Lung physiology

• Airway patency


  The ability of the airway to remain open and allow for adequate airflow
• Ways in which airway patency can be compromised

  • Edema
  • Crushing injuries to bone and cartilage
  • Inhalation/swallowing of a foreign object
  • Deviated nasal septum
  • Nasal polyps
  • Inflammation of mucous membranes/airway allergic reactions/vocal chord changes
  • Spasms of smooth muscle
  • Deficiency on surfactant
  • Tumours
• Observations of compromised airway patency

  • Stridor with breathing (noisy)
  • Secretions
  • Snoring
  • Difficulty inhaling/exhaling
  • Coughing
  • Changes in respiratory status (i.e. ↓ oxygen saturation)

  
• Interventions to bypass obstructions

  • Tracheostomy
  • Endotracheal intubation
• Pulmonary circulation

  The lungs receive blood via two sets of arteries: Pulmonary arteries carry deoxygenated blood from the right heart to the lungs for oxygenation, and Bronchial arteries branch from the aorta and deliver oxygenated blood to the lungs primarily perfusing the muscular walls of the bronchi and bronchioles

  
• Ventilation-Perfusion Coupling

  Blood flow to each area of the lungs matches the extent of airflow to alveoli in that area. In the lungs, vasoconstriction in response to hypoxia diverts pulmonary blood from poorly ventilated areas of the lungs to well-ventilated regions. In all other body tissues, hypoxia causes dilation of blood vessels to increase blood flow.

  
• Respiration

  1. Pulmonary ventilation (breathing)

  2. External (pulmonary) respiration

  3. Internal (tissue) respiration

  
• Pulmonary ventilation

  The movement of air between the atmosphere and the alveoli, consisting of inhalation and exhalation, driven by alternating pressure differences

  
• Boyle's Law

  The pressure of a gas in a closed container is inversely proportional to the volume of the container

  
• Pulmonary ventilation - Breathing

  1. Inhalation is an active process requiring contraction of the diaphragm and external intercostal muscles (rib elevation)

  2. Exhalation during quiet breathing is a passive process (no muscle contractions) due to elastic recoil of the chest wall and lungs

  3. During forceful breathing (exercise) contraction of the abdominal muscles and internal intercostal muscles is required for exhalation

  
• Factors affecting pulmonary ventilation

  • Alveolar surface tension
  • Compliance of the lungs
  • Airway resistance

  
• Surface tension

  Causes the alveoli to assume the smallest possible diameter - must be overcome during breathing. Accounts for 2/3 of elastic recoil which decreases size alveoli during exhalation. Surfactant in alveoli reduces surface tension below that of water and prevents alveoli collapse. Premature infants lack surfactant and thus are prone to alveoli collapse (respiratory distress syndrome), which may require CPAP (continuous positive airway pressure).
• Compliance


  How easy it is to stretch the lungs - high compliance means easy, low compliance means it's hard. Depends on elasticity and surface tension. Scar tissue formation (tuberculosis) would reduce compliance by reducing elasticity.
• Airway resistance


  Airflow equals the pressure difference between the alveoli and atmosphere divided by the resistance. Bronchioles expand during inhalation, decreasing airway resistance. Airway diameter also controlled by smooth muscle in the walls of the airways. Sympathetic input induces muscle relaxation and bronchodilation and decreased airway resistance. Conditions which narrow or obstruct airways increases resistance – Chronic Obstructive Pulmonary Disease (COPD).

  
• Lung volumes - spirometry

  At rest, average ~12 breaths per minute, ~500 ml of air per breath (tidal volume). Minute ventilation (MV) = 12 breaths per minute x 500 ml = 6 L/min. However only ~350 ml (70%) of this reaches the respiratory zone, ~150 ml (30%) remains in the conducting zone or anatomic (respiratory dead space). Alveolar ventilation rate is the volume of air per min that actually reaches the respiratory zone (~4.2 L/min).

