Breathing and Exchange of Gases

O₂ is needed to break down food for energy and CO₂ is the harmful by-product. Breathing is the exchange of atmospheric O₂ with CO₂ produced by the cells.

Mechanisms of breathing vary among animals. Sponges/coelenterates use simple diffusion; insects use a network of tracheal tubes; aquatic arthropods/molluscs use gills (branchial respiration); terrestrial vertebrates use lungs (pulmonary respiration). Frogs also use moist skin (cutaneous respiration).

Human respiratory system: external nostrils → nasal chamber → pharynx → larynx (with epiglottis) → trachea → primary, secondary, tertiary bronchi → bronchioles → alveoli. Larynx (sound box) is supported by cartilage. The trachea divides at the 5th thoracic vertebra.

The lungs are covered by a double-layered pleura with pleural fluid in between to reduce friction. The thoracic chamber is air-tight — bounded by vertebrae (dorsally), sternum (ventrally), ribs (laterally) and the dome-shaped diaphragm (below).

Two functional parts: conducting part (nostrils → terminal bronchioles) transports, cleans, humidifies and warms the air; respiratory/exchange part (alveoli + ducts) is where O₂ and CO₂ actually move across the membrane.

Breathing has two stages: inspiration (air drawn in) and expiration (air pushed out). Air moves because of a pressure gradient between the lungs (intra-pulmonary pressure) and the atmosphere.

During inspiration, the diaphragm contracts (increases antero-posterior volume) and the external intercostal muscles lift ribs and sternum (dorso-ventral increase). Thoracic and lung volume rise; intra-pulmonary pressure falls → air enters.

During expiration, the diaphragm and intercostal muscles relax. Diaphragm returns to its dome shape; ribs lower. Thoracic and pulmonary volume decrease; intra-pulmonary pressure rises above atmospheric → air goes out. A healthy human breathes 12–16 times per minute.

Respiratory volumes — Tidal Volume (TV): air inspired/expired during normal breathing ≈ 500 mL. Inspiratory Reserve Volume (IRV): extra inspiration ≈ 2500–3000 mL. Expiratory Reserve Volume (ERV): extra expiration ≈ 1000–1100 mL. Residual Volume (RV): air left after a forceful expiration ≈ 1100–1200 mL.

Respiratory capacities (derived) — Inspiratory Capacity IC = TV+IRV; Expiratory Capacity EC = TV+ERV; Functional Residual Capacity FRC = ERV+RV; Vital Capacity VC = ERV+TV+IRV; Total Lung Capacity TLC = RV+ERV+TV+IRV = VC+RV.

Alveoli are the primary site of gas exchange. Exchange between blood and tissues also occurs by simple diffusion driven by pressure (concentration) gradients. Solubility of gases and thickness of the diffusion membrane affect the rate.

Partial pressures (mm Hg): Atmospheric pO₂ ≈ 159, pCO₂ ≈ 0.3; Alveolar pO₂ ≈ 104, pCO₂ ≈ 40; Deoxygenated blood pO₂ ≈ 40, pCO₂ ≈ 45; Oxygenated blood pO₂ ≈ 95, pCO₂ ≈ 40; Tissues pO₂ ≈ 40, pCO₂ ≈ 45.

Solubility of CO₂ is 20–25 times higher than that of O₂, so even with a tiny pressure difference CO₂ diffuses much faster across the membrane.

The diffusion membrane has three layers: thin squamous epithelium of alveoli, endothelium of alveolar capillaries, and basement substance in between. Total thickness < 1 mm. This thin barrier favours rapid gas exchange.

Blood transports O₂ and CO₂. About 97% of O₂ is carried by RBCs and 3% dissolved in plasma. About 20–25% of CO₂ is carried by RBCs (as carbamino-haemoglobin), 70% as bicarbonate (HCO₃⁻) and 7% dissolved in plasma.

Haemoglobin is a red, iron-containing pigment in RBCs. Each Hb molecule can carry up to 4 molecules of O₂ reversibly to form oxyhaemoglobin. Binding depends mainly on pO₂; pCO₂, H⁺ and temperature also affect it.

The Oxygen Dissociation Curve is sigmoid. In alveoli (high pO₂, low pCO₂, low H⁺, lower T) → favours formation of oxyhaemoglobin. In tissues (low pO₂, high pCO₂, high H⁺, higher T) → favours dissociation. Every 100 mL of oxygenated blood delivers about 5 mL O₂ to tissues.

CO₂ transport via bicarbonate: at the tissue, CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺ catalysed by carbonic anhydrase in RBCs. At alveoli the reaction reverses to release CO₂. Every 100 mL of deoxygenated blood delivers about 4 mL CO₂ to alveoli.

The respiratory rhythm centre in the medulla primarily maintains respiratory rhythm. The pneumotaxic centre in the pons can moderate it by reducing inspiration duration.

A chemosensitive area adjacent to the rhythm centre is highly sensitive to CO₂ and H⁺. Increased levels activate it; it signals the rhythm centre to eliminate CO₂. Receptors at the aortic arch and carotid artery also detect CO₂/H⁺. Role of O₂ in regulation is insignificant.

Asthma — difficulty in breathing causing wheezing due to inflammation of bronchi and bronchioles. Emphysema — chronic disorder where alveolar walls are damaged, reducing respiratory surface; major cause is cigarette smoking.

Occupational respiratory disorders — long exposure to dust (e.g., stone-grinding) causes inflammation that progresses to fibrosis (proliferation of fibrous tissues) and serious lung damage. Workers should wear protective masks.