S2) Ventilation and Lung Mechanics Flashcards
Breathing rate and depth is controlled to allow for certain processes.
Identify 5 processes
- Eating/drinking
- Speech
- Defecation
- Parturition
- Change in metabolic requirements (sleep/exercise)
Which two structures both need to function to ensure healthy respiration?
- Lung parenchyma
- Respiratory airways
Inspiration is an active form of tidal breathing.
Describe the processes involved
- Diaphragm contracts and moves down
- Ext. intercostals contract and elevate ribs
- Thoracic cavity expands
- Intrathoracic pressure falls below atmospheric pressure and air flows in
Expiration is an passive form of tidal breathing.
Describe the processes involved
- Muscle contraction ceases
- Muscles relax
- Elastic recoil of the lungs results in return to the resting end-expiratory level
- Air flows out
What is resting expiratory level?
Resting expiratory level refers to the state of equilibrium in the respiratory system before you breathe in and after you breathe out
Identify and describe the forces acting on the lung at the equilibrium position at the end of quiet expiration
- Inward: lung’s elasticity and surface tension generate an inwardly directed force that favours small lung volumes
- Outward: elastic elements of muscles and various connective tissue associated with the rib favour the outward movement of the chest wall
Result = opposing forces balance each other and create a negative pressure gradient in intrapleural space
What is tidal volume?
Tidal volume is the lung volume representing the normal volume of air displaced between normal inhalation and exhalation when extra effort is not applied
What is inspiratory reserve volume?
Inspiratory reserve volume is the additional air that can be forcibly inhaled after the inspiration of a normal tidal volume
What is expiratory reserve volume?
Expiratory reserve volume is the additional air that can be forcibly exhaled after the expiration of a normal tidal volume
What is residual volume?
Residual volume is the volume of air still remaining in the lungs after the expiratory reserve volume is exhaled
What is Inspiratory Capacity?
Inspiratory capacity is the maximum amount of air that can be inspired i.e. inspiratory reserve + tidal volumes
What is Functional Residual Capacity?
Functional residual capacity is the volume of air in the lungs at the end of a passive exhalation
What is vital capacity?
- Vital capacity is the total amount of air that can be expired after fully inhaling
- Vital capacity = inspiratory capacity + expiratory reserve OR inspiratory reserve volume + TV + expiratory reserve volume
What is Total lung volume?
- Total lung volume is the maximum amount of air that can fill the lungs
- Total lung volume = vital capacity + residual volume
What determines the functional residual capacity?
- The balance of elastic forces of the chest wall, favouring outward expansion
- The elasticity and surface tension of the lung, favouring a smaller lung volume
What role do the pleural membranes have in ventilation
- The pleural membranes constitute the pleural seal which holds the lungs to the chest wall
- Hence, as the chest wall expands, the lung is forced to follow
Describe the structure and function of the pleural membranes
- Structure: double-walled sacs enclosing each lung
- Function: slide over each other to enable smooth expansion of the lung
What is the pleural space/cavity?
- The pleural space is the space between the visceral and parietal pleural membranes
- Contains 10-20 ml pleural fluid (lubricant)
Identify the muscles of quiet inspiration and expiration
- Inspiration: diaphragm and external intercostal muscles
- Expiration: due to elastic recoil, no muscles used
Describe the mechanism of quiet expiration and the role of elastic recoil
In quiet expiration, when muscle contraction ceases, the elastic recoil of the lung results in the thoracic cavity and the lung returning to the original equilibrium position (passive process)
Identify the accessory muscles of forced inspiration
- Sternocleidomastoid
- Scalene
- Pectoralis major & minor
- Trapezius
Identify the accesory muscles of forced expiration
- Internal intercostals
- Muscles of the abdominal wall
In the older adult, resting end-expiratory level (FRC) is relatively higher than in the young person.
Why?
- The balance between the chest wall (recoiling out) and the lung (recoiling in) has changed
- Lung tissue has lost its elasticity
What is compliance?
Compliance is the relationship between pressure and volume
C = Δvolume/Δpressure
Describe how the stiffness/slackness of the lungs influences compliance
- Stiff lungs = low compliance (fibrosis)
- Slack lungs = high compliance (emphysema – less elastin)
Identify the factors which affect compliance of the lung
- Elastic fibres (elastin)
- Fibrosis (disease)
- Surface tension
What effect does surface tension have?
- Makes inflation of alveolus harder
- Makes smaller alveoli tend to collapse into larger ones
Surfactant reduces surface tension.
How does it do this?
- Allows the lung to inflate more easily (increased compliance)
- Helps to regulate alveolar size
- Prevents alveolar collapse
Describe the structure of surfactant
- Complex mixture of lipids and proteins
- Secreted by alveolar cells
Describe the production and depletion of surfactant
- Production begins between 25-28 weeks gestational age
- Diminished in Acute Respiratory Distress Syndrome (ARDS)
Ventilation is measured through minute ventilation.
What is the minute ventilation for 15 breaths per minute with a tidal volume of 0.5 L?
Minute ventilation = 15 x 0.5 L/min
= 7.5 L/min
Where in the bronchial tree is the main site of airways resistance?
Upper respiratory tract
How can airways resistance be increased?
- Increased mucus
- Hypertrophy of the smooth muscle and/or oedema
- Loss of radial traction
A small change in radius makes a big difference in resistance.
How is resistance of airways calculated?
- Resistance = Pressure ÷ Flow (units kPa.L-1.s)
- Resistance α 1/r4
Provide examples of clinical conditions which manifest the following:
- Increased mucus
- Hypertrophy of the smooth muscle and/or oedema
- Loss of radial traction
- Increased mucus: chronic bronchitis
- Hypertrophy of the smooth muscle and/or oedema: asthma
- Loss of radial traction: emphysema
Why does air flow into the pleural cavity during pneumothorax?
- Normally, a negative pressure gradient in the pleural space keeps the two pleurae together & facilitates ventilation
- When punctured, air flows from high→low pressure, from the atmosphere into the space until it reaches equilibrium
Why do the lungs recoil when air enters the pleural cavity (in pneumothorax)?
- If air enters the pleural space between the parietal and visceral pleura, the -4cm H2O pressure gradient disappears
- This gradient normally keeps the lung against the chest wall, hence, the lung recoils and collapses
A tube is placed in the pleural space to drain the air in pneumothorax.
How could damage be minimised during this procedure?
- Tube is inserted through the intercostal space under ultrasound guidance
- Tube is inserted above superior border of lower rib as intercostal nerves and vessels run along the inferior border (4/5th intercostal space, mid-axillary line)
Identify 4 intra-abdominal structures that could be damaged due to a stab wound to the chest
- Nerves (intercostal and phrenic)
- Intercostal vessels (veins and arteries)
- Diaphragm
- Aorta
Why would lack of surfactant cause difficulty breathing?
- Less surfactant increases surface tension in alveoli and they don’t inflate properly
- Lungs are stiffer and compliance is lower
What is Respiratory distress of the newborn?
- Neonatal respiratory distress syndrome is a condition occurring in premature infants caused by developmental insufficiency of pulmonary surfactant production and structural immaturity in the lungs
-There is an insufficient amount of surfactant to inflate and ventilate the lungs
Why would a premature baby with Respiratory Distress Syndrome have intercostal recession?
- Intercostal muscles are immature not strong enough to expand the ribcage
- Hence, soft tissues between the lungs draw inwards against the negative pressure
