Mechanics of breathing Flashcards
What is respiration if this context and what does it contribute to?
• Respiration: exchanges of gases (oxygen and carbon dioxide) between atmosphere, blood and cells
- Contributes to homeostasis
- Regulates the pH of the internal environment
What are the three steps respiration takes place in?
- Pulmonary ventilation (breathing): inspiration and expiration of air between atmosphere and lungs (alveoli)
- External (pulmonary) respiration: Exchange of gases between alveoli and blood in pulmonary capillaries. Blood gains oxygen and loses carbon dioxide
- Internal (tissue) respiration: Exchange of gases between blood in systemic capillaries and tissue cells. Blood loses oxygen and gains carbon dioxide. Carbon dioxide is generated from cellular respiration
What is pulmonary ventilation and what are the two parts it is made up of?
- Physical movement of air in and out of the alveoli of the lungs
Movement in = inspiration
Movement out = expiration
What are the two physical principles pulmonary ventilation relies on?
Boyles Law
Air flows from an area of high pressure to that of low pressure (down a pressure gradient)
What is Boyle’s law?
- When the temperature of a gas is constant, the pressure of the gas varies inversely with volume
- Volume decreases, pressure increases
- Volume increases, pressure decreases
What is the atmospheric pressure value and what does it do?
760 mm Hg
- it compresses our body and everything around us
What are the pressure differences between atmospheric and inter-pulmonary when someone is at rest (just before the beginning of inspiration)?
no air is flowing into or out of the lungs therefore our atmospheric pressures and inter-pulmonary/alveolar pressures are equal (760 mm Hg (0))
How does the pressure change during inspiration, why and how does this affect air movement?
Lung volume increases
Intrapulmonary pressure decreases (now 759 mm Hg (-1))
Air moves into the lungs
How does the pressure change during expiration, why and how does this affect air movement?
Lung volume decreases
Intrapulmonary pressure increases (791 mm Hg (+1))
Air moves out
What is the intrapleural pressure, how is it created and how does it compare to atmospheric and intrapulmonary pressures?
- Refers to the pressure within the pleural cavity
- Always lower than atmospheric and intrapulmonary pressures (negative in comparison) – because of relationship between lungs and chest wall
- Created by elastic recoil of the lungs – creates pressure and allows lungs to expand and contract
- pleural cavity is sort of like a suction pas sticking to both the chest wall and the superior surface of the diaphragm
What are the two forces that cause resistance to breathing that need to be overcome?
- lung/ pulmonary compliance: the ease with which the lungs can be expanded
- airway resistance
What affect pulmonary compliance?
Elasticity of the lung tissue – connective tissue structure
Mobility of chest wall
Surface tension (alveoli)
How does the intrapleural pressure vary during inspiration and expiration?
• As our lung volume increases our intra-pleural pressure become more negative and as our lung volume decreases our intra-pleural pressure becomes less negative
- This isn’t a straightforward pressure change because of the forces it has to overcome
What does elastic recoil of the lung oppose and aid?
Opposes inspiration and aids expiration (where we are trying to decrease the volume of the lungs)
What does our airway resistance oppose and if there was no airway resistance what would this affect?
- Opposes inspiration and expiration
- If no airway resistance then our intra-pleural pressure would be a linear (much smoother) process
What is the elasticity of lung tissue?
- Measure of elastic recoil
- A measure of (lung) volume changes resulting from a given change in pressure
- Very much linked to compliance
What will a more compliant lung do?
stretch further with an increase in pressure than a less compliant lung
How do you calculate compliance?
Compliance = change in lung volume/ change in lung pressure (litres/cmH20)
Calculate the compliance of the lung with these figures:
- A patient inhales 500 ml of air inhaled on a spirometer
- Intrapleural pressure before inspiration is -5 cm H2O and -8 cm H2O at the end of inspiration
- = 0.5L/ (-5cm H2O – (-8cm H2o)) = 3 cm H2O
- = 0.5L/3 cm H2O
- = 0.1667 L/cm H2O
What does the mobility of the thoracic cage do and what may have an affect of the mobility of the thoracic cage?
- Allows expansion of lungs
- A rib fracture of damage to the rib may have an affect on the mobility of the thoracic cage
What is surface tension?
