Respiratory anatomy, physiology and mechanics of the thorax

1Airways, lobes and segments

• Upper airway (nose, pharynx, larynx) warms, humidifies and filters air. The trachea divides at the carina, at the level of the sternal angle (about T4–T5).
• The right main bronchus is wider, shorter and more vertical than the left, so inhaled objects and aspirated fluid usually go right.
• About 23 generations of branching: generations 0–16 are the conducting zone (no gas exchange; anatomical dead space about 150 mL); 17–23 are the respiratory zone (respiratory bronchioles, alveolar ducts and sacs), with hundreds of millions of alveoli.
• Alveolar cells: type I (thin, for gas exchange) and type II (make surfactant, which lowers surface tension and stops small alveoli collapsing); alveolar macrophages clear particles.
• Mucociliary escalator: cilia sweep a thin mucus layer upwards to the throat; slowed by smoking, dry gas and infection.
• Lobes: right lung has 3 (upper, middle, lower; oblique and horizontal fissures), left has 2 (upper with the lingula, and lower; oblique fissure).

Surface markings (for hand placement and auscultation)

• Oblique fissure: from about the T3 spinous process at the back, round the side, to the 6th costal cartilage at the front. So the lower lobes are mostly at the back, the upper lobes mostly at the front.
• Horizontal fissure (right): along the 4th costal cartilage; the middle lobe lies between it and the oblique fissure, at the front.
• Lower lung border in quiet breathing: 6th rib in the mid-clavicular line, 8th in the mid-axillary line, 10th at the back. The pleura reaches about two ribs lower (8th, 10th, 12th).

Nerves: vagus (bronchoconstriction, mucus secretion); sympathetic β2 (bronchodilation, the target of salbutamol); phrenic nerve C3–C5 to the diaphragm; intercostal nerves to the intercostals. Blood supply: the low-pressure pulmonary circulation (pulmonary artery pressure about 25/10 mmHg) for gas exchange, and bronchial arteries for the airway walls.

2Mechanics of breathing and biomechanics of the thorax

• The diaphragm is the main muscle of quiet breathing (often quoted as about three-quarters of the tidal volume). Where it lies against the inside of the lower ribs (the zone of apposition), its contraction, with the abdominal contents as a fulcrum, also lifts and widens the lower ribs.
• In hyperinflation (COPD) the diaphragm is flat and short, the zone of apposition shrinks, and contraction can pull the lower ribs in (Hoover's sign). This is why forward-lean positions, which push the diaphragm back into a dome, ease breathlessness.
• Posture: slumped sitting and lying flat reduce lung volume; kyphosis, scoliosis and a stiff chest wall (ankylosing spondylitis) restrict it.

• Pressures: pleural pressure is about −5 cmH₂O at rest and about −7.5 cmH₂O at the end of a quiet breath in; alveolar pressure is about −1 cmH₂O during inspiration (air flows in) and +1 cmH₂O during expiration.
• Compliance (stretchiness: volume change per pressure change): about 200 mL/cmH₂O for the lungs alone and about 110 mL/cmH₂O for lungs and chest wall together. Low in fibrosis, oedema, obesity and chest wall deformity; high lung compliance in emphysema (loss of elastic recoil).
• Airway resistance sits mainly in the medium-sized bronchi; it rises with bronchospasm, mucus and airway collapse.
• Work of breathing takes only about 3–5% of the body's energy at rest, but much more in lung disease, so breathing muscles compete with leg muscles for oxygen during exercise.

3Lung volumes and capacities

FRC matters most to physiotherapists: it falls after surgery, with pain, obesity and lying flat, and small airways at the bases then close (collapse). Upright positions, deep breaths and walking restore it. Minute ventilation = tidal volume × rate (about 6 L/min); alveolar ventilation subtracts the dead space.

4Gas exchange and transport

• Oxygen and CO₂ cross the very thin alveolar–capillary membrane by diffusion, which slows if the membrane thickens (fibrosis, oedema) or its area falls (emphysema, lung resection).
• Oxygen is carried about 98.5% bound to haemoglobin and 1.5% dissolved; 1 g of haemoglobin carries about 1.34 mL of oxygen, so anaemia lowers oxygen delivery even with a normal SpO₂.
• CO₂ is carried as bicarbonate (about 70%), bound to proteins as carbamino compounds (about 23%) and dissolved (about 7%).
• Bohr effect: CO₂ and acid make haemoglobin release oxygen in the tissues. Haldane effect: oxygen binding makes haemoglobin release CO₂ in the lungs (one reason high oxygen raises CO₂ in COPD).

Ventilation–perfusion matching and West's zones are on the positioning card.

5Control of breathing

6Physiology behind physiotherapy techniques

7Exam corner

Not asked on its own in the 20 papers (2016–2026); the related questions are on the zones of the lung (S25) and causes of altered V/Q (S23) on the positioning card, and spirometry in obstructive and restrictive disease (W20, S19) on the investigations card.