Chapter 3 – Breathing & Gas Exchange

Respiration vs. Breathing

Respiration
- Chemical (oxidation) process that releases energy from food, especially glucose.Breathing
  Mechanical process of inhaling oxygen and exhaling carbon dioxide, essential for facilitating gas exchange in the lungs.
- Occurs inside every living cell (mitochondria) ⇢ links to cellular metabolism & ATP production introduced in earlier chapters.
Breathing (Ventilation)
- Physical movement of air into (inhalation) and out of (exhalation) the lungs.
- Creates the external supply/removal of gases needed for respiration.
- Governed by pressure gradients: $P{high} \rightarrow P{low}$ .
- Foundation for later discussions on respiratory disorders, gas laws, and exercise physiology.

Protective Framework & Gross Anatomy of the Thorax

Ribcage
- Bony cage ⮕ protects lungs/heart and anchors intercostal muscles.
- 12 pairs of ribs; movement changes thoracic volume.
Intercostal Muscles
- External intercostals: contract ⮕ ribs move up & out (↑ volume).
- Internal intercostals: contract primarily during forced exhalation ⮕ ribs move down & in (↓ volume).
Diaphragm
- Dome-shaped sheet of muscle + fibrous centre separating thorax from abdomen.
- Contraction → flattens; relaxation → domed.
- Major contributor (~75 %) to tidal volume during quiet breathing.

The Bronchial Tree (Air-Conduction Pathway)

Trachea (Windpipe)
- Connects larynx/mouth/nose to thoracic cavity.
- Lined with ciliated epithelium & mucus-secreting goblet cells.
- C-shaped cartilage rings keep lumen patent during pressure changes.
Bronchi (Primary Bronchus per Lung)
- Initial split of trachea; contain cartilage plates (not full rings).
Bronchioles
- Progressively narrower tubes arising from bronchi.
- No cartilage; smooth muscle regulates airway diameter ⮕ important in asthma.
Alveolar Ducts & Alveoli
- Terminal sacs (≈ $3\times10^8$ alveoli per adult) providing enormous surface area (> $70\,\text{m}^2$ ).
- Site of gas exchange with pulmonary capillaries.

Pleural Membranes & Pleural Cavity

Visceral & Parietal Pleura
- Two continuous, thin, moist membranes enveloping lungs & lining thoracic wall.
Pleural Cavity
- Very narrow space containing pleural fluid.
- Functions:
- Lubrication → reduces friction during breathing.
- Airtight seal → lungs follow thoracic wall movements (negative intrapleural pressure).
- Clinical link: pleurisy, pneumothorax disrupt this seal ⇢ lung collapse.

Airway Cleaning Mechanisms

Mucus
- Sticky secretion trapping dust, pathogens, smoke particles.
Cilia
- Microscopic hair-like projections beating synchronously ⮕ waft mucus towards pharynx to be swallowed or expectorated.
Importance
- First line of defence against infection; impairment → ↑ risk of bronchitis & pneumonia.
Smoking Impact
- Hot smoke + toxins paralyse/destroy cilia.
- Accumulated mucus stasis fosters bacterial growth → chronic cough, COPD.

Mechanics of Ventilation (Pressure–Volume Changes)

Fundamental principle: $P V = \text{constant}$ (Boyle’s Law) for a closed amount of gas.

Inhalation (Inspiration)

Diaphragm contracts & flattens.
External intercostal muscles contract → ribs lift up/out.
Thoracic volume ↑ → intrapulmonary pressure ↓ below atmospheric pressure.
Air flows in along gradient (≈0.5 L during quiet inspiration).

Exhalation (Expiration)

Diaphragm relaxes (domes).
External intercostals relax → ribs move down/in; internal intercostals may aid during forceful exhalation.
Thoracic volume ↓ → intrapulmonary pressure ↑ above atmospheric.
Air is forced out.

Summary Flow

\text{(\uparrow V{thorax})} \Rightarrow \downarrow P{lungs} \Rightarrow \text{Air in}
\text{(\downarrow V{thorax})} \Rightarrow \uparrow P{lungs} \Rightarrow \text{Air out}

Alveolar Adaptations for Efficient Gas Exchange

Large Surface Area
- Millions of alveoli + folded structure yields huge interface.
Thin Diffusion Barrier
- Both alveolar epithelium & capillary endothelium are 1-cell thick (~ $0.5\,\mu m$ ).
Rich Blood Supply
- Dense capillary network maintains steep concentration gradients (high $\text{CO}2$ in blood, high $\text{O}2$ in alveoli).
Ventilation–Perfusion Coupling
- Continuous air renewal keeps alveolar $\text{O}2$ high / $\text{CO}2$ low.
Moist Lining
- Allows gases to dissolve → faster diffusion (Henry’s Law relevance).
Elastic Fibres
- Permit stretch/recoil aiding passive exhalation.

Smoking: Chemical Components & Pathophysiological Effects

Tar
- Mixture of carcinogens ⮕ initiates mutations, causing lung & other cancers.
- Deposits in bronchi → chronic bronchitis.
- Destroys alveolar walls → emphysema (part of COPD).
Nicotine
- Psychoactive, highly addictive.
- Stimulates adrenal medulla → ↑ adrenaline.
- Vasoconstriction → ↑ blood pressure & heart rate, straining cardiovascular system.
Carbon Monoxide (CO)
- Binds to haemoglobin with ≈200× affinity vs. $\text{O}_2$ forming carboxyhaemoglobin.
- Reduces blood’s $\text{O}_2$ -carrying capacity.
- Leads to tissue hypoxia, triggers polycythaemia.
- Major contributor to coronary heart disease & stroke risk.
Overall Disease Links
- COPD (chronic bronchitis + emphysema) – progressive, irreversible airflow limitation.
- Coronary heart disease – atherosclerosis acceleration, thrombosis.
- Multiple cancers – lung, oral, laryngeal, pancreatic, etc.

Ethical, Social & Public-Health Considerations

Smoking impacts not only users but also passive smokers (second-hand smoke).
Economic burden: healthcare costs, lost productivity.
Regulatory responses: age restrictions, taxation, advertising bans, plain packaging.
Preventive education ties back to understanding basic respiratory physiology.

Key Terms & Definitions (Quick Reference)

Aerobic respiration – energy release using $\text{O}_2$ .
Alveoli – microscopic air sacs for gas exchange.
Bronchioles – small airway branches lacking cartilage.
COPD – chronic obstructive pulmonary disease; umbrella term.
Pleural cavity – fluid-filled space between pleural membranes.
Ventilation – mechanical process of moving air.