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PY12.1-10 | Integrated Physiology — Self-Directed Learning

Temperature Regulation — The Hypothalamic Thermostat

Normal Body Temperature

Figure: Temperature Regulation — The Hypothalamic Thermostat

Core body temperature is maintained within a narrow range of 36.5–37.5°C (oral), with a circadian variation of ~0.5–1.0°C (lowest at ~4 AM, highest at ~6 PM). Rectal temperature is ~0.5°C higher than oral; axillary temperature is ~0.5°C lower. The shell (skin, subcutaneous tissue) temperature varies widely (20–40°C) depending on environment and vasomotor tone, while core temperature (deep organs, blood) is tightly regulated.

The Thermoregulatory Centre

The hypothalamus is the body's thermostat, specifically:
- Preoptic area and anterior hypothalamus (PO/AH): the primary thermoregulatory centre; contains warm-sensitive neurons (3× more than cold-sensitive neurons) that increase firing rate when blood temperature rises
- Posterior hypothalamus: integrates cold responses; contains the 'set-point' mechanism; coordinates shivering and vasoconstriction

Figure: The Thermoregulatory Centre

Temperature Sensors (Afferent Limb)

1. Peripheral thermoreceptors: in skin — predominantly cold receptors (A-delta fibres, active 10–40°C) and warm receptors (C fibres, active 30–46°C). Cold receptors outnumber warm receptors 10:1 (hence we detect cold more readily). Information travels via lateral spinothalamic tract to hypothalamus.



Figure: Temperature Sensors (Afferent Limb)

1. Central thermoreceptors: in the PO/AH, responsive to blood temperature changes of as little as 0.01°C. Also present in spinal cord, abdominal viscera, and great veins.

Heat Balance Equation

Heat production = Heat loss ± Heat storage
- If production > loss → core temperature rises
- If loss > production → core temperature falls
- Thermoregulation aims to keep heat storage at zero (steady state)

Figure: Heat Balance Equation

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Mechanisms of Heat Production (Thermogenesis)

1. Basal metabolic rate (BMR): ~60–75% of total heat production; mainly from liver, brain, heart, and skeletal muscle at rest



Figure: Mechanisms of Heat Production (Thermogenesis)

2. Muscular activity: exercise can increase heat production 10–20× above basal
3. Shivering thermogenesis: involuntary rhythmic contraction of skeletal muscles (4–8 Hz); can increase heat production 4–5× above basal; controlled by the 'shivering centre' in the posterior hypothalamus
4. Non-shivering thermogenesis (NST): primarily in brown adipose tissue (BAT); UCP1 (uncoupling protein 1, thermogenin) in inner mitochondrial membrane dissipates the proton gradient as heat instead of ATP. Important in neonates (who have abundant BAT in interscapular, axillary, and perirenal regions) and during cold acclimatization. Stimulated by noradrenaline via β₃-adrenergic receptors.
5. Specific dynamic action (SDA) of food: diet-induced thermogenesis; protein has the highest SDA (~30% of caloric content released as heat)
6. Thyroid hormones: T₃ increases BMR by stimulating Na⁺/K⁺-ATPase activity and mitochondrial oxidation (chronic cold → ↑ TSH → ↑ T₃/T₄ → ↑ BMR)
7. Sympathetic stimulation: catecholamines increase metabolic rate

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Mechanisms of Heat Loss (Thermolysis)

1. Radiation (~60% of heat loss at rest): Infrared electromagnetic radiation emitted from exposed body surface to cooler surrounding objects. Does NOT require air flow or contact. Effective when ambient temperature < skin temperature.



Figure: Mechanisms of Heat Loss (Thermolysis)

1. Convection (~12%): Heat transfer to moving air or fluid in contact with the body. Wind increases convective loss dramatically (wind-chill factor). Blood convection from core to shell is regulated by cutaneous vasomotor tone.

1. Conduction (~3%): Direct heat transfer by contact with a cooler surface. Normally minor (air is a poor conductor), but significant in water (25× more conductive than air — hence rapid hypothermia in cold water immersion).

