Ventilation: Physics of Breathing Flashcards
What are the non-respiratory functions of ventilation?
- expulsion of foreign bodies
- defence against infection/disease
What is alveolar ventilation
the rate at which new air reaches the areas of gas exchange
What is dead space air?
- air that is breathed in but never reaches gas exchange areas
- fills respiratory passages
- nose
- pharynx
- trachea
What are the ways that the lungs can be expanded and contracted?
- downward and upward movement of the diaphragm to lengthen/shorten chest cavity (quiet breathing)
- elevation and depression of the ribs to increase/decrease anteroposterior diameter of chest cavity
What are the important muscles that raise the rib cage?
- external intercostals
- sternocleidomastoid (life sternum upwards)
- anterior serrati (lift many ribs)
- scaleni (lift first 2 ribs)
What are the important muscles that lower the rib cage?
- abdominal recti
- internal intercostals
What creates the suction effect in the lungs?
- lymph drainage of excess fluid between the lung pleural membrane and pleural surface of thoracic wall
- lungs held against the thoracic wall
What is intrapleural pressure?
- pressure of fluid in the thin space between lung pleura and chest wall pleura
- usually slightly negative pressure
- varies over length of lungs
Explain how intrapleural pressure changes during respiration
- inspiration: expansion of chest cage pulls lungs outward
- negative pressure increases to about -7.5cm H2O
- air sucked into lungs
- expiration is opposite
What is alveolar pressure?
pressure of air inside lung alveoli
Explain how alveolar pressure changes during respiration?
- when glottis is open and no air flowing, pressure in all parts of respiratory tree is equal to atmospheric pressure
- inspiration: chest wall expansion, alveolar pressure decreases to about -1cm H2O
- pulls 0.5L into lungs
- expiration: opposite occurs
What is transpulmonary pressure?
- pressure difference between that in the alveoli and that on the outer surfaces of the lungs
- measure of elastic forces that tend to collapse lungs
What processes in respiration is energy needed for?
- contracting muscles for inspiration (in quiet breathing diaphragm comprises 75% of energy expenditure)
- stretch elastic elements
- overcome airway resistance
- overcome frictional forces arising from viscosity of the lung and chest wall
- overcome intertia of air and tissues
What has the greatest effect on airway resistance?
airway radius
In what conditions is turbulent flow more likely?
high velocities and large diameter airways
What is static compliance?
- the extent to which the lungs will expand for each unit increase in transpulmonary pressure
Determined by :
- elastic forces of lung tissue itself (determined by elastin and collagen fibres among lung parenchyma)
- elastic forces caused by surface tension of fluid that lines alveoli
What is the correlation between elastic recoil and compliance?
high compliance means low elastic recoil
Describe the composition of lung tissue when inflated/deflated?
- deflated: fibres are contracted and kinked
- inflated: fibres stretched and unkinked
Describe the changes to the lung in pulmonary fibrosis
- deposition of fibrous tissue making lung stiff
- lung compliance decreased
- smaller changes in lung volume for small changes in transpulmonary pressure
- patients breathe more shallowly and rapidly
- decreases in RV, FRC and TLC
Describe the changes to the lung in emphysema (COPD)
- alveolar and capillary walls progressively destroyed, elastic tissue
- lung compliance increased
- larger changes in lung volume for smaller changes in transpulmonary pressure
- airway resistance increased
- patients breathe more slowly and deeply
- increases in RV, FRC and TLC
Describe the lung changes in chronic bronchitis (COPD)
- mucus and airway inflammation produce an increase in airway resistance
- increases in RV, FRC and TLC
- compliance is normal
What is surface tension?
measure of the force acting to pull a liquid’s surface molecules together at an air-liquid interface
Describe the production of surfactant
- lipid components enter type II cell from bloodstream
- secreted by type II alveolar epithelial cells
- alveolar macrophages help in degrading surfactant
- type II cells take up rest and recycle/destroy it
What makes up surfactant mixture?
- DPPC
- proteins: surfactant apoproteins (SP-A/B/C/D)
Describe the role of surfactant
- reduces surface tension of H2O minimising fluid accumulation in alveolus
- increases compliance to it is easier to inflate the lungs
- reduces pressure 4.5 times
- helps keep alveolus size relatively uniform during respiratory cycle
What is the method for studying pulmonary ventilation?
spirometry
What is tidal volume?
volume of air inspired or expired with each normal breath
What is inspiratory/expiratory reserve volume?
- inspiratory reserve volume: extra volume of air that can be inspired over and above normal tidal volume
- expiratory reserve volume: max extra volume of air that can be expired by forceful expiration after end of normal tidal expiration
What is residual volume?
volume of air remaining in lungs after most forceful expiration
