🦠 Breathing & Exchange of Gases Study Notes

4.1 Respiratory Organs

Breathing exchanges atmospheric O₂ for the CO₂ made by cells, in two phases — inspiration and expiration. Animals respire by different structures depending on habitat and body plan:

• Body surface — sponges, coelenterates, flatworms.
• Moist skin / cuticle — earthworms (cutaneous).
• Tracheal tubes — insects.
• Gills (branchial) — fishes, prawns.
• Lungs (pulmonary) — terrestrial vertebrates.

Human air passage:

• The larynx (sound box) helps produce sound; the epiglottis guards the glottis during swallowing.
• The trachea and bronchi have incomplete cartilage rings to stop collapse; the trachea divides at about the 5th thoracic vertebra.
• Lungs sit in the thoracic chamber, covered by a double-walled pleura with lubricating pleural fluid.

4.2 Mechanism of Breathing & Lung Volumes

Air movement follows Boyle's law (pressure ∝ 1/volume at constant temperature):

A healthy human breathes 12–16 times/min. Volumes are measured with a spirometer.

4.3 Exchange of Gases

Gas exchange at the alveoli (and at tissues) happens by simple diffusion, driven by partial-pressure gradients. The diffusion membrane has three thin layers — alveolar squamous epithelium, capillary endothelium and the basement substance between (~0.2 mm).

O₂ diffuses alveoli → blood → tissues; CO₂ diffuses tissues → blood → alveoli. CO₂ is ~20–25× more soluble than O₂, so it diffuses readily despite a smaller gradient.

4.4 Transport of Gases

Oxygen: ~97% is carried as oxyhaemoglobin (each haemoglobin binds up to 4 O₂); ~3% dissolves in plasma. Binding is favoured by high pO₂, low pCO₂, low H⁺ and low temperature (in the lungs).

The oxygen-dissociation curve is sigmoid. In tissues (low pO₂, high pCO₂, high H⁺, higher temperature) it shifts right and O₂ is unloaded — the Bohr effect.

Carbon dioxide is transported three ways:

Every 100 mL of oxygenated blood delivers ~5 mL O₂ to tissues; every 100 mL of deoxygenated blood carries ~4 mL CO₂ to the alveoli.

4.5 Regulation of Respiration

The body precisely controls breathing through centres in the brain:

• Respiratory rhythm centre (medulla oblongata) — sets the basic rhythm.
• Pneumotaxic centre (pons) — can reduce the duration of inspiration, altering the rate.
• Chemosensitive area (near the rhythm centre) — highly sensitive to CO₂ and H⁺; a rise signals adjustments to expel CO₂. O₂ has little direct role.

4.6 Disorders of the Respiratory System

• Asthma: difficulty in breathing with wheezing, due to inflammation and spasm of the bronchi/bronchioles (often allergic).
• Emphysema: alveolar walls are damaged, lowering the respiratory surface area; cigarette smoking is a major cause.
• Occupational respiratory disorders: long dust exposure (stone-grinding, mining) inflames and scars the lungs — e.g. silicosis and asbestosis.