Physiology · Short notes

Respiratory Physiology: Short Notes & Cheat Sheet

Breathing mechanics and surfactant, lung volumes and spirometry, gas exchange, oxygen and carbon dioxide transport, and control of breathing.

Attempt Respiratory Physiology MCQs

Short notes

Read these before you attempt the topic's MCQs.

Lung volumes and capacities

Capacities are sums of volumes. Anything that includes residual volume cannot be measured by spirometry alone.

Mechanics

Inspiration is active (diaphragm and external intercostals); quiet expiration is passive recoil.
Surfactant from type II pneumocytes lowers surface tension and stops alveoli collapsing. Premature babies lacking it develop respiratory distress syndrome; antenatal steroids speed its production.
Obstructive disease (asthma, COPD) lowers FEV1/FVC below 0.7. Restrictive disease (fibrosis) lowers volumes with a normal or high FEV1/FVC.
Anatomical dead space is about 150 mL. Alveolar ventilation = (tidal volume − dead space) × respiratory rate.
Spirometry cannot measure residual volume, so FRC and TLC need helium dilution or body plethysmography.

Gas transport

About 98.5% of oxygen is carried bound to haemoglobin (about 1.34 mL O₂ per gram); very little is dissolved.
The oxygen dissociation curve is sigmoid, with a P50 of about 26–27 mmHg.
Right shift (lower affinity, more O₂ released): more CO₂, more acid (Bohr effect), higher temperature, more 2,3-BPG.
Left shift (higher affinity): fetal haemoglobin, carbon monoxide, alkalosis, cold.
CO₂ is carried about 70% as bicarbonate (made by carbonic anhydrase in red cells, with the chloride shift), about 20% bound to proteins (carbamino) and about 10% dissolved.
CO₂ diffuses about 20 times faster than O₂.

Control of breathing

Breathing is driven by centres in the medulla and pons.
Central chemoreceptors respond to CO₂ (through H⁺ in CSF); this is the main drive in healthy people.
Peripheral chemoreceptors (carotid and aortic bodies) respond mainly to low oxygen (below about 60 mmHg).
In chronic CO₂ retainers (some COPD patients), high-flow oxygen can reduce breathing, so oxygen is titrated to a target saturation (about 88–92%).
Ventilation–perfusion ratio is about 0.8 overall: higher at the lung apex, lower at the base.

Cheat sheet

High-yield facts in one place. Cover the right-hand column and test yourself.

Lung volumes (adult, approx.)

Volume / capacity	Value
Tidal volume	≈ 500 mL
Inspiratory reserve volume	≈ 3000 mL
Expiratory reserve volume	≈ 1100 mL
Residual volume	≈ 1200 mL
Vital capacity (IRV + TV + ERV)	≈ 4.6 L
Functional residual capacity (ERV + RV)	≈ 2.3 L
Total lung capacity	≈ 5.8–6 L

Dissociation curve shifts

Shift	Causes
Right (O₂ released)	↑ CO₂, ↑ H⁺, ↑ temperature, ↑ 2,3-BPG, exercise
Left (O₂ held)	Fetal Hb, CO, alkalosis, cold

Memory aids

Right shift = Release (CADET face Right): CO₂, Acid, 2,3-DPG, Exercise, Temperature shift the curve right, releasing oxygen to tissues.

Exam traps

Residual volume cannot be measured by simple spirometry.
In healthy people CO₂, not oxygen, is the main stimulus to breathe.
Fetal haemoglobin shifts the curve left (it holds oxygen more tightly).
An FEV1/FVC below 0.7 indicates obstruction, not restriction.

Revised? Now test yourself.

Attempt the Respiratory Physiology MCQs while the notes are fresh. Anything you get wrong is saved to your incorrect list for the next revision round.