  
• Factors affecting lung volumes and capacities

  • Gender
  • Height
  • Age
  • Disease

  
• Inspiratory Reserve Volume (IRV)


  The maximal volume that can be inspired in addition to a tidal inspiration
• Expiratory Reserve Volume (ERV)


  The maximal volume that can be expired in addition to tidal expiration
• Residual Volume (RV)


  The volume remaining in the lungs at the end of a maximal expiration

  
• Minimal volume


  The air remaining in the lungs when the thoracic cavity is opened, providing a medical and legal tool for determining whether a baby is born dead (stillborn) or died after birth - autopsy.
• Inspiratory Capacity (IC)


  The maximal volume inspired following a normal expiration
• Vital Capacity (VC)

  The maximal volume that can be expired following a maximal inspiration, i.e. the largest possible breath you can make
• Functional Residual Capacity (FRC)

  The volume in the lungs at the end of normal expiration when all the muscles of breathing are relaxed

  
• Total Lung Capacity (TLC)


  The volume in the lungs at the end of a maximal inspiration

  
• Respiration

  1. External respiration (pulmonary) - gas exchange between the alveoli and the blood

  2. Internal respiration (tissue) - gas exchange between the systemic capillaries and the tissues of the body

  
• Dalton's Law


  Each gas in a mixture of gases exerts its own pressure as if no other gases were present. The pressure of a specific gas in a mixture is called its partial pressure (Px) and can be calculated by multiplying the percentage of gas in the mixture by the total pressure. The total pressure of the mixture is calculated simply by adding all of the partial pressures.

  
• Henry's Law


  The ability of a gas to stay in solution is greater when its partial pressure is higher and when it has a high solubility in water. The quantity of a gas that will dissolve in a liquid is proportional to the partial pressure of the gas and its solubility.
• Scuba diving - nitrogen narcosis or "rapture of the deep"


  As the PN2 is higher in compressed air, more N₂ will dissolve in body fluids. Excessive amounts of dissolved N₂ may produce giddiness. If a diver comes to the surface slowly, the dissolved N₂ can be eliminated by exhalation. If the ascent is too rapid, N₂ comes out of solution too quickly and forms gas bubbles in the tissues, resulting in decompression sickness (the bends).
• Respiration rate

  The rate of pulmonary and systemic gas exchange depends on several factors, including partial pressure difference of the gases - e.g. altitude sickness: ↑ altitude - ↓ total atmospheric pressure ↓ PO2 (159 mmHg/sea level, 110 mmHg/4,500 m)

  
• Henry's Law


  The quantity of a gas that will dissolve in a liquid is proportional to the partial pressure of the gas and its solubility
• When we open a fizzy drinks bottle

  Henry's Law can explain what happens
• Scuba diving - nitrogen narcosis or "rapture of the deep"


  • As the PN2 is higher in compressed air, more N₂ will dissolve in body fluids
  • Excessive amounts of dissolved N₂ may produce giddiness
  • If a diver comes to the surface slowly, the dissolved N₂ can be eliminated by exhalation
  • If the ascent is too rapid, N₂ comes out of solution too quickly and forms gas bubbles in the tissues, resulting in decompression sickness (the bends)
• Symptoms of decompression sickness include joint pain, dizziness, shortness of breath, extreme fatigue, paralysis, and unconsciousness
• Respiration

  • The rate of pulmonary and systemic gas exchange depends on:
  • Partial pressure difference of the gases
  • Surface area available for gas exchange
  • Diffusion distance
  • Molecular weight and solubility of gases

  
• O₂ has lower molecular weight than CO₂ but CO₂ has about 24x greater solubility, so net outward CO₂ diffusion occurs 20x more rapidly than net inward O₂ diffusion
• External respiration (pulmonary)


  Gas exchange between the alveoli and the blood

  
• Internal respiration (tissue)


  Gas exchange between the systemic capillaries and the tissues of the body

  
• At rest, tissue cells need only 25% of the available O₂ in oxygenated blood; despite its name, deoxygenated blood retains 75% of its O₂ content
• During exercise, more O₂ diffuses from the blood into metabolically active cells for ATP production, causing the O₂ content of deoxygenated blood to drop below 75%
• O₂ transport in blood

  About 1.5% is dissolved in the plasma, 98.5% is carried attached to haemoglobin (Hb)