Human Physiology

• Cells need oxygen for aerobic respiration and must remove carbon dioxide (a waste product)
• Single-celled organisms exchange gases by diffusion across their cell surface
• Larger organisms need specialised gas exchange surfaces (lungs, gills, tracheal systems)

• Trachea: supported by C-shaped cartilage rings (prevents collapse)
• Bronchi: two branches from trachea, one to each lung
• Bronchioles: smaller branches within each lung
• Alveoli: tiny air sacs at the end of bronchioles — the site of gas exchange
• Lungs are enclosed by the pleural membranes and pleural fluid (reduces friction)

Inspiration (Breathing In)

1. External intercostal muscles contract → ribs move up and out
2. Diaphragm contracts → flattens and moves down
3. Thoracic cavity volume increases
4. Pressure inside lungs decreases (below atmospheric pressure)
5. Air is drawn in through the trachea

Expiration (Breathing Out)

1. External intercostals relax → ribs move down and in (gravity and elastic recoil)
2. Diaphragm relaxes → returns to dome shape (moves up)
3. Thoracic cavity volume decreases
4. Pressure inside lungs increases (above atmospheric)
5. Air is pushed out

Note: Ventilation is caused by pressure changes resulting from volume changes. Air flows from high to low pressure.

Forced Expiration

• Uses internal intercostal muscles (contract to pull ribs down) and abdominal muscles (push diaphragm up)

Adaptations for Efficient Gas Exchange

Direction of Gas Movement

• Oxygen: alveolar air (high $pO_2$) → blood (low $pO_2$) by diffusion
• Carbon dioxide: blood (high $pCO_2$) → alveolar air (low $pCO_2$) by diffusion
• In blood: O₂ binds to haemoglobin → oxyhaemoglobin; CO₂ transported mainly as hydrogen carbonate ions ($HCO_3^-$) in plasma

Type I Pneumocytes

• Extremely thin, flat cells that make up the alveolar wall
• Adapted for gas exchange — short diffusion distance
• Cover ~95% of the alveolar surface area

Type II Pneumocytes

• Cuboidal cells scattered among Type I cells
• Secrete pulmonary surfactant — a mixture of phospholipids and proteins
• Surfactant reduces surface tension of the moisture lining the alveoli
• Without surfactant, alveoli would collapse (atelectasis) during expiration
• Premature babies may lack surfactant → respiratory distress syndrome

A spirometer can measure lung volumes:

• Tidal volume: volume of air in a normal breath (~500 mL)
• Vital capacity: maximum volume of air that can be exhaled after maximum inhalation
• Residual volume: air remaining in lungs after maximum exhalation (cannot be measured by spirometer)
• Ventilation rate = tidal volume × breathing frequency

Question: Explain how the alveoli are adapted for efficient gas exchange. (4 marks)

Solution: Alveoli have a very large total surface area (~70 m²) provided by approximately 300 million alveoli, maximising the area available for diffusion. The alveolar walls are extremely thin (type I pneumocytes are flattened), and together with the capillary wall, the total diffusion distance is less than 1 μm. Each alveolus is surrounded by a dense capillary network, which maintains a steep concentration gradient by continuously bringing deoxygenated blood and removing oxygenated blood. The alveoli are ventilated — breathing constantly refreshes the air, maintaining the gradient. A moist lining (with surfactant from type II pneumocytes) allows gases to dissolve for diffusion and prevents alveolar collapse.

• Ventilation: intercostal muscles and diaphragm change thoracic volume → pressure changes → air flow.
• Alveoli are adapted for gas exchange: large surface area, thin walls (type I pneumocytes), rich blood supply, moist lining.
• Type II pneumocytes produce surfactant to reduce surface tension and prevent collapse.
• O₂ diffuses from alveoli → blood; CO₂ from blood → alveoli (driven by concentration gradients).
• Ventilation rate = tidal volume × breathing frequency.