What physiology topics are tested in the PADI Instructor Exam?

The PADI Instructor Exam physiology paper covers: the respiratory drive and carbon dioxide, dead air space, voluntary and involuntary hyperventilation, shallow water blackout, CNS and pulmonary oxygen toxicity (VENTID-C), barotrauma during descent and ascent, lung overexpansion injuries (arterial gas embolism, pneumothorax, mediastinal emphysema, subcutaneous emphysema), decompression sickness (Types I and II), silent bubbles, DCS risk factors, carbon monoxide poisoning, carotid sinus reflex, ear and sinus injuries, and aquatic life injuries.

What is the primary trigger for the urge to breathe?

The urge to breathe is triggered primarily by rising carbon dioxide (CO2) in the blood, not by falling oxygen. Chemoreceptors in the brain and carotid bodies detect rising CO2 and signal the respiratory muscles. This is why oxygen-depleted air can cause sudden loss of consciousness with no preceding sensation of suffocation — the normal warning signal is absent.

What causes shallow water blackout in breath-hold diving?

Shallow water blackout is caused by falling oxygen levels, not rising CO2. When a breath-hold diver deliberately hyperventilates before a dive, CO2 is flushed out — removing the normal urge to surface. Oxygen continues to be consumed throughout the dive. On ascent, as ambient pressure drops, the partial pressure of oxygen falls sharply. At a critical threshold, consciousness is lost without warning.

What are the four lung overexpansion injuries?

The four lung overexpansion injuries are: arterial gas embolism (AGE) — the most serious, where air enters the pulmonary veins and travels to the brain or heart; pneumothorax, where air escapes into the pleural space and collapses the lung; mediastinal emphysema, where air collects in the chest cavity around the heart; and subcutaneous emphysema, where air accumulates under the skin at the neck and collarbone. All are caused by holding the breath on ascent.

What factors increase the risk of decompression sickness?

Factors that increase DCS risk include dehydration, obesity, fatigue, alcohol consumption before or after diving, strenuous exercise before or after diving, cold water, patent foramen ovale (PFO), flying after diving too soon, repetitive dives over multiple days, and a history of previous DCS. These factors affect how efficiently the body absorbs and eliminates nitrogen during and after a dive.

Physiology of Diving - PADI IDC and Divemaster Theory

Physiology is one of the six written subjects in the PADI Instructor Exam. These questions test your understanding of how pressure and gas laws affect the human body — from the mechanics of the respiratory drive to why decompression sickness occurs and how lung overexpansion injuries happen. Work through both topic pages, then test yourself with the practice quizzes.

Part 1

Breathing, Hyperventilation & Oxygen Toxicity

Greek word roots — your exam shortcut
The respiratory drive and CO₂
Dead air space
Voluntary and involuntary hyperventilation
Shallow water blackout
CNS oxygen toxicity and VENTID-C
Pulmonary oxygen toxicity

Start studying →

Part 2

Barotrauma, Lung Injuries & Decompression Sickness

Barotrauma — squeeze and expansion injuries
Lung overexpansion injuries — AGE, pneumothorax, mediastinal and subcutaneous emphysema
Decompression sickness — types, causes and treatment
Silent bubbles and DCS risk factors
Carbon monoxide poisoning
Ear and sinus injuries
Aquatic life injuries

Start studying →

Practice

Physiology Practice Exams

Two full practice quizzes
32 questions with full explanations
Covers all physiology exam topics
Instant scoring with answer feedback

Start practising →