Ventilation and Lung Mechanics Flashcards
Define resting expiratory level
Defined by volume of air in the lungs at the end of a passive exhalation
Explain the forces acting on the lung at the equilibrium position at the end of a quiet expiration
- Inward – lung’s elasticity and surface tension generate an inwardly directed force that favours small lung volumes
- Outward – muscles and various connective tissues associated with the rib cage also have elasticity
- These components favour outward movement of the chest wall
- Net effect – at rest two opposing forces balance each other and also creates a negative pressure within the intrapleural space relative to atmospheric pressure
Describe the mechanism of normal quiet inspiration and the role of inspiratory muscles
- In tidal breathing, inspiration is active
- Diaphragm contracts and moves down
- External intercostal muscles contract and elevate the ribs
- Pleural seal ensures that the lungs expand along with the thorax
- Thoracic cavity expands, pressure inside the lungs falls below atmospheric pressure
- Air flows in through the nose or mouth until lung pressure is atmospheric
Describe the mechanism of quiet expiration and the role of elastic recoil
- Tidal expiration is a passive process
- Muscle contraction ceases, muscles relax
- Elastic recoil of the lungs results in return to resting end-expiratory level
State the accessory muscles of inspiration
- Aid forced inspiration
- Sternocleidomastoid
- Scalene
- Pectoralis major & minor
- Trapezius
- Serratus Anterior
State the accessory muscles of expiration
- Aid in forced expiration such as cough
- Internal intercostals
- Muscles of the abdominal wall
Explain the importance of the pleural seal in respiration
- Pleural seal ensure that the lungs expand along with the thorax
- Pleural membrane are essentially double-walled sacs enclosing each lung
- Slide over each other to enable smooth expansion of the lung
- Pleural space contains pleural fluid which acts as a lubricant to reduce friction
Explain why the pressure within the pleural cavity is lower than atmospheric pressure at rest and changes in pleural pressure during respiratory cycle
- At end expiration, pressure within the pleural space is slightly negative (-3 cmH2O)
- Forces the lungs to adhere against the chest wall to prevent collapse due to the recoil nature of lung tissue
- Pleural pressure becomes more negative when breathing in (-7 cmH2O)
Explain how a pneumothorax occurs and why this results in collapse of the lung
- Pneumothorax is when air enters the pleural space, causing the lung to collapse
- Lose the pleural seal
Define the term compliance
- Compliance is the measure of ability of lungs to stretch and expand
- Compliance is the volume change per unit pressure change
- C = ∆volume/∆pressure
- Stiff lungs = low compliance (fibrosis)
- Slack lungs = high compliance (emphysema)
Describe the factors which affect compliance of the lungs
- Elastic fibres reduced in aging and lungs become slacker (increased compliance)
- In the older adult, resting end-expiratory level (FRC) is relatively higher than in the young person
- Diseases such as fibrosis and emphysema
- Surface tension
Explain the effect of surface tension in the alveoli
- A gas-liquid interface wants to achieve a minimum surface area
- Surface tension makes inflation harder and makes smaller alveoli tend to collapse into larger ones (alveolar collapse)
Describe the role of surfactant
- Surfactant acts to reduce surface tension
- Allows the lung to inflate more easily – increased compliance
- Helping to regulate alveolar size
- Prevents alveolar collapse
Describe the structure of surfactant
- Surfactant is a complex mixture of phospholipids and proteins
- Secreted by alveolar cells
- Adequate amount produced at about 35 weeks gestational age
- Problems if baby born too early as lung may collapse
- Water molecules exhibit hydrogen bonding between them
- Surfactant molecules have hydrophilic ends in the fluid to disrupt interacts between surface molecules and there by reducing surface tension
Describe Laplace’s law and apply it to how surfactant works
- Pressure = 2 x T/r
- Where T = surface tension and r = radius
- If surface tension was unchanging, the pressure within a small bubble would exceed that in a large bubble – so the smaller bubbles empty into the larger ones
- But the surfactant molecules are spread more thinly as a bubble expands, so surfactant is less effective and surface tension increases in the bigger bubbles
- Net effect is that pressure is the same in bigger and smaller bubbles, so overall structure remains stable
Describe how air resistance differs in trachea and bronchioles
Upper respiratory tract has higher airway resistance than the lower respiratory tract as total cross sectional area of bronchioles is bigger than cross sectional area of trachea
Define resistance and its relationship with radius
- Resistance = pressure / flow
- Units – kPa/L.s
- Resistance proportional to 1/r4
- Small change in radius males a big difference in resistance
Describe how resistance changes across a respiratory cycle
- Flow reaches a peak early, then gradually falls to zero at residual volume
- As lung volume goes down, the airways narrow
- They will begin to close as the subject approaches RV
- Airways resistance increases as lung volume decreases
Describe the pathogenesis of respiratory distress syndrome of the newborn
- Seen in premature babies, particularly less than 30 weeks old due to lack of surfactant
- Without surfactant, the surface tension of the alveolar sacs is high, leading to an increased tendency of the alveoli to collapse
- Typically present with respiratory distress – cyanosis, grunting, intercostal recession (spaces between intercostal muscles sucked inwards as baby attempts to decrease pressure inside)
- Treatment – surfactant replacement via endotracheal tube with oxygen and assisted ventilation
Define tidal volume, inspiratory reserve volume, expiratory reserve volume, residual volume
- Tidal volume – volume change of lungs during tidal breathing
- Inspiratory reserve volume – total inspiration possible above tidal inspiration
- Expiratory reserve volume – total exhalation possible below tidal exhalation
- Residual volume – air that cannot be breathed out
Define inspiratory capacity, functional residual capacity, vital capacity and total lung volume
- Inspiratory capacity – total inspiration possible
- End of quiet expiration to maximum inspiration
- Functional residual capacity – volume of air in the lungs at the end of a passive exhalation
- Vital capacity = inspiratory capacity + expiratory reserve
- = inspiratory reserve volume + TV + expiratory reserve volume
- Total lung volume = vital capacity + residual volume
Define anatomical (serial), alveolar (distributive) and physiological dead space
- Not all the respired volume is available for gas exchange
- Anatomical deadspace
- Upper respiratory tract between mouth and respiratory bronchioles
- About 150 ml in adults
- Alveolar deadspace
- Where alveoli are ventilated but not perfused, or very poorly perfused
- Physiological deadspace
- Combination of both
Define and calculate pulmonary ventilation rate (the minute volume) and alveolar ventilation rate
- Pulmonary ventilation (minute volume) = respiratory frequency x tidal volume
- eg. 15 breaths per minute x 0.5L = 7.5 L/min
- Alveolar ventilation = respiratory frequency x volume available for gas exchange
- eg. Resting tidal volume = 500ml
- Amount of anatomical deadspace = 150ml
- Tidal volume entering gas exchange region of the lung = 500ml – 150ml = 350ml
- Alveolar ventilation = 15 breaths per minute x 0.35L = 5.25 L/min
- eg. Resting tidal volume = 500ml
