NEW Added to close a syllabus gap — the 2024 syllabus adds "physiology of pregnancy, childhood, aging" explicitly under Physiology.

Physiology at the extremes

Pregnancy, the neonate and the elderly — the three physiological states the syllabus names separately from their clinical anaesthesia.

Pregnancy at term

System	Change	Why it matters
Respiratory	FRC ↓20% · minute ventilation ↑50% (tidal volume, not rate) · O₂ consumption ↑20–60% · PaCO₂ falls to ~4.1 kPa with compensatory ↓HCO₃⁻ (to ~20)	Very rapid desaturation. A compensated respiratory alkalosis is normal
Cardiovascular	CO ↑40–50% (rate and stroke volume) · SVR ↓ · BP falls in 2nd trimester · aortocaval compression from 20 weeks	Left lateral tilt; a further 20–50% CO rise occurs in labour and immediately post-delivery
Haematological	Plasma volume ↑45%, red cell mass ↑25% → dilutional anaemia · hypercoagulable: ↑fibrinogen, ↑VII–X, ↓protein S	Physiological anaemia is normal; VTE risk is markedly raised
Gastrointestinal	↓ Lower oesophageal sphincter tone · ↑intragastric pressure · gastrin from placenta	Aspiration risk — antacid prophylaxis and RSI
Renal	GFR ↑50% · ↓urea and creatinine · glycosuria common	"Normal" creatinine may mask impairment
Other	MAC ↓30% · plasma cholinesterase ↓25% · ↓albumin · reduced local anaesthetic requirement (engorged epidural veins, ↓CSF volume)	Reduce doses across the board

Fetal haemoglobin — two α and two γ chains, binds 2,3-DPG poorly, so the curve is shifted LEFT with a P50 of about 2.5 kPa versus 3.5 in the adult. This favours placental oxygen uptake. HbF is ~70–80% at birth, falling to adult proportions by ~6 months.
Fetal shunts: ductus venosus (bypasses liver), foramen ovale (right to left atrium), ductus arteriosus (pulmonary artery to aorta).
At birth: lung expansion → PVR falls → pulmonary blood flow rises → LA pressure exceeds RA → foramen ovale closes functionally. Rising PaO₂ and falling prostaglandins close the ductus arteriosus within 24–48 h. Clamping the cord closes the ductus venosus.
Persistent fetal circulation can be re-established by hypoxia, acidosis and hypothermia — reverting to right-to-left shunt.
Neonatal physiology: alveolar ventilation:FRC 5:1 · rate-dependent cardiac output · non-shivering thermogenesis via brown fat · immature hepatic enzymes and renal function · blood volume 90 ml/kg.

System	Change with age	Anaesthetic consequence
Respiratory	Closing capacity rises, exceeding FRC at ~44 supine and ~66 erect · ↓chest wall compliance · ↓elastic recoil · ↓PaO₂ · blunted hypoxic and hypercapnic drive	Shunt and V/Q mismatch; higher risk of postoperative respiratory complications
Cardiovascular	↓Arterial compliance → systolic hypertension and widened pulse pressure · LV hypertrophy · diastolic dysfunction · reduced β-receptor responsiveness · fixed stroke volume	Dependent on preload and on atrial kick — AF is poorly tolerated
Renal	GFR falls ~1%/year after 40 · ↓concentrating ability · ↓renin and aldosterone	Drug accumulation; poor tolerance of fluid shifts
Nervous	↓Neuronal density · ↓neurotransmitters · MAC falls ~6% per decade after 40 · autonomic dysfunction	Reduce all doses; high risk of postoperative delirium
Body composition	↓Total body water · ↑fat · ↓albumin · ↓muscle mass · ↓hepatic blood flow	Altered Vd — larger for lipophilic drugs, smaller for water-soluble; higher free fraction
Thermoregulation	Impaired — reduced vasoconstriction and shivering	Rapid intraoperative hypothermia

The overall rule: reduced physiological reserve in every system, so smaller doses, slower titration, and less tolerance of error.
Postoperative delirium and postoperative cognitive dysfunction are distinct: delirium is acute and fluctuating within days; POCD is a subtler, longer-lasting decline.