Key Points

Breathing is the process of exchanging atmospheric oxygen ($\text{O}_2$) with carbon dioxide ($\text{CO}_2$) produced by cells. Cellular respiration is the metabolic process where cells use $\text{O}_2$ to break down glucose and produce energy.

Mechanisms of breathing vary; lower invertebrates use their body surface, earthworms use moist cuticle, insects have tracheal tubes, aquatic animals use gills, and terrestrial vertebrates use lungs.

Air travels from the external nostrils through the pharynx, larynx (sound box), trachea, bronchi, and bronchioles, finally reaching the alveoli in the lungs.

The conducting part, from nostrils to terminal bronchioles, transports, filters, and humidifies air. The exchange part, consisting of alveoli, is the site of actual gas exchange between blood and air.

Inspiration is an active process initiated by the contraction of the diaphragm and external intercostal muscles. This increases thoracic volume, creating a negative pressure in the lungs that draws air in.

Normal expiration is a passive process where the diaphragm and intercostal muscles relax. This decreases thoracic volume, increasing intra-pulmonary pressure and forcing air out of the lungs.

Tidal Volume (TV) is the air volume in a normal breath (approx. 500 mL). Residual Volume (RV) is the air remaining in the lungs after forceful expiration (1100-1200 mL).

Vital Capacity (VC) is the maximum air volume breathed out after a forced inspiration (ERV + TV + IRV). Total Lung Capacity (TLC) is the total air in the lungs after a forced inspiration (VC + RV).

Exchange of $\text{O}_2$ and $\text{CO}_2$ occurs by simple diffusion, driven by partial pressure gradients. Partial pressure is the pressure contributed by an individual gas in a mixture.

In alveoli, the partial pressure of oxygen ($\text{pO}_2$) is high (104 mmHg) and carbon dioxide ($\text{pCO}_2$) is low (40 mmHg), facilitating diffusion of $\text{O}_2$ into blood and $\text{CO}_2$ out of blood.

Approximately 97% of $\text{O}_2$ is transported by red blood cells (RBCs) bound to haemoglobin as oxyhaemoglobin. The remaining 3% is carried dissolved in blood plasma.

This S-shaped (sigmoid) curve shows haemoglobin's affinity for oxygen. High $\text{pCO}_2$, high $\text{H}^+$ concentration, and higher temperatures shift the curve to the right, promoting $\text{O}_2$ release to tissues.

Carbon dioxide is transported in three forms: as bicarbonate ions ($\text{HCO}_3^-$) (70%), bound to haemoglobin as carbamino-haemoglobin (20-25%), and dissolved in plasma (7%).

This enzyme in RBCs rapidly converts $\text{CO}_2$ and $\text{H}_2\text{O}$ into carbonic acid, which then forms $\text{H}^+$ and $\text{HCO}_3^-$. The reaction is $CO_2 + H_2O \leftrightarrow H_2CO_3 \leftrightarrow H^+ + HCO_3^-$.

Respiration is regulated by a respiratory rhythm centre in the medulla oblongata. A pneumotaxic centre in the pons can moderate the rhythm centre's function, altering the respiratory rate.

A chemosensitive area adjacent to the rhythm centre is highly sensitive to $\text{CO}_2$ and hydrogen ions ($\text{H}^+$). An increase in these substances stimulates the centre to adjust breathing and eliminate them.

Common disorders include Asthma (inflammation of bronchi), Emphysema (damage to alveolar walls, reducing respiratory surface), and Occupational Respiratory Disorders (fibrosis due to dust exposure in industries).