Breathing and Exchange of Gases

Key Concepts

Introduction to Respiration

Respiration (or breathing) is the process of exchange of oxygen ($O_2$) from the atmosphere with carbon dioxide ($CO_2$) produced by the cells during catabolic activities.

• Respiratory Organs: Differ across animal groups. Sponges/Coelenterates use simple diffusion; Earthworms use moist cuticle; Insects use tracheal tubes; Aquatic animals use gills; Terrestrial animals use lungs.

Human Respiratory System

1. Conducting Part: From external nostrils to terminal bronchioles. It transports air, clears it of foreign particles, humidifies, and brings it to body temperature.
2. Respiratory/Exchange Part: Alveoli and their ducts. This is the site of actual diffusion of $O_2$ and $CO_2$ between blood and atmospheric air.

• Larynx: A cartilaginous "sound box."
• Epiglottis: A thin elastic cartilaginous flap that prevents food from entering the larynx.
• Pleura: Double-layered membrane covering the lungs, with pleural fluid to reduce friction.

Mechanism of Breathing

Breathing involves creating a pressure gradient between the lungs and the atmosphere.

• Inspiration: Diaphragm and external intercostal muscles contract, increasing thoracic volume and decreasing intra-pulmonary pressure, forcing air in.
• Expiration: Muscles relax, decreasing thoracic volume and increasing intra-pulmonary pressure, expelling air.

Respiratory Volumes and Capacities

• Tidal Volume (TV): Volume of air inspired/expired during normal breathing ($\approx 500$ mL).
• Inspiratory Reserve Volume (IRV): Forcible inspiration ($\approx 2500–3000$ mL).
• Expiratory Reserve Volume (ERV): Forcible expiration ($\approx 1000–1100$ mL).
• Residual Volume (RV): Air remaining after forced expiration ($\approx 1100–1200$ mL).
• Vital Capacity (VC): Maximum volume of air one can breathe out after forced inspiration ($VC = ERV + TV + IRV$).
• Total Lung Capacity (TLC): Total air in lungs after forced inspiration ($TLC = VC + RV$).

Exchange of Gases

Occurs by simple diffusion based on partial pressure/concentration gradients.

• Partial Pressure: Pressure contributed by an individual gas in a mixture ($pO_2$, $pCO_2$).
• Diffusion Membrane: Composed of thin squamous epithelium of alveoli, endothelium of alveolar capillaries, and the basement substance. Total thickness is less than 1 mm.
• Solubility: $CO_2$ is 20–25 times more soluble than $O_2$.

Transport of Gases

• Oxygen Transport: 97% by RBCs (as oxyhaemoglobin) and 3% dissolved in plasma. Each haemoglobin molecule can carry 4 $O_2$ molecules.
• Oxygen Dissociation Curve: A sigmoid curve plotting percentage saturation of haemoglobin against $pO_2$. Low $pH$, high $pCO_2$, and high temperature favor dissociation at tissues.
• Carbon Dioxide Transport: 20–25% by RBCs (carbamino-haemoglobin), 70% as bicarbonate ($HCO_3^-$), and 7% dissolved in plasma.
• Carbonic Anhydrase: Enzyme in RBCs that facilitates the conversion of $CO_2$ and $H_2O$ to bicarbonate and vice-versa.

Regulation and Disorders

• Regulation: Controlled by the Respiratory Rhythm Centre (medulla) and Pneumotaxic Centre (pons). Chemosensitive areas respond to $CO_2$ and $H^+$ concentration.
• Asthma: Inflammation of bronchi and bronchioles causing wheezing.
• Emphysema: Damage to alveolar walls (major cause: smoking), reducing respiratory surface.
• Occupational Disorders: Long-term exposure to dust (e.g., stone breaking) leads to fibrosis (lung damage).

Activities

• Measure your breathing rate at rest vs. after exercise.
• Draw a diagram of the human respiratory system and label its parts.
• Discuss the impact of smoking on the respiratory surface area (Emphysema).