Unit 05 — Respiratory · Question Bank

What it is

Surfactant is a phospholipid (mainly dipalmitoyl-phosphatidylcholine) secreted by type II alveolar (pneumo)cytes; it adsorbs to the alveolar air–liquid interface and lowers surface tension.

Physiological significance

• ↓Surface tension → ↑lung compliance → easier inflation, less work of breathing.
• Stabilises alveoli of different sizes — its effect is greater in small alveoli, so small ones don't empty into large ones (anti-Laplace).
• Prevents atelectasis (alveolar collapse) at end-expiration.
• Keeps alveoli dry — reduced surface tension lowers the pull drawing fluid into alveoli (anti-oedema).

Clinical link

Premature babies lacking surfactant develop neonatal respiratory distress syndrome (stiff lungs, atelectasis, hypoxia).

Shape and meaning

A sigmoid (S-shaped) curve of O₂ saturation vs PO₂ (cooperative binding). The flat upper plateau makes loading in the lungs safe despite falls in PO₂; the steep lower part allows large O₂ unloading for small PO₂ falls in tissues. P₅₀ ≈ 26–27 mmHg.

Right shift — ↓affinity → unloading

Caused by ↑CO₂, ↑H⁺ (↓pH), ↑temperature, ↑2,3-DPG, exercise (the Bohr effect) — helps deliver O₂ to active tissue.

Left shift — ↑affinity → loading

Caused by ↓CO₂, ↓H⁺ (↑pH), ↓temperature, ↓2,3-DPG, CO and fetal Hb — holds O₂ more tightly.

Obstructive (asthma, COPD)

• Problem: increased airway resistance — difficulty getting air OUT.
• FEV₁ markedly reduced; FVC slightly reduced; FEV₁/FVC low (<70%).
• Lung volumes: air trapping → ↑RV, ↑TLC.

Restrictive (fibrosis, NRDS, chest-wall disease)

• Problem: reduced compliance/expansion — difficulty getting air IN.
• FEV₁ and FVC both reduced proportionally; FEV₁/FVC normal or increased.
• Lung volumes: small stiff lungs → ↓RV, ↓TLC.

Key discriminator

The FEV₁/FVC ratio: low = obstructive, preserved/high = restrictive.

Three forms

• Bicarbonate (~70%): CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻ (carbonic anhydrase in red cells). HCO₃⁻ leaves the cell as Cl⁻ enters (the chloride shift); the H⁺ is buffered by haemoglobin.
• Carbamino-haemoglobin (~23%): CO₂ bound to the amino groups of Hb.
• Dissolved (~7%): physically dissolved in plasma.

Haldane effect

Oxygenation of Hb in the lungs reduces its ability to hold CO₂, promoting CO₂ release — the counterpart of the Bohr effect.

Respiratory centre

The basic rhythm is generated in the medulla oblongata (dorsal and ventral respiratory groups); the pons (pneumotaxic/apneustic centres) modulates it. The medulla is also the vital centre for cardiovascular and GI control.

Central chemoreceptors

In the medulla — the most important drive; they respond to arterial PCO₂ (via H⁺ generated in the CSF). A rise in PCO₂ powerfully increases ventilation.

Peripheral chemoreceptors

The carotid and aortic bodies — respond mainly to a fall in arterial PO₂ (hypoxia), and also to ↑CO₂/↓pH.

Clinical note

In chronic CO₂ retainers the hypoxic drive dominates, so high-flow O₂ can suppress breathing.