MECHANISM OF GASEOUS EXCHANGE IN MAMMALS | FORM TWO TULL NOTES

The mechanism of gaseous exchange in mammals is the process by which oxygen (O₂) enters the body and carbon dioxide (CO₂) is removed. It takes place mainly in the alveoli (tiny air sacs) of the lungs through diffusion.

1. Ventilation (Breathing)

• During inhalation, the diaphragm contracts and flattens while the rib muscles lift the ribs upward and outward.

• This increases the volume of the chest cavity, lowering air pressure in the lungs.

• Air rich in oxygen enters through the nose or mouth, passes through the trachea, bronchi, and bronchioles, and reaches the alveoli.

• During exhalation, the diaphragm relaxes and becomes dome-shaped, and the rib muscles relax.

• The chest cavity volume decreases, increasing air pressure, forcing air rich in carbon dioxide out of the lungs.

2. Exchange of Gases in the Alveoli

• The alveoli have thin, moist walls and are surrounded by many blood capillaries.
• Oxygen concentration is higher in the alveoli than in the blood, so oxygen diffuses across the alveolar wall into the blood.
• Carbon dioxide concentration is higher in the blood than in the alveoli, so carbon dioxide diffuses into the alveoli to be exhaled.

3. Transport of Oxygen

• Oxygen combines with haemoglobin in red blood cells to form oxyhaemoglobin.
• The blood carries oxygen to body tissues.
• In tissues, oxyhaemoglobin releases oxygen, which diffuses into cells for cellular respiration.

4. Transport of Carbon Dioxide

• Carbon dioxide produced during respiration diffuses from body cells into the blood.
• Most CO₂ is transported as bicarbonate ions in the plasma, while some is carried by haemoglobin or dissolved in plasma.
• In the lungs, bicarbonate ions are converted back to carbon dioxide, which diffuses into the alveoli and is exhaled.

Adaptations of the Alveoli for Efficient Gas Exchange

• Large surface area: Millions of alveoli provide a vast area for diffusion.
• Thin walls: Only one cell thick, allowing rapid diffusion.
• Moist lining: Dissolves gases before they diffuse.
• Rich blood supply: Maintains a concentration gradient by continuously carrying away oxygen and bringing carbon dioxide.
• Continuous ventilation: Keeps oxygen levels high and carbon dioxide levels low in the alveoli.

Summary

Gas exchange in mammals occurs by diffusion across the alveoli. Oxygen moves from the alveoli into the blood and is transported to body cells, while carbon dioxide moves from the blood into the alveoli and is expelled during exhalation. Efficient ventilation, a large alveolar surface area, thin moist walls, and a rich blood supply ensure rapid and effective gas exchange