- Caused by intermolecular forces between molecules in a liquid
- Air-fluid interface surface of fluid is under tension like a thin membrane being stretched
Like the thin fluid layer between the alveolar cells and the air
What is Laplace’s law?
describes the relationship between pressure (P), surface tension (T) and the radius (r) of an alveolus (bubble):
P = 2T/r
What happens at equilibrium with an alveolus?
At equilibrium the tendency of increased pressure to expand the alveolus balances the tendency of surface tension to collapse it
What does pulmonary surfactant do?
Pulmonary surfactant greatly reduces surface tension increasing compliance. It also equalizes the pressure differences between small and large alveoli by reducing the surface tension – meaning that there won’t be collapsing of small alveoli
Give a list of how surfactant helps to keep uniform alveolar size and how else it helps:
More concentrated in smaller alveoli (per mm s. area)
Lower surface tension helps to equalise pressure among alveoli of different sizes
Easier to inflate smaller alveoli
Work needed to expand alveoli with each breath greatly reduced
Allows alveoli to dynamically adjust the rates of inflation and deflation
What is neonatal respiratory distress syndrome?
Lack of surfactant secretion in premature babies (28-32 weeks gestation)
Reduced compliance
Alveoli collapse on exhalation
Difficult to inflate lungs
50% die without rapid treatment (surfactant replacement)
What is airway resistance and what is it determined by?
- Major ‘non-elastic’ source of resistance to gas flow
- Increased resistance leads to decreased gas flow
- Resistance mainly determined by radius of conducting airways: trachea, bronchi etc
How does airway resistance change across the respiratory pathway and why?
- Resistance decreases going from trachea -> terminal bronchioles
This is because even though the diameter of the terminal bronchioles gets smaller the overall system is large enough to reduce the resistance to airflow
What are the two factors affecting airway resistance?
- lung volume
- bronchial smooth muscle
How does the lung volume affect airway resistance?
Bronchi dilate as lungs expand
Reduces resistance
How does the bronchial smooth muscle affect airway resistance?
Parasympathetic nerves lead to bronchoconstriction (when we are relaxed and don’t have a huge demand for airflow) – leads to increased resistance
Sympathetic nerves and adrenaline lead to bronchodilation (when we need an increased demand for oxygen) – leads to decreased resistance
Smoke, dust, irritants and histamine cause a reflex bronchoconstriction response to try and keep the substance out of our system
What two things do we use to measure airway resistance and how is it expressed?
forced vital capacity (FVC) and forced expiratory volume in 1 second (FEV1)
FVC – forcibly breathing out vital capacity. Usually has little difference with vital capacity.
FEV1 – the volume of air expired in 1 second
Used to assess changes in resistance to airflow, for e.g. asthma patients
FEV1 most frequently used and expressed as a % of FVC (80% is healthy)
What are the three ways you can assess lung function?
- breath sounds
- pulmonary function tests
- using a spirometer
How does listening to breath sounds assess lung function?
- Presence of mucous/fluid
- Absence of breath sounds: collapsed lung?
- Listen using stethoscope
How do pulmonary function tests work?
- Peak flow meter: measures the speed at which you are able to breathe air out
- Used by chronic asthmatics on a regular basis
What can we measure using a spirometer?
- We can measure maximum inspiration and expiration
- We can measure inspiratory reserve volume, tidal volume, expiratory reserve volume and residual volume (the amount of air left in our lungs after maximum expiration)
- Residual volume is an estimate
What is our normal low volume called?
Our tidal volume
What are the spirometry findings for total lung capacity and how is this found?
Total lung capacity is 6.0 litres for males and 4.2 litres for females
Found by adding up the inspiratory reserve volume, tidal volume, expiratory reserve volume and residual volume
What is the tidal volume?
Volume of air moved per quiet breath (500 ml under resting conditions)
What is the functional residual capacity, what does it do and why can’t it be measured by spirometry?
Volume of air left in the lungs after a normal, passive exhalation (larger than residual volume)
Cannot be measured by spirometry (as it includes the residual volume)
(FRC) helps to stabilize the composition of alveolar air
What is the expiratory reserve volume (ERV)?
