Mechanics of Breathing Flashcards
What is the definition of breathing?
The bodily function that leads to ventilation of the lungs (external respiration)
Define ventilation
The process of moving gases in (inspiration) and out (expiration) of the lungs
Define the mechanics of breathing
Describes the structural and physiological bases of ventilation
What 2 types of diseases affect lung volumes/capacities?
- Restrictive lung diseases - pulmonary fibrosis (intrinsic) and pneumothorax (extrinsic)
- Obstructive lung diseases - asthma, COPD, emphysema
What does the movement of gas in and out of the lungs depend on?
Pressure differentials between atmosphere and alveoli for movement of gas.
Atmospheric pressure remains constant so flow is determined by changes in pressure within the lungs.
What change in pressure occurs for inspiration to happen?
Muscles of inspiration tense and volume of chest increases as lungs expand. Alveoli pressure falls bellow atmospheric pressure and air is drawn in
Pa < Pb
What change in pressure occurs for expiration to happen?
Muscles of inspiration relax and volume of chest and lungs decreases. Alveoli pressure exceeds atmospheric pressure and air is blown out
Pa > Pb
What do changes in alveolar pressure occur secondary to?
Changes in thoracic volume
What is atmospheric pressure sometimes referred to as?
Barometric pressure (Pb)
What inspiratory muscles are involved in quiet breathing?
- Most work done by diaphragm (moves down)
- External intercostal muscles stabilise rib cage
What inspiratory muscles are involved in breathing with increased effort?
- Diaphragm
- External intercostal muscles lift and expand rib cage
- Accessory muscles used (neck muscles and shoulder girdle muscles)
What does movement of external intercostal muscles result in?
Move ribcage up and out to increase the lateral and antero-posterior diameter of the thorax
Where does force that causes change in volume of chest and lungs come from?
- Contraction
2. Elastic properties of lungs and chest (similar to balloon)
What is tendency of lung?
To collapse unless held inflated by pressure difference between inside and outside
What 2 forces does the pleural pressure reflect?
- The force required to keep lung inflated against its elastic recoil
- The force required to cause airflow in and out of lung
What is result of fixing the shoulder girdle? When is this seen?
Pectoralis major and latissimus dorsi muscles will pull ribcage outwards
Seen in patients with respiratory distress or athletes at end of hard race
How are abdominal muscles involved in forced expiration?
Pulls ribcage downwards and medially in forced expiration
How are expiratory muscles involved in quiet breathing?
- Little voluntary effort required due to elastic recoil of tissues
- Muscles of inspiration relax
How are expiratory muscles involved in increased effort breathing?
- Active contraction of internal intercostals
- Abdominal wall muscles help to push diaphragm back up
What are intercostals innervated by?
The intercostal nerves which arise from the anterior rami of the thoracic spinal nerves from T1 to T11 (respective nerve from thoracic spinal cord)
What is diaphragm innervated by?
Phrenic nerves (C3, C4, C5)
In embryos, where is the origin of the diaphragm? How does this affect its innervation?
- Origin in the neck that starts off as membrane that becomes muscle
- Has nerves coming from cervical cord (phrenic nerves)
- Supply from 3rd, 4th and 5th cervical segments
How are pleura involved in lung expansion?
Transmit thoracic cage expansion into lung volume expansion
Pressure in pleura always precedes the pressure change in alveolus
Thoracic cage expansion exerts an increasing negative pressure on intrapleural space (less than atmospheric so air drawn in)
Why are some neck injuries survivable?
If damage to cervical cord is below the origins of the phrenic nerves (3rd, 4th and 5th) then diaphragm not paralysed
What is change in pressure in pleura affected by?
Movement of chest
How is volume of air moving in and out of lungs during ventilation measured?
Spirometer
- Breathes in and pen moves up
- Breathes out and pen moves down
How is pressure measured?
cm of H20
Negative –> less than atmospheric
Positive –> more than atmospheric
At the end of inspiration/expiration, what has happened pressure wise?
