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