The amount of air than can be forcibly exhaled after a normal tidal volume exhalation
Males = 1000mls
Females = 700mls
What is the inspiratory reserve volume?
Amount of air that can be forcibly inhaled after a normal tidal volume inhalation
Males = 3300 mls
Females =1900 mls
What is the vital capacity (VC)?
VC = TV + IRV + ERV
Maximum amount of air than can be expired after a maximum inspiratory effort
Males = 4800ml
Females = 4200 ml
What is the inspiratory capacity (IC)?
IC = TV + IRV
Maximum amount of air than can be inspired after a normal expiration
What is the functional residual capacity (FRC) and how do you calculate it?
FRC = RV + ERV
Volume of air remaining in the lungs after a normal tidal expiration
What is the total lung capacity (TLC)?
Maximum amount of air contained in lungs after a maximum inspiratory effort
Males = 6000mls
Females = 4200mls
How does the respiratory system adapt to changing oxygen demands?
by varying:
Number of breaths per minute (respiratory rate)
Volume of air moved per breath (tidal volume)
What is the pulmonary ventilation work and how do you work it out?
Also called respiratory minute volume (amount of air moved per minute)
= Tidal volume x breathing frequency
500 ml x 12/min
= 6 litres/ min
What is the difference between pulmonary ventilation and alveolar ventilation?
- For every 500 ml Tidal volume, 150 ml doesn’t reach alveolar exchange surface
- Because there is a volume of air in conducting passages that does not participate in gas exchange = anatomical dead space
- alveolar ventilation is the amount of air reaching alveoli each minute
How do you work out alveolar ventilation?
- Respiratory rate x (TV – anatomical dead space)
- 12 x (500 – 150) = 4.2 L/ min
What does a single respiratory cycle consist of?
An inhalation and expiration
What are the most important muscles involved in breathing?
the diaphragm and external intercostals – primary respiratory muscles
When do the accessory muscles become active?
when the depth and frequency of breathing must be greatly increased
Inhalation is an active process. What happens during it?
- Contracting of the diaphragm takes place
- Contraction of the internal intercostal muscles raises the ribs
- Contractions of accessory muscles can assist the external intercostal muscles in elevating the ribs
Is exhalation active or passive?
- passive in quiet breathing
- active in forced breathing
What happens when exhalation is active?
- The internal intercostal muscle and transversus thoracis depress the ribs. This action reduces the width and depth of the thoracic cavity
- The abdominal muscles can assist the internal intercostal muscles in exhalation by compressing the abdomen forcing the diaphragm upwards
What happens in costal breathing?
the thoracic volume changes because the rib cage alters it’s shape
What happens when the muscles of inhalation relax?
the elastic components recoil
How does the size of the pressure gradient between intrapulmonary pressure and atmospheric pressure change when you breathe heavily?
It increases
Why can’t the elastic fibres contract enough to recoil the lungs?
is because they are not strong enough to overcome the fluid bond between the parietal and visceral pleurae. The elastic fibres remain stretched even after a full exhalation. For this reason intrapleural pressure remain below atmospheric pressure
What does the thin surface layer of surfactant interact with and what does this do?
interacts with the water molecules keeping the alveoli open
What increases compliance?
The loss of supporting connective tissue of the lungs. This results in the alveoli increasing beyond their normal size
How do you calculate the anatomic dead space?
Multiply the tidal volume by 0.3
Why does the air in the alveoli contain less oxygen and more carbon dioxide than the atmospheric air?
because the inhaled air mixes with the ‘used’ air in the conducting passageways on the way in
What is respiratory distress syndrome?
If pneumocytes type II produce inadequate amounts of surfactant due to injury or genetic abnormalities the alveoli collapse after each exhalation. This makes breathing difficult. Inspiration must be forceful enough to open the alveoli.
What is transpulmonary pressure?
- 5 CM H20 at rest
- A pressure that is always positive throughout the normal breathing cycle
- Elastic recoil pressure of the lung
- Increases when inhaling
Difference between respiratory tract pressure and pleural cavity pressure
What is intrapulmonary pressure?
- Pressure within the alveoli
- Increases when exhaling
- Decreases when inhaling
- Pressure inside the respiratory tract
