Gas Transfer in the Lung and Lung function testing Flashcards
what is Flicks principle
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
describe the permeability coefficient in terms of oxygen and carbon dioxide in the lungs
- 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.
when is gas transfer from the alveoli to capillaries is reduced
- 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
- increased thickness of alveolar membrane with pulmonary fibrosis, alveolar proteinosis, and acute lung injury scarring inflammation
- reduced oxygen concentration as in high altitude (reduced partial pressure)
- inadequate time for gas transfer if there is lung disease – takes time for gases to diffuse across the membrane
what is the average partial pressure of oxygen in alveolar gas
100mmHg
describe the partial pressure of oxygen in the lung apices compared to the lung bases
- 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
what is the average partial pressure of oxygen in alveolar gas in the apical part of the lung
135 mm Hg
what is the average partial pressure of oxygen in alveolar gas in the base part of the lung
92 mm Hg
what happens to the blood passing through the apices and bases
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.
describe how the partial pressure of oxygen changes in the pulmonary capillaries
- 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
why won’t changing breathing rate not normally effect the arterial oxygen saturation
Haemoglobin will be effectively saturated with oxygen even if the partial pressure in the pulmonary capillary blood is as low as 80 mm Hg
what will change the breathing rate effect
- 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.
what two forces keeps the thoracic wall and lungs in close opposition
- the intrapleural fluid cohesiveness
- the negative intrapleural pressure
describe intrapleural cohesiveness
- 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
describe negative intrapleural pressure
- 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
where do the two layers of the pleura fuse
- they fuse at the hilum of the lung
how do the parietal pleura and visceral pleura stay attached to each other
- they are attached to each other by surface tension as they secrete a fluid that allows them to stay attached to each other
what happens if you stop breathing with an open glottis
- the pressure inside your alveoli equals the atmospheric pressure
what happens in when the open glottis
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
Define Boyle/’s Law
Boyle’s Law states that at any constant temperature the pressure exerted by a gas varies inversely with the volume of the gas
define poiseuille’s law
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
describe what happens to pressure in the alveoli during inspiration
- 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
describe what happens to the pressure in the alveoli during expiration
- 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
what happens at the end of inspiration and expiration
alveoli pressure = atmospheric pressure
what are the two factors that cause the alveoli to recoil during expiration
1) Elastic connective tissue in the lungs
2) Alveolar surface tension
what is the alveolar surface tension
- 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
what does Laplace’s Law mean
The smaller alveoli (with smaller radius ) naturally will have a higher tendency to collapse during expiration.
what happens without a surfactant
During expiration smaller alveoli collapse completely, larger alveoli change little
what happens with a surfactant
during expiration both alveoli decrease proportionality in size
what is them pulmonary surfactant made up of
is a complex mixture of lipid and protein,
what secretes the pulmonary surfactant
type II pneumocytes (alveolar cells)
what does the surfactant do
- 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
How many different lipoproteins make up different forms of surfactant
4
- A,B,C,D
what does surfactant containing lipoproteins B and C do
reduces surface tension and ensure proper lung function.
what does surfactant containing lipoproteins A and D do
coat bacteria and viruses and help the immune system deal with them.
What is a particular risk for pulmonary tuberculosis
surfactant D
why is there no mucus in the alveoli
- 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
What is respiratory distress syndrome of the new born
- 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.
what is spirometry used to do
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.
How do you measure the total lung capacity
- 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
what is peak flow used to test
- used to assess airway resistance
- could be used to monitor the effectiveness of treatment
- best of 3 attempts is used
what patients is peak flow used for
those with obstructive lung diseases such as asthma and COPD
how does the peak flow work
It is measured by the patient taking a full inspiration and then giving a short sharp blow into the peak flow meter
what does peak flow vary in
- varies in age and height
How do you test ventilation and perfusion matching
isotope scanning
- has to be done in a ventilation lab
what does reduced ventilation cause
Reduced ventilation and normal perfusion in pneumonia causes hypoxaemia
what does reduced perfusion cause
Reduced perfusion with normal ventilation in pulmonary emboli also causes hypoxaemia
what happens to the V/Q in COPD
in COPD Regional V/Q mismatch reduces the efficiency of gas transfer.
how do you test the measurement of gas transfer in the lung
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
how is exercise testing does
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
