Physiology

• Air passes through


  1. Nose

  2. Pharynx

  3. Larynx

  4. Trachea

  5. Bronchial tree

  6. Lungs

  
• Airways


  • Divided into generations
  • Each generation getting smaller, leading to increase in surface area
• Zones of the airways


  • Conducting zone – larynx to terminal bronchioles
  • Respiratory zone – respiratory bronchioles and alveolar sacs

  
• Bronchi branching


  1. Primary bronchi branch into secondary bronchi

  2. Secondary bronchi branch into tertiary or segmental bronchi

  3. Tertiary bronchi branch into multiple bronchioles

  4. Bronchioles branch into terminal bronchioles

  5. 1 tertiary bronchus = 6500 terminal bronchioles

  6. Each tertiary bronchus supplies air to a single bronchopulmonary segment
• Bronchopulmonary segments


  • Right lung has 10
  • Left lung has 8 or 9

  
• Conducting zone


  Larynx to terminal bronchioles
• Respiratory zone


  Respiratory bronchioles and alveolar sacs

  
• Alveoli


  • Approx. 300 million
  • Lined with a film of lipoprotein (surfactant)
  • Surfactant lowers surface tension at air/liquid interface, prevents collapse of alveoli, increases lung compliance
  • Premature babies may not have sufficient – respiratory distress syndrome of the newborn

  
• Alveolar epithelial cells


  • Type 1 alveolar – thin cell walls, gas exchange
  • Type II alveolar – secrete surfactant, microvilli
  • Alveolar macrophage
  • Capillary endothelium

  
• Boyle's law


  As pressures change, air moves
• Air flow


  Flow = Pressure difference
• Resistance


  For air flow in/out of lungs: pressure difference = atm pressure – alv pressure

  
• Breathing: inspiration


  1. Brain signals the phrenic nerve

  2. Phrenic nerve stimulates the diaphragm (muscle) to contract

  3. Diaphragm contraction makes the thoracic cavity larger

  4. Volume increases, pressure inside decreases

  5. Air moves from higher pressure (atmosphere) to lower pressure (lungs)

  
• Boyle's law


  P1V1 = P2V2
• Respiratory muscles


  • Diaphragm
  • External intercostal muscles of the ribs
  • Accessory respiratory muscles (activated when respiration increases significantly)

  
• Breathing: exhalation


  1. Phrenic nerve stimulus stops

  2. Diaphragm relaxes and moves up in the chest

  3. Volume decreases, intrapulmonary pressure increases

  4. Air flows out of the lungs to the lower atmospheric pressure

  
• Elastic recoil


  An object's ability to reclaim its original shape once forces are removed
• Lung compliance


  Lung stretchability, determined by elasticity and surface tension forces

  
• Transpulmonary pressure = Alveolar pressure – Pleural pressure

  
• Respiratory capacities


  • Inspiratory capacity
  • Functional residual capacity (FRC)
  • Vital capacity
  • Total lung capacity

  
• External respiration


  Pulmonary gas exchange, diffusion of O2 and CO2 across alveolar-capillary membrane

  
• Internal respiration


  Exchange of O2 and CO2 between systemic capillaries and tissue cells

  
• During resting, only 25% of oxygen is used by body; 75% is retained even in the deoxygenated blood
• Partial pressures of CO2 and O2 in the body


  • Deoxygenated blood: PO2 = 40 mmHg, PCO2 = 45 mmHg
  • Oxygenated blood: PO2 = 100 mmHg, PCO2 = 40 mmHg
  • Alveolar air: PO2 = 105 mmHg, PCO2 = 40 mmHg
  • Atmospheric air: PO2 = 159 mmHg, PCO2 = 0.3 mmHg
  • Systemic tissue cells: PO2 = 40 mmHg, PCO2 = 45 mmHg
• Transport of CO2 and O2 in the blood


  • CO2 more soluble than O2
  • 98.5% of O2 in blood bound to Hb in the erythrocyte, 1.5% dissolved in blood plasma
  • Erythrocytes play important role with haemoglobin, carbonic anhydrase, Cl-HCO3 exchanger
  • Erythrocytes "mop up" CO2 and O2 to maximise concentration gradients
• Oxygen dissociation curve


  Relationship between PO2 and Hb-O2 saturation
• PO2 most important factor in promoting O2 binding and separation
• Deoxygenated blood


  PO2 = 40 mmHg, PCO2 = 45 mmHg
• Oxygenated blood


  PO2 = 100 mmHg, PCO2 = 40 mmHg

  
• Alveolar air


  PO2 = 105 mmHg, PCO2 = 40 mmHg

  
• Atmospheric air


  PO2 = 159 mmHg, PCO2 = 0.3 mmHg
• Systemic tissue cells


  PO2 = 40 mmHg, PCO2 = 45 mmHg
• CO2 more soluble than O2
• 98.5% of O2 in blood bound to Hb in the erythrocyte, 1.5% dissolved in blood plasma
• Erythrocytes


  • Play an important role in transport of CO2 and O2
  • Mop up CO2 and O2
  • Maximise concentration gradients
• Haemoglobin


  Haem contains 4 Fe atoms
• Carbonic anhydrase


  Enzyme involved in CO2 transport
• Cl-HCO3 exchanger "Cl-shift"


  Mechanism for CO2 transport
• Bulk flow in blood


  Mechanism for CO2 and O2 transport
• If all Hb is converted to Hb-O2 - fully saturated