8.2 - Mechanisms of Breathing

Breathing Mechanism of Insects

• Air movement into and out of the trachea is assisted by relaxation and contraction of the abdominal muscles.
• Inhalation:
  • Abdominal muscles relax.
  • Air pressure in the trachea decreases.
  • Air enters the trachea through the spiracles.
• Exhalation:
  • Abdominal muscles contract.
  • Air pressure in the trachea increases.
  • Air is forced out through the spiracles.

Breathing Mechanism of Fish

• Mouth and operculum movements produce ventilation, increasing water flow over the respiratory surface.
• Inhalation:
  • Mouth opens and the floor of the buccal cavity lowers.
  • Opercular cavity enlarges and the operculum opening closes.
  • Pressure in the buccal cavity decreases.
  • Water containing dissolved oxygen enters the mouth.
• Exhalation:
  • Mouth closes and the floor of the buccal cavity rises.
  • Water flows through the gill lamellae; gaseous exchange between blood and water occurs through diffusion.
  • Operculum muscles relax and the opercular cavity becomes smaller.
  • Buccal-cavity volume decreases; its pressure becomes higher than the external pressure.
  • Water flows out through the open operculum opening.

Breathing Mechanism of Frogs

• An active frog breathes through the buccal cavity and lungs.
• Inhalation — filling the buccal cavity:
  • Mouth and glottis close; nostrils open.
  • Floor of the buccal cavity lowers.
  • Low pressure draws air through the nostrils into the buccal cavity.
• Inhalation — forcing air into the lungs:
  • Glottis opens; nostrils close.
  • Floor of the buccal cavity rises.
  • Increased pressure forces air into the lungs.
• Exhalation:
  • Lungs contract, assisted by abdominal pressure and lung elasticity.
  • Air is expelled from the lungs.
  • Some air leaves through the nostrils; the remainder mixes with air in the buccal cavity.

Breathing Mechanism of Humans

• Inhalation:
  • External intercostal muscles contract; internal intercostal muscles relax.
  • Ribcage moves upwards and forwards.
  • Diaphragm muscles contract; the diaphragm moves downwards and becomes flat.
  • Thoracic-cavity volume increases; thoracic-cavity pressure decreases.
  • Higher external atmospheric pressure forces air into the lungs.
• Exhalation:
  • External intercostal muscles relax; internal intercostal muscles contract.
  • Ribcage moves downwards and inwards.
  • Diaphragm muscles relax; the diaphragm curves upwards into a dome.
  • Thoracic-cavity volume decreases; thoracic-cavity pressure increases.
  • Air is forced out of the lungs.

Comparison of Breathing Mechanisms in Humans and Animals

• Similarities:
  • Special muscular structures expand and contract the respiratory cavity.
  • Breathing involves changes in respiratory-cavity volume and pressure.
• Respiratory apertures:
  • Insect: spiracles.
  • Fish: mouth and operculum.
  • Frog: nostrils.
  • Human: nostrils.
• Structures and actions assisting breathing:
  • Insect: thorax and abdomen; contraction and relaxation of abdominal muscles.
  • Fish: operculum and muscular buccal-cavity floor; movements of the buccal-cavity floor and operculum.
  • Frog: muscular buccal-cavity wall; rapid movement of the buccal-cavity floor and lung elasticity.
  • Human: diaphragm, ribcage and intercostal muscles; contraction and relaxation of intercostal and diaphragm muscles with ribcage movements.