4. Evaporation (~25% at rest, up to 100% in extreme heat): The ONLY mechanism that works when ambient temperature > skin temperature.
- Insensible perspiration: ~600–800 mL/day from skin and lungs; not regulated; ~15 kcal/mL water evaporated
- Sweating: active secretion by eccrine sweat glands (2–4 million glands, highest density on palms, soles, forehead); controlled by sympathetic cholinergic fibres (acetylcholine, NOT noradrenaline — unique exception); each litre of sweat evaporated dissipates ~580 kcal
- Maximum sweat rate: ~1.5–2 L/hour (acclimatized); sweat is hypotonic (NaCl ~40–60 mEq/L)
- Evaporation depends on humidity — at 100% humidity, evaporation ceases → heat dissipation fails → heat stroke risk
- Panting: respiratory evaporative cooling (important in dogs; minor in humans)

1. Behavioural responses: seeking shade, removing clothing, fanning (humans rely heavily on behavioural thermoregulation)

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Response to Cold (Hypothalamic Cold Response)

When core temperature falls below set-point:
- Cutaneous vasoconstriction (sympathetic noradrenergic) → reduces skin blood flow → reduces radiative and convective heat loss
- Shivering (posterior hypothalamus → motor pathway)
- Non-shivering thermogenesis (sympathetic → BAT)
- Piloerection ('goosebumps' — vestigial in humans; effective in furred animals)
- Behavioural: curling up, adding clothing, seeking shelter
- Chronic cold: ↑ thyroid hormones, ↑ adrenal cortisol

Figure: Response to Cold (Hypothalamic Cold Response)

Response to Heat (Hypothalamic Heat Response)

When core temperature rises above set-point:
- Cutaneous vasodilation → ↑ skin blood flow (up to 7–8 L/min in extreme heat, from ~300 mL/min at rest) → ↑ radiative and convective heat loss
- Sweating (sympathetic cholinergic)
- Decreased heat production (↓ muscle tone, lethargy)
- Behavioural: seeking shade, drinking cold fluids, removing clothing

Figure: Response to Heat (Hypothalamic Heat Response)

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Fever vs Hyperthermia — A Critical Distinction

Feature	Fever	Hyperthermia
Set-point	Raised (e.g., 39°C)	Normal (37°C)
Mechanism	Pyrogens reset hypothalamic set-point upward	Heat production exceeds dissipation capacity
Sweating	Initially absent (body is 'cold' relative to raised set-point → shivering/vasoconstriction to reach new set-point); resumes when set-point returns to normal	Sweating is maximal initially, then fails (gland fatigue or dehydration)
Antipyretics	Effective (lower set-point back to normal)	Ineffective and contraindicated
Upper limit	Usually self-limited <41°C	Can exceed 42–43°C → fatal
Examples	Infection, inflammation, malignancy	Heat stroke, malignant hyperthermia, thyroid storm

Mechanism of Fever

Figure: Fever vs Hyperthermia — A Critical Distinction

1. Exogenous pyrogens (bacterial LPS, viruses) → activate macrophages
2. Macrophages release endogenous pyrogens: IL-1, IL-6, TNF-α, IFN-γ
3. These cytokines act on the organum vasculosum of the lamina terminalis (OVLT) — a circumventricular organ with a leaky blood-brain barrier
4. OVLT cells produce PGE₂ (prostaglandin E₂) via COX-2 (cyclooxygenase-2)
5. PGE₂ acts on EP3 receptors in the PO/AH → raises the set-point
6. Body now perceives normal temperature (37°C) as 'too cold' → activates heat-generating and heat-conserving mechanisms (shivering, vasoconstriction) until core temperature reaches the new set-point
7. When pyrogen stimulus is removed → set-point returns to normal → body is now 'too hot' → sweating and vasodilation (the 'crisis' or 'defervescence')

Antipyretics (aspirin, paracetamol, ibuprofen) work by inhibiting COX → ↓ PGE₂ → set-point returns to normal. They do NOT work in hyperthermia because the set-point is already normal — the problem is heat overload.

Heat Stroke: core temperature >40°C with CNS dysfunction (confusion, seizures, coma). Sweating ceases (classic heat stroke) or may persist (exertional heat stroke). Treatment: rapid external cooling (cold water immersion, ice packs to groin/axillae/neck), IV fluids, monitoring for DIC, rhabdomyolysis, AKI, and hepatic failure. Antipyretics are contraindicated.

Hypothermia: core temperature <35°C. Classified as mild (35–32°C: shivering, tachycardia), moderate (32–28°C: shivering stops, bradycardia, atrial fibrillation), severe (<28°C: ventricular fibrillation risk, coma). Treatment: passive external rewarming (blankets), active core rewarming (warm IV fluids, warm humidified O₂, peritoneal lavage). Avoid rapid rewarming of extremities (causes 'afterdrop' — cold peripheral blood returns to core).

Exercise Physiology — Cardiovascular and Respiratory Responses

Classification of Exercise

• Isotonic (dynamic/aerobic): muscle length changes, joint movement occurs; sustained moderate effort; e.g., walking, running, swimming, cycling



Figure: Classification of Exercise

• Isometric (static): muscle tension increases without significant change in length; e.g., pushing against a wall, holding a heavy object, handgrip dynamometry
• Most real-world activities are a combination of both

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Cardiovascular Responses to Exