 11

• However, CO2 excretion is slightly less efficient. The CO2 partial pressure is about 46 mm Hg in pulmonary arterial blood and 40 mm Hg in alveoli. - Changing the breathing rate WILL affect the excretion of CO2,
• this enables the lungs to adjust CO2 excretion (by changing breathing rate) to keep blood pH at desired level, without reducing oxygen saturation of arterial blood.

Question 12

what two forces keeps the thoracic wall and lungs in close opposition

Answer 12

• the intrapleural fluid cohesiveness

- the negative intrapleural pressure

Question 13

describe intrapleural cohesiveness

Answer 13

• this is when water molecules in the intrapleural fluid are attracted to each other and therefore resist being pulled apart
• this means the pleural membranes stick together
• this means that the pleural fluid may contain molecules that increase the surface tension of the fluid

Question 14

describe negative intrapleural pressure

Answer 14

• the sub-atmospheric intrapleural pressure creates a pressure gradient between the lung wall and chest wall
• this holds the outer surface of the lung against the inner surface of the thorax

Question 15

where do the two layers of the pleura fuse

Answer 15

• they fuse at the hilum of the lung

Question 16

how do the parietal pleura and visceral pleura stay attached to each other

Answer 16

• they are attached to each other by surface tension as they secrete a fluid that allows them to stay attached to each other

Question 17

what happens if you stop breathing with an open glottis

Answer 17

• the pressure inside your alveoli equals the atmospheric pressure

Question 18

what happens in when the open glottis

Answer 18

In this condition, the intrapleural pressure (between the two layers of pleura) is about 4 mm Hg less than atmospheric. This small negative pressure is enough to hold the two layers firmly together

Question 19

Define Boyle/’s Law

Answer 19

Boyle’s Law states that at any constant temperature the pressure exerted by a gas varies inversely with the volume of the gas

Question 20

define poiseuille’s law

Answer 20

Poiseuille’s law states that as the diameter increases, the flow of gas into the alveoli increases as the fourth power of the radius of the alveolar ducts and respiratory bronchioles

Question 21

describe what happens to pressure in the alveoli during inspiration

Answer 21

• During inspiration the increase in the size of the lungs makes the intra-alveolar pressure fall (Boyle’s Law).
• The decrease in alveolar pressure during inspiration in quiet breathing is SMALL; only about 1mmHg.
• The small pressure difference means that the lungs are highly efficient and only a small amount of work needs to be done done to inflate them

Question 22

describe what happens to the pressure in the alveoli during expiration

Answer 22

• Expiration is a normally a passive process involving no muscle work.
• It is brought about by relaxation of inspiratory muscles
• The chest wall and stretched lungs recoil to their preinspiratory size because of their elastic properties
• The recoil of the lungs makes the intra-alveolar pressure rise above atmospheric, but again, only by 1 mm Hg.
• The air then leaves the lungs down its pressure gradient until the intra-alveolar pressure become equal to atmospheric pressure

Question 23

what happens at the end of inspiration and expiration

Answer 23

alveoli pressure = atmospheric pressure

Question 24

what are the two factors that cause the alveoli to recoil during expiration

Answer 24

1) Elastic connective tissue in the lungs

2) Alveolar surface tension

Question 25

what is the alveolar surface tension

Answer 25

- Alveolar surface tension is the attraction between water molecules at the water-air interface - this produces a force which tends to make them collapse, but the alveoli are lined with a surfactant that stops them from collapsing

Question 26

what does Laplace's Law mean

Answer 26

The smaller alveoli (with smaller radius ) naturally will have a higher tendency to collapse during expiration.

Question 27

what happens without a surfactant

Answer 27

During expiration smaller alveoli collapse completely, larger alveoli change little

Question 28

what happens with a surfactant

Answer 28

during expiration both alveoli decrease proportionality in size

Question 29

what is them pulmonary surfactant made up of

Answer 29

is a complex mixture of lipid and protein,

Question 30

what secretes the pulmonary surfactant

Answer 30

type II pneumocytes (alveolar cells)

Question 31

what does the surfactant do

Answer 31

- it lowers alveolar surface tension by interspersing between the water molecules lining the alveoli - it prevents the smaller alveoli from collapsing and emptying their air contacts into the larger alveoli, with surfactant alveoli shrink evenly during expiration

Question 32

How many different lipoproteins make up different forms of surfactant

Answer 32

4 | - A,B,C,D

Question 33

what does surfactant containing lipoproteins B and C do

Answer 33

reduces surface tension and ensure proper lung function.

Question 34

what does surfactant containing lipoproteins A and D do

Answer 34

coat bacteria and viruses and help the immune system deal with them.

Question 35

What is a particular risk for pulmonary tuberculosis

Answer 35

surfactant D

Question 36

why is there no mucus in the alveoli

Answer 36

- this would impair gas exchange but it means that they are more likely to be infected so there are macrophages that move around inside the alveolus and phagocytose invading bacteria

Question 37

What is respiratory distress syndrome of the new born

Answer 37

- Fetal lungs are unable to synthesize surfactant until late in pregnancy - Premature babies (before ~28 weeks) may not have enough pulmonary surfactant - This causes respiratory distress syndrome (RDS) of the new born; lungs are hard to inflate and some alveoli may fail to open at all during inspiration - The baby has to make very strenuous inspiratory efforts in an attempt to overcome the high surface tension and inflate the lungs.

Question 38

what is spirometry used to do

Answer 38

Assess the prognosis of respiratory disease in a patient. Assess whether lung disease is present at an early stage, i.e. prior to overt clinical disease. Assist in quantifying the severity of airway disease. Assess the effect of therapy, such as corticosteroids, bronchodilators Delineate risk factors, e.g. the odds of developing future respiratory disease, or operative risks. Monitor whether the pattern of lung growth or aging is normal.

Question 39

How do you measure the total lung capacity

Answer 39

- total lung capacity cannot be measured directly by spirometry - measure by the inspiration of a fixed volume of gas that includes helium - then rebreathing the mixture happens until the helium is evenly distributed in the lung gases - helium is not absorbed into the blood therefore we sample the rebreathed gas and it contains lower concentration of the helium than the original inspired gas as it has penetrated the residual volume

Question 40

what is peak flow used to test

Answer 40

- used to assess airway resistance - could be used to monitor the effectiveness of treatment - best of 3 attempts is used

Question 41

what patients is peak flow used for

Answer 41

those with obstructive lung diseases such as asthma and COPD

Question 42

how does the peak flow work

Answer 42

It is measured by the patient taking a full inspiration and then giving a short sharp blow into the peak flow meter

Question 43

what does peak flow vary in

Answer 43

- varies in age and height

Question 44

How do you test ventilation and perfusion matching

Answer 44

isotope scanning | - has to be done in a ventilation lab

Question 45

what does reduced ventilation cause

Answer 45

Reduced ventilation and normal perfusion in pneumonia causes hypoxaemia

Question 46

what does reduced perfusion cause

Answer 46

Reduced perfusion with normal ventilation in pulmonary emboli also causes hypoxaemia

Question 47

what happens to the V/Q in COPD

Answer 47

in COPD Regional V/Q mismatch reduces the efficiency of gas transfer.

Question 48

how do you test the measurement of gas transfer in the lung

Answer 48

The efficiency of oxygen transport across the alveolar membrane can be measured by comparing alveolar oxygen levels (PAO2) with arterial levels (PaO2) - avelolar oxygen partial pressure is obtained by analysis with an oxygen meter of the end tidal expired gas - arterial oxygen pressure is measured with a pulse oximeter which measures the saturation of haemoglobin with oxygen

Question 49

how is exercise testing does

Answer 49

Physiotherapy and OT assessment Assess breathlessness on exercise with visual analogue score Treadmill or bicycle exercise tests show ventilation as a function of work and anaerobic threshold, which correlates with cardiopulmonary capacity and operative risk