Lecture on Breathing Mechanics and Control

Introduction to Breathing Mechanics

• Breathing: Essential life-sustaining task performed continuously without conscious thought.
• Control of Breathing: Can be controlled consciously, but often happens automatically (discussed later).

Function and Anatomy of the Lungs

• Purpose of Breathing: Exchange of carbon dioxide (CO2) and oxygen (O2) with the bloodstream.
• Lung Anatomy:
  • Right lung: 3 lobes (superior, middle, inferior).
  • Left lung: 2 lobes (superior, inferior).
  • The heart prevents a third lobe in the left lung.

Mechanisms of Air Exchange

• Thoracic Cavity: Increase volume to allow air intake.
• Volume Increase:
  • Vertically by pulling diaphragm down.
  • Horizontally by expanding ribs out and up.
• Pleurae:
  • Parietal pleura: Membrane on thoracic wall.
  • Visceral pleura: Membrane on lung tissue.
  • Fluid between pleurae reduces friction and keeps lung adherent to the wall.

Breathing Phases and Muscles Involved

Inspiration (Breathing In)

• Main Muscles:
  • Diaphragm: Contracts, pulls downward, increases thoracic cavity's vertical volume.
    • Moves 1 cm during relaxed breathing, up to 10 cm during active breathing.
  • External Intercostals: Elevate ribs, increase thoracic cavity width.
  • Accessory Muscles (Engaged during deep breathing):
    • Sternocleidomastoid: Elevates sternum.
    • Scalenes (anterior, middle, posterior): Elevate first two ribs.
    • Pectoralis minor: Attaches to ribs 3-5, aids in elevating ribs.

Expiration (Breathing Out)

• Passive Mechanism: Normal quiet breathing mainly involves relaxing diaphragm and external intercostals.
• Elastic Recoil: Lungs and muscles naturally recoil to help expel air.
• Active Mechanism (during deep breathing): Involves expiratory muscles like internal intercostals and abdominal muscles:
  • Internal Intercostals: Pull ribs down.
  • Abdominal Muscles: (e.g., Rectus abdominis, external oblique, internal oblique) compress the thoracic cavity, aiding air expulsion.

Influence of Brain and Control Centers

• Medulla Oblongata: Primary respiratory centers located in the brainstem.
  • Dorsal Respiratory Group: Controls normal quiet breathing muscles (diaphragm, external intercostals).
  • Ventral Respiratory Group: Controls forced breathing muscles (accessory muscles for inspiration and expiration).
• Conscious Control: Cerebral cortex can override medulla to control breathing voluntarily.
• Emotional Influence: Hypothalamus regulates breathing in response to emotions (laughter, crying).