8.2 - Mechanisms of Breathing

Breathing is the repetitive process of taking in oxygen and discharging carbon dioxide in living organisms. The ventilation/breathing mechanisms in animals and humans are different.

Breathing Mechanism of Insects

Figure 6 - The abdominal muscles contract and relax during breathing. When an insect inhales , the abdominal muscles relax and the spiracles open. Air pressure within the tracheae decreases and air is drawn in. On the other hand, when the insect exhales , the muscles contract. This increases the air pressure within which forces air out through the spiracles. The oxygen is transported directly to the body cells and hence no blood system is involved.

Breathing Mechanism of Fish

Fish ventilate by swimming and by opening and closing the operculum. Freshwater is drawn into their mouths which passes over the gills. The process of ventilation helps to increase the flow of water over the respiratory surface.

Figure 7 - During inhalation , the floor of the buccal cavity lowers. At the same time, the opercular cavity becomes larger and the operculum closes. This causes reduced pressure in the buccal cavity. Water containing dissolved oxygen is drawn into the mouth.

During exhalation , the mouth closes which causes the floor of buccal cavity to rise. Water passes through the lamellae and exchange of gases happens between the blood capillaries and water. Meanwhile, the opercular cavity becomes smaller. The pressure within the buccal cavity is higher than the pressure outside. The high pressure forces the operculum to open and water to flow out.

Breathing Mechanism of Frogs

Figure 8 - During inhalation, air is breathed in through the nostrils. This causes the buccopharyngeal floor to lower and the glottis to close. Air is drawn into the buccopharyngeal cavity. When the glottis opens, the nostrils close and the buccopharyngeal floor rises. This increases the air pressure in the cavity which forces the air into the lungs.

During exhalation , when the lung muscles contract, air is forced out from the lungs. This process is facilitated by the abdominal pressure and elasticity of the lungs. While some air escapes through the nostrils, some are mixed with the air in the buccopharyngeal cavity.

Breathing Mechanism of Humans

Figure 9 - During inhalation , the external intercostal muscles contract while the internal intercostal muscles relax. This causes the rib cage to move upwards and outwards. Meanwhile, the diaphragm muscles contract and the diaphragm becomes flattened. These lead to an increased in the volume of the thoracic cavity and the pressure of the thoracic cavity drops. High atmospheric pressure outside forces the air into our lungs.

Figure 10 - During exhalation, the external intercostal muscles relax while the internal intercostal muscles contract. This causes the rib cage to move downwards and inwards. At the same time, the diaphragm muscles relax and the diaphragm curves upwards (dome-shaped). These cause the volume of the thoracic cavity to decrease, which increases the pressure within the cavity. The higher pressure inside the lungs forces the air out of the lungs.

Similarities and Differences of Breathing Mechanisms in Humans and Animals

• Similarities

  • Have special muscular structures that expand and contract the respiratory cavity.
  • Changes in the volume and pressure of the respiratory cavity during breathing.

• Differences

  • Insects

    • Respiratory openings: Spiracles
    • Other structures which help in breathing: Thorax and abdomen
    • Breathing mechanism: Involves contraction and relaxation of abdominal muscles

  • Fish

    • Respiratory openings: Mouth and operculum
    • Other structures which help in breathing: Operculum and the muscular walls of the floor of buccal cavity
    • Breathing mechanism: Involves movements of the floor of buccal cavity and operculum

  • Amphibians

    • Respiratory openings: Nostrils
    • Other structures which help in breathing: Muscular walls of the buccopharyngeal floor
    • Breathing mechanism: Involves the fast movement of the buccopharyngeal cavity floor and elasticity of the lungs

  • Humans

    • Respiratory openings: Nostrils
    • Other structures which help in breathing: Diaphragm, rib cage and intercostal muscles
    • Breathing mechanism: Involves the contraction and relaxation of the intercostal muscles, diaphragm muscles and the movements of the rib cage