Gaseous Exchange and Mechanisms of Breathing

The Mechanical Process of Gaseous Exchange

• Breathing is defined as a mechanical process necessary for gaseous exchange.
• The fundamental mechanism involves the movement of air into and out of the lungs.
• This movement occurs as a direct result of differences between the atmospheric air pressure and the air pressure existing inside the lungs.
• The process specifically involves the coordination of different muscles and changes in the volume and pressure within the thoracic cavity.

The Mechanism of Inhalation

• Definition: Inhalation is the phase of breathing characterized by air moving into the lungs.
• Muscle Activity:
  • The diaphragm muscle contracts, which causes it to flatten and move downwards.
  • The external intercostal muscles, located between the ribs, contract.
• Skeletal and Cavity Changes:
  • The ribcage lifts upwards and pushes outwards.
  • These actions result in an increase in the volume of the chest (thoracic cavity).
• Pressure Dynamics:
  • As the volume of the chest increases, the air pressure within the lungs decreases.
  • Because the atmospheric pressure becomes greater than the pressure inside the lungs, air naturally flows into the lungs to fill the space.
• Process Nature: Inhalation is considered an active process because it necessitates the contraction of muscles.

The Mechanism of Exhalation

• Definition: Exhalation is the phase of breathing characterized by air being pushed out of the lungs.
• Muscle Activity:
  • The diaphragm relaxes and moves upwards, returning to its resting position.
  • The external intercostal muscles relax.
• Skeletal and Cavity Changes:
  • The ribcage moves downward and inward.
  • These actions result in a reduction in the volume of the chest (thoracic cavity).
• Pressure Dynamics:
  • As the chest volume reduces, the air pressure inside the lungs increases relative to the outside atmospheric air pressure.
  • Consequently, air is forced out of the lungs to equalize the internal and external pressures.
• Process Nature: Exhalation is considered a passive process because it occurs through the relaxation of muscles.

Comparative Analysis of Inhaled and Exhaled Air

• The internal atmospheric composition changes between inhalation and exhalation depending on the physiological state of the body (e.g., rest vs. exercise).

• Nitrogen ($N_2$) Levels:

  • Inhaled air: $78\%$
  • Exhaled air from a sleeping person: $78\%$
  • Exhaled air from a person exercising: $78\%$
  • Note: Nitrogen is not utilized in the human breathing system.

• Oxygen ($O_2$) Levels:

  • Inhaled air: $21\%$
  • Exhaled air from a sleeping person: $16\%$
  • Exhaled air from a person exercising: $12\%$
  • Note: Oxygen is consumed by cells for use in the process of cellular respiration.

• Carbon Dioxide ($CO_2$) Levels:

  • Inhaled air: $0.04\%$
  • Exhaled air from a sleeping person: $4\%$
  • Exhaled air from a person exercising: $9\%$
  • Note: Carbon dioxide is a byproduct of cellular respiration and must be removed from the cells.

Physiological Summary of Gases

• Nitrogen ($N_2$): Remains constant because it is inert in regards to human respiratory metabolism.
• Oxygen ($O_2$): Decreases during exhalation because it is used by cells for energy production. The concentration is lower during exercise as the body's demand for energy increases.
• Carbon Dioxide ($CO_2$): Increases significantly during exhalation because it is produced by cells. The concentration is higher during exercise as the rate of cellular respiration increases to meet higher energy demands.