Physiology and Systemic regulation

Syllabus Unit: Pre & Para Clinical Subjects

Why it matters

Chapter overview

Physiology is the mechanistic 'why' behind every clinical finding tested elsewhere in the exam — the cardiac cycle's pressure-volume choreography, the nephron's filtration-reabsorption-secretion machinery, the ionic basis of synaptic transmission and muscle contraction, and the Henderson-Hasselbalch logic that turns a single blood gas into a diagnosis — and AIIMS PG expects these mechanisms recalled with the same precision as the diseases they explain.

Postgraduate Medical Exam Syllabus Pre & Para Clinical Subjects 50% weightage

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1Isovolumetric Contraction

2Ejection

3Isovolumetric Relaxation

4Filling (rapid + slow + atrial systole)

1Cardiovascular & Respiratory Physiology

The cardiac cycle's pressure-volume relationships and the oxygen-haemoglobin dissociation curve are the two frameworks AIIMS PG uses to test integrated CVS/respiratory function.

Learning Objectives

Sequence the cardiac cycle's phases against valve states and pressure changes.
State the factors that shift the oxygen-haemoglobin dissociation curve rightward and their physiological benefit.
Calculate cardiac output using the Fick principle and state normal resting values.
Explain the physiological basis of regional variation in the ventilation-perfusion ratio in the upright lung.

Key Terms

Cardiac CycleThe sequence of mechanical events per heartbeat — isovolumetric contraction (both valve sets closed, pressure rises), ejection (semilunar valves open), isovolumetric relaxation, then rapid and slow filling followed by atrial systole.

Bohr EffectRightward shift of the oxygen-haemoglobin dissociation curve with increased CO2, H+, temperature or 2,3-DPG, reducing haemoglobin's oxygen affinity so more oxygen unloads to actively metabolising tissue.

Fick PrincipleCardiac output equals oxygen consumption (VO2) divided by the arteriovenous oxygen content difference; used to measure cardiac output from the arteriovenous O2 difference.

V/Q RatioThe ventilation-perfusion ratio, ideally around 0.8; the apex of the upright lung has a high V/Q (relatively over-ventilated) while the base has a low V/Q (relatively over-perfused), since gravity affects blood flow more than airflow.

Cardiovascular and respiratory physiology are tested through their quantitative relationships — a named phase of the cardiac cycle tied to specific valve states, and a named factor tied to a specific direction of shift on the oxygen dissociation curve.

The cardiac cycle

Isovolumetric Contraction → Ejection → Isovolumetric Relaxation → Rapid Filling → Slow Filling (Diastasis) → Atrial Systole

The oxygen-haemoglobin dissociation curve

Key Point2,3-DPG rises in chronic hypoxia and high altitude, shifting the curve right and aiding oxygen unloading to tissue despite a lower overall saturation — a key adaptive mechanism.

Cardiac output

By the Fick principle, cardiac output = VO2 / (arterial O2 content − venous O2 content); normal resting cardiac output is approximately 5 L/min in an average adult.

Ventilation-perfusion matching

In the upright lung, gravity pulls more blood to the base than the apex, and while ventilation also increases toward the base, the effect on perfusion is greater — giving the apex a high V/Q (~3) and the base a low V/Q (~0.6).