Mechanisms of respiration

• Boyle's law


  The pressure and volume of a gas have an inverse relationship, when temperature is held constant
• Negative pressure breathing


  • There are no muscles attached to lungs
  • Lungs are passive elastic structures (like balloons)
• Pleural sac


  • Pleural cavity
  • Parietal pleura
  • Visceral pleura

  
• Intrapleural fluid


  Lubricates pleural surfaces and creates intrapleural pressures
• Intercostal muscles


  Involved in breathing
• Intra-pleural & intra-alveolar pressure changes during inspiration & expiration

  1. Thoracic cavity expands

  2. Lung volume increases

  3. P alveolar < P atmosphere

  4. Air flow into lung until P alveolar = P atmosphere

  5. Diaphragm and "inspiratory" intercostals contract

  
• Breathing cycle - Expiration/exhalation (breath out)

  1. Thoracic cavity decreases

  2. Lung volume decrease

  3. P alveolar > P atmosphere

  4. Air flow out of lung until P alveolar = P atmosphere

  5. Diaphragm and "inspiratory" intercostals relax

  
• Branch of pulmonary artery

  • Arterioles
  • Capillary beds
  • Branch of pulmonary vein
• Terminal bronchioles


  • Bronchoconstriction
  • Bronchodilation

  
• Extrinsic & intrinsic regulators


  • ↓CO2
  • ↑CO2
  • ↑O2 vasodilation
  • ↓O2 vasoconstriction
• Central chemoreceptors in medulla oblongata

  Monitor pH (CO2) of cerebrospinal fluid (CSF)
• Peripheral chemoreceptors in aortic and carotid bodies

  Monitor O2, CO2 and pH of blood
• Hering-Breuer inflation reflex


  Respiratory centre input
• Defensive respiratory reflex


  Central controller
• Breathing depends on cyclical respiratory muscle excitation

  1. INSPIRATORY MOTOR NEURONS SEND nerve impulse (2 sec)

  2. STOP nerve impulse (3 sec)

  3. Fine-tune inspiration

  4. Fine-tune expiration

  
• Small increases in PCO2 cause marked increased in respiration
• Decreases in blood PO2 directly stimulate breathing only when the blood PO2 is lower than 60 mmHg