System in equilibrium –> equal pressure between alveoli and atmosphere and no movement of gas
Describe pressure changes during inhalation
- Patient breathes in
- Chest expands and volume increases
- Intrapleural pressure decrease
- Alveolar pressure decreases and becomes negative
- Generates pressure gradient so air sucked in
Describe pressure changes during exhalation
- Patient breathes out
- Volume in chest decreases
- Intrapleural pressure increases
- Alveolar pressure increases and becomes positive
- Air blown out
What is pneumothorax?
Air in the cavity between the lungs and chest wall (between layers of the pleura)
Causes collapse of lung
What is pleural effusion?
Fluid builds up in space between layers of pleura
What are the 4 lung volumes?
- Tidal volume
- Inspiratory reserve volume
- Expiratory reserve volume
- Residual volume
What is tidal volume? What are typical values at rest/during exercise?
The volume of air moved in or out of the lungs during normal breathing
o At rest 6-7 ml/Kg
o During exercise 15 ml/Kg
A typical value for tidal volume of an adult male is 500 ml
What is the inspiratory reserve volume?
After normal expiration, take as deep a breath in as possible
Typical value for a 70kg male is 3000ml
What is the expiratory reserve volume?
After a normal inspiration, breathe out as deeply as possible
Typical value for a 70kg male is 1500ml
What is the residual volume?
Air that remains in lungs after maximal expiration
About 1.0 L
Why does air remain in lungs after maximal expiration?
Because of the rigid nature of the thorax and the pleural attachments of the lungs to the chest wall that prevent complete emptying of the lungs
How is residual volume measured?
- Can’t be measured using spirometry
- Measurement of RV based on the fact air contains around 80% nitrogen neither absorbed nor produced in the body
- By washing all the N2 out of the lungs and measuring its volume, the volume of air that was in the lungs can be estimated.
- To wash the N2 from the lungs, the subject breathes O2 (N2-free) gas and breathes out through a turbine volume-flow meter for a few minutes.
- Normally 2 minutes are required in healthy adults but longer (7 minutes) may be necessary for people with asthma or emphysema.
- The expired gas is collected and its N2 concentration measured and its volume determined.
What is the functional residual capacity?
Volume of air left in the lungs at end of normal expiration
- The opposing elastic recoil forces of lungs and chest wall are in equilibrium
- No exertion by diaphragm etc
What is the recoil pressure of the lung?
Pressure of alveoli - Pressure of pleura
What is the recoil pressure of the chest wall?
Pressure of pleura - Pressure of atmosphere
Define compliance
The change in lung volume per unit exchange in intrathoracic pressure (i.e. measure of lung’s ability to stretch and expand_
What is FRC dependent on?
Compliance of lungs and chest wall
Describe lung with low compliance
Stiff lung with high elastic recoil (seen in fibrosis). Lots of energy required to increase volume.
Describe lung with high compliance
Pliable lung with low elastic recoil (seen in emphysema). Elastic tissue damaged by enzymes (produced by leukocytes in response to inhaled irritant). Little energy required to increase volume
What is volume in lung after maximal expiration? What is recoil pressure of lung?
Residual volume
Intrapleural pressure is as positive as it gets (-3) and alveolar pressure is 0
Recoil pressure –> 0 - -3 = +3 cm H20
What is recoil pressure of lung at end of expiration during tidal breathing?
Relaxed breathing and intrapleural pressure is around -5
Recoil pressure –> 0 - -5 = +5 cm H20
What is the volume in lung at end of normal tidal breathing?
Functional Residual Capacity
Wha is recoil pressure of lung during peak inspiration?
Big breath in and hold it
Generates very negative pressure in intrapleural space (-30) so air drawn in
Recoil pressure –> 0 - -30 = +30 cm H20
What does alveolar pressure remain at?
0 so recoil dependent on pleural pressure
What is volume of chest wall after maximal expiration?
Residual volume
What is recoil pressure of chest at maximal expiration?
Close glottis and relax muscles
Intrapleural pressure is negative (-30) during exhalation
Recoil pressure of chest wall –> -30 - 0 = -30 cm H20
What is atmospheric pressure?
Always 0
What is volume of chest after expiration normal breathing?
Functional residual capacity
What is recoil pressure of chest after expiration normal breathing?
Open glottis with muscles still relaxed
Intrapleural pressure becomes less negative (-5)
Recoil pressire –> -5 - 0 = -5 cm H20
What is recoil pressure of chest at maximal inspiration?
Close glottis and relax muscles
Intrapleural pressure becomes +3
Recoil pressure –> +3 - 0 = +3 cm H20
What is FRC in terms of recoil pressures?
The relaxation point of the respiratory system when chest wall and lung recoil pressures are equal but opposite
What is scoliosis and how does it affect lungs?
Abnormal lateral curvature of spine which causes abnormal alignment of ribcage
How can severe burns affect lungs?
Scar formation causes tightening of chest
How do basal alveoli compare to upper alveoli?
Spaces at top are bigger than spaces at bottom so have different compliance
When you apply an equal pressure change, there is a bigger change in basal alveoli
Basal alveoli are better ventilated
What is the closing capacity?
The volume in the lungs at which its smallest airways, the respiratory bronchioles, collapse. The alveoli lack supporting cartilage so depend on other factors to keep them open
How is relationship between closing capacity and FRC?
Closing capacity is normally less than the FRC (amount of gas that normally remains in lungs) so there is normally enough air within lungs to keep these airways open during inspiration and expiration
What happens to closing capacity as you age?
Gradual increase in closing capacity so small airways begin to collapse at higher volumes/before expiration is complete
What are type 1 alveolar cells responsible for?
Gas exchange
What are type 2 alveolar cells responsible for?
Secrete surfactant
What is function of surfactant?
Acts as detergent to reduce alveolar surface tension
- Increases pulmonary compliance
- Minimises alveolar fluid
What is deficient in infant respiratory distress syndrome?
Type 2 alveoli cells not producing surfactant
What happens to surfactant as alveolar volume increases?
Surfactant becomes more dispersed and molecules less tightly packed
What is the difference in compliance in the lungs due to?
Additional energy required to overcome surface tension forces during inspiration and to recruit and inflate additional alveoli
What happens to resistance as lungs expand?
The calibre (diameter) of the bronchioles expands so there is less resistance to flow - Decreasing radius of bronchus by half reduces flow by 16 fold
What is laminar flow? Where does it occur?
Air flowing through tube in parallel layers with no disruption.
Occurs more in distal (lower) branches of alveolar tree (in smaller bronchi the overall resistance is less)
What is turbulent flow? Where is it localised to?
Air not flowing in parallel layers and becomes chaotic.
Mainly upper airways
How does turbulent flow affect resistance?
Turbulence leads to greater difference in pressure (between outside air and inside lungs) being needed to maintain air flow which, in turn, increases resistance
Intercostal muscles and diaphragm need to work harder to expand and contract lungs
What happens during asthma attack?
- Airways constrict due to increased smooth muscle tone and inflammation within mucosa
- Can decrease diameter causing resistance to airflow to become high –> turbulent flow
During clinical examination, where are breath sounds generated?
In the large, upper airways (high flow rate - turbulent flow)
What is a vitalograph spirometer used to measure?
- The forced vital capacity (FVC)
2. The force expiratory volume in 1 second (FEV1)
What is a peak flow meter used to measure?
Peak expiratory flow rate (PEFR)
What is FEV1/FVC ratio used to distinguish between?
Obstructive and restrictive conditions
- Ratio < 0.7 = obstructive
- Ratio > 0.7 = restrictive
What is PEFR used for?
- Convenient way of measuring airways obstruction (not as good as spirometry) as patients can monitor condition at home
- Wide variations occur, with highest readings in evening and lowest in early morning
Describe reversibility of airway constriction in asthma and COPD
Asthma –> reversible
COPD –> irreversible
Where is the energy used in inspiration to overcome elastic forces stored?
As potential energy which is dissipated in expiration
What is breathing like in restrictive/constructive conditions?
Restrictive –> rapid small volume breaths
Obstructive –> large volume slow breaths
What happens do the work done during a respiratory cycle?
- About half is dissipated during inspiration (as heat) to overcome resistance to airflow
- The remaining energy is stored as potential energy in the elastic structures of the lungs and chest wall, and this stored energy is the driving force for normal expiration
What is a flow-volume loop?
Describes the pressure and volume changes during inspiration and expiration. The amount of work done is given by the area within the loop.
Flow-volume loops measure the velocity of air flowing through the airways in relation to the volume of air moved during inspiration or expiration.
What do the measurements of a flow-volume loop then represent?
The measurements therefore reflect both the condition of the conducting airways and lung capacity. Can give idea of where airflow limitation occurs in lungs. Di
How can flow-volume loops be used in diagnosis?
Different pulmonary diseases give different flow-volume loops
What is the purpose of the recoil of the elastic tissues in the lungs?
To expel air from the alveoli and airways
What is job of external intercostal muscles?
Move the ribcage upwards and outwards to increase the lateral and antero-posterior diameter of the thorax
What does fixing the shoulder girdle do? When can this be seen?
The pectoralis major and latissimus dorsi muscles pull the ribcage outwards.
Seen hard at work in patients with respiratory distress as in acute asthma, and in athletes at the end of a hard race
How else are ribs elevated?
- Neck muscles pull the ribcage upwards
- Sternocleidomastoids elevate the sternum
- The scalenus major and minor muscles will elevate both the first two ribs and the sternum
What muscles pull the ribcage downwards?
The oblique, transversus and rectus abdominis muscles
How much does diaphragmatic/ribcage movement contribute to the increase in thoracic volume?
Diaphragmatic –> 75%
Ribcage –> 25%
Why is less diaphragmatic movement required to expand the thoracic cavity in the upright position than the supine position?
In an upright posture, the abdominal contents sink under gravity, as does the diaphragm. The diaphragm flattens out, increasing its cross-sectional area so the diaphragmatic movement required to achieve expansion of the thoracic cavity is smaller than that required to achieve the same expansion in the supine posture, when the diaphragm is more dome-shaped.
What is shape of diaphragm in supine/upright position?
More dome-shaped
Flattens out
Describe the airways in the upright posture
The airways in the dependent, lowest, parts of the lungs are compressed and narrowed by the weight of the overlying tissues
In these regions, expiration will reduce the volume of air in an airway to a point where the airway closes (closing capacity)
What happens when the closing capacity exceeds the FRC?
Begins to compromise gas exchange
What is the total lung capacity (TLC)?
The maximum amount of gas the lungs can accommodate –> the sum of the 4 volumes (VC + RV). About 6 L
What is the forced vital capacity (FVC) or vital capacity (VC)?
The maximum possible breath that can be taken (typically 5 L)
Made up of 3 volumes (inspiratory reserve volume, expiratory reserve volume, tidal volume)
Why is intrapleural pressure normally subatmospheric?
Because of the elastic recoil of the lungs continually trying to separate the 2 layers
What is the lung volume?
The amount of gas that can be moved with respiration (depends on inspiratory and expiratory effort)
Definition of pulmonary ventilation
The amount of gas moved in or out of the lungs per minute
At rest, what is the work performed by the muscles of respiration?
Between 2% and 5% of the resting O2 consumption
In normal individuals with maximal hyperventilation, what is the work of breathing?
About 30% of the resting O2 consumption
What is most of the resistance to airflow in the lungs offered by?
The large airways, trachea and bronchi (of more than 2mm diameter) rather than the smaller airways
Why do the smaller airways offer less resistance?
There are many more smaller airways in parallel so the effective cross-sectional area is much larger at lower points in the tracheobronchial tree
What are the 8 functions of the lungs?
- Gas exchange
- Act as reservoir of blood
- Conversion of angiotensin I to II
- Expulsion of air to produce sound and speech
- Filtration of small blood clots from blood to stop them entering systemic circulation
- Secretion of immunoglobulin (mainly IgA) into bronchial mucus in response
- Local production of surfactant to decrease surface tension in alveolar cells
- Maintenance of acid-base balance by excretion of CO2
What are the intercostal nerves?
Part of the somatic nervous system
Arise from the anterior rami of the thoracic spinal nerves from T1 to T11
What are the intercostals supplied/drained by?
Intercostal arteries/veins
What is FEV1?
Forced expiratory volume in one second, is the volume of breath exhaled with effort in that timeframe
What is FVC?
Forced vital capacity, is the full amount of air that can be exhaled with effort in a complete breath
What is the FEV1/FVC ratio?
A measurement of the amount of air you can forcefully exhale from your lung
