Respiratory Physiology

• Respiratory air flow is determined by the pressure difference between mouth and alveoli. Flow results from either an upstream rise (positive pressure breathing) or a downstream fall in pressure (negative pressure breathing).
• Pressure gradients during normal respiratory cycle:

  A) ATM

  B) < ATM

  C) Inspiration

  
• Pressure gradients during normal respiratory cycle:

  A) ATM

  B) > ATM

  C) Expiration

  
• Pip = intrapleural pressure (aka intrathoracic pressure)
• Palv = Alveolar pressure
• Ptp = Transpulmonary pressure (Palv - Pip)
• Patm = atmospheric pressure
• Palv - Pip = Ptp
• Positive pressure - increase atmospheric pressure relative to lung pressure - air forced into lungs
• Negative pressure - decrease lung pressure relative to atmospheric pressure - air drawn into lungs
• Humans are negative pressure breathers under normal circumstances.
• Assisted breathing utilises positive pressure - if individuals are unconscious we increase the outside pressure to force air into their lungs.
• Inspiration - lungs at lower pressure than atmosphere therefore air is forced into lungs
• Expiration - increased pressure inside the lungs to create a gradient and force air out
• Barometric (atmospheric) pressure ~ 750 mmHg
• Atmospheric pressure is the pressure in the atmosphere.

  Alveolar pressure is the pressure in the alveoli
• Two layers of pleura

  • parietal pleura is connected to chest wall (ribcage)
  • visceral pleura is connected to lung tissue
• Intrapleural cavity - space between two pleural layers containing pleural fluid which provides lubrication and surface tension to pull the two layers together - as the chest wall goes out the parietal membrane goes out and pulls the visceral membrane with it.
• Intrapleural pressure - pressure between two pleural membranes
• The difference between the alveolar and intrapleural pressures is the transpulmonary pressure.
• Intrapleural pressure is always negative (lower than atmospheric) due to counter recoil of chest wall and alveoli - the ribcage wants to pull outwards and alveoli want to pull inwards. By extending and pulling the two membranes apart, you decrease the pressure inside that chamber - therefore negative in relation to the atmosphere.
• Boyle's law - if you increase the space without changing the volume, you will decrease the pressure (and vice versa)
• Inspiration

  • inspiratory muscles contract
  • diaphragm goes from dome to flat shape
  • Ribs come up and out

  Therefore thoracic cavity size increased.
• Physiology of inspiration

  • parietal membrane pulled out with chest wall, due to attraction the visceral membrane is also pulled out but not to the same degree
  • intrapleural pressure decreases because the space gets bigger
  • transpulmonary pressure increases (difference between alveolar and intrapleural pressures)
  • Bigger Ptp - bigger alveoli
  • therefore alveolar pressure decreases - creates a gradient between the atmosphere and the alveoli so air goes IN
• Expiration is passive under normal conditions i.e. unless there is exercise or respiratory distress
• Expiration

  • relaxation of inspiratory muscles
  • chest wall recoils
  • space between visceral and parietal membranes decreases
  • intrapleural pressure becomes less negative
  • transpulmonary pressure decreases (difference between alveolar and intrapleural pressures)
  • alveoli become smaller
  • increase alveolar pressure
  • create gradient - positive to force air out of lungs
• Collagen and elastin fibres help the lung to recoil
• alveolar interdependence means inner alveoli open as well as outer. The outer alveoli are affected by the change in intrapleural pressure. This pulls the next layer and then the next layer etc. Alveolar parenchyma in between is pulled.
• Pneumothorax

  • Pleural seal broken, with pathway either:
  • inwards through lung tissue
  • outwards through chest wall
  • no connection between chest wall and lungs
  • lung tissue wants to come in, chest wall wants to go out, so they separate
  • lungs recoil and collapse
  • elastic recoil of alveoli
• Work of Breathing: 2 factors to overcome

  • resistance
  • compliance
• resistance

  • resistance of respiratory tract to airflow during inspiration and expiration
  • poiseuille's law of fluid dynamics
  • look at inspiration, expiration - usually expiration issue
• compliance

  • measure of the lung's ability to stretch and expand (distensability of elastic tissue)
  • not the elastic properties/elasticity
• Resistance becomes a problem with:

  • airway narrowing (e.g. asthma, croup)
  • asthma is predominantly an expiratory problem
  • increasing respiratory rate
  • eventually the body will try to increase volume of breath but first will increase respiratory rate as it is easier
• children are at greater risk with reduced resistance because they have smaller airways and higher resting respiratory rates.
• Obstructive pulmonary disease affects resistance. Larger to smaller airway diameter.
• compliance

  • lung tissue thickens through disease process e.g. fibrosis
  • restriction to ability to expand
  • expandability of lungs (and chest wall)
  • low compliance - fibrosis
  • high compliance - emphysema
  • value varies as lung inflates
  • low compliance is bad as you need to put more work in
• Restrictive pulmonary disease affects compliance. Reduced ability of chest wall to expand
• Understanding and quantifying resistance in conduction zone is difficult.

  • branching of airways, narrowing of airways, dispensible, compressible - all lead to dynamic resistance
  • airflow within the conduction zone also changes (laminar, turbulent, transitional)
• Presuming air flows through a rigid, smooth bored tube - governed by Poiseuille's law

  dP = V x R1

  (dP = pressure difference, V = airflow, R1 = resistance)
• POISEUILLE'S LAW

  Resistance is directly proportional to viscosity of fluid and the length of tube and inversely proportional to the fourth power of the radius of the tube

  • change in diameter of airway has a massive impact on resistance