Breathing and Exchange of Gases.

BREATHING AND EXCHANGE OF GASES

1. The Human Respiratory System

The pathway starts at the External Nostrils and ends at the Alveoli, which are the primary sites of gas exchange.

  • Conducting Part: From nostrils to terminal bronchioles. It humidifies the air, brings it to body temperature, and filters out foreign particles.

  • Respiratory Part: The Alveoli and their ducts. This is where the actual exchange of gases occurs between the air and the blood.

2. Mechanism of Breathing

Breathing involves two stages: Inspiration (active) and Expiration (passive during normal breathing). This relies on a pressure gradient between the lungs and the atmosphere.

  • Inspiration: The Diaphragm contracts (moves down) and External Intercostal Muscles lift the ribs. This increases the thoracic volume, lowers the intra-pulmonary pressure, and air rushes in.

  • Expiration: Muscles relax, the diaphragm moves up, thoracic volume decreases, pressure increases, and air is pushed out.

3. Respiratory Volumes (The "Numbers" Part)


Respiratory volumes are different volumes of air involved in breathing. Tidal Volume (TV) is the volume of air inhaled or exhaled during normal breathing, which is about 500 mL. Inspiratory Reserve Volume (IRV) is the extra amount of air that can be forcefully inhaled after a normal inhalation, and it is about 2500–3000 mL. Expiratory Reserve Volume (ERV) is the extra amount of air that can be forcefully exhaled after a normal exhalation, which is approximately 1000–1100 mL. Residual Volume (RV) is the amount of air that remains in the lungs even after a forceful exhalation, and it is about 1100–1200 mL. Vital Capacity (VC) is the maximum amount of air that can be exhaled after taking a deep breath, and it is the sum of tidal volume, inspiratory reserve volume, and expiratory reserve volume.

VC = TV + IRV + ERV


4. Exchange and Transport of Gases

Gases move by Simple Diffusion based on a concentration gradient (Partial Pressure).

Oxygen Transport

  • 97% is carried by Hemoglobin in RBCs as Oxyhemoglobin.

  • 3% is dissolved in Plasma.

  • The oxygen dissociation curve is a sigmoid (S-shaped) curve that shows how oxygen binds to hemoglobin in the blood.
    When carbon dioxide (CO₂), hydrogen ions (H⁺), or temperature increase, the curve shifts to the right.
    This right shift helps hemoglobin release more oxygen to body tissues.

Carbon Dioxide Transport

Carbon dioxide is much more soluble than oxygen (O₂) and is transported in the blood in three ways:

  1. As bicarbonate ions (HCO₃⁻) – about 70%

    • This is the main method of CO₂ transport.

    • The reaction is helped by the enzyme carbonic anhydrase in red blood cells.

  2. As carbamino-hemoglobin – about 20–25%

    • CO₂ combines with hemoglobin in red blood cells to form carbamino-hemoglobin.

  3. Dissolved in plasma – about 7%

    • A small amount of CO₂ is transported directly dissolved in blood plasma.

5. Disorders of the Respiratory System

  • Asthma: Inflammation of bronchi and bronchioles (wheezing).

  • Emphysema: Alveolar walls are damaged (mostly due to cigarette smoking), decreasing the surface area for gas exchange.

  • Occupational Disorders: Silicosis or Asbestosis (caused by long-term exposure to dust in industries).

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