The Respiratory System

• Inspiration is an active mechanical process:
1. Diaphragm: The radial muscles of the dome-shaped diaphragm contract, pulling it downward into a flattened horizontal position, increasing the vertical length and volume of the thoracic chest cavity.
2. External Intercostal Muscles: Simultaneously contract, pulling the ribs and the breastbone (sternum) upward and outward, increasing the anterior-posterior and lateral diameter of the thorax.
• Result: The overall thoracic volume increases significantly.
• By Boyle's Law, the pressure inside the lungs (intrapulmonary pressure) drops below atmospheric pressure (creating negative suction), causing atmospheric air to rush into the lungs to equalize pressure.

• Preventing Collapse: The firm hyaline cartilaginous rings provide structural rigidity, preventing the trachea from collapsing inwards under the negative suction pressure created during deep inhalation.
• Incomplete C-Shape: The rings are incomplete on the posterior (back) side, where the trachea rests directly against the soft, muscular Oesophagus (food pipe).
• This gap allows the oesophagus to expand freely and bulge forward when swallowing a large food bolus without painful obstruction.

• (i) Tidal Volume (TV): The volume of air inspired or expired during a normal, relaxed, unforced breath. Value: $\approx 500\text{ mL}$.
• (ii) Vital Capacity (VC): The maximum volume of air a person can expel from the lungs after a maximum, deepest forced inspiration ($\text{TV} + \text{IRV} + \text{ERV}$). Value: $\approx 3500 - 4500\text{ mL}$.
• (iii) Residual Volume (RV): The volume of air that permanently remains inside the lungs even after the most forceful exhalation. Value: $\approx 1200\text{ mL}$.

• The lungs never collapse because of the Residual Volume ($\approx 1200\text{ mL}$) of air that permanently remains inside the alveoli and bronchial tree.
• Furthermore, the intrapleural fluid pressure within the sealed pleural cavity remains continuously negative relative to the atmosphere, acting as an outward suction that holds the elastic lung walls expanded against the thoracic rib cage.
• This ensures that gas exchange continues smoothly and uninterrupted between breaths.

• (i) Oxygen Transport:
- $97\%$ is transported chemically bound to Hemoglobin in red blood cells as unstable Oxyhemoglobin ($\text{Hb}(\text{O}_2)_4$).
- Remaining $3\%$ is carried dissolved in physical solution in blood plasma.
• (ii) Carbon Dioxide Transport:
- $\approx 70\%$ is transported dissolved as Bicarbonate ions ($\text{HCO}_3^-$) in blood plasma.
- $\approx 23\%$ is transported bound to the globin part of hemoglobin as Carbaminohemoglobin ($\text{HbCO}_2$).
- $\approx 7\%$ is transported dissolved in physical solution in plasma.

• Oxygen ($\text{O}_2$): Inspired air contains $21\%$; Expired air contains $16\%$ (consumed by cellular respiration).
• Carbon Dioxide ($\text{CO}_2$): Inspired air contains $0.04\%$; Expired air contains $4.4\%$ (over $100\times$ higher; excreted from tissues).
• Nitrogen ($\text{N}_2$): Inspired air contains $78\%$; Expired air contains $78\%$ (inert; unchanged).
• Expired air is also saturated with water vapor and warmed to body temperature ($37^\circ\text{C}$).

• The epiglottis is a flexible, leaf-shaped flap of elastic cartilage positioned at the entrance of the larynx.
• During swallowing (deglutition), the larynx rises and the epiglottis folds downward to seal the Glottis (tracheal entrance).
• This acts as a mechanical diverter valve that guides the food bolus safely into the posterior Oesophagus, completely preventing food or liquids from entering the respiratory windpipe (which would cause choking or fatal aspiration).

• Breathing through the nose provides three vital conditioning steps that the mouth cannot perform:
1. Filtration: Nasal hairs and sticky mucus trap airborne dust, soot, pollen, and bacterial pathogens.
2. Warming: An extensive network of warm blood capillaries lining the nasal conchae warms cold incoming air to internal body temperature ($37^\circ\text{C}$) before it reaches delicate lung tissue.
3. Humidification: Mucus evaporates water into dry air, humidifying it to prevent drying of alveolar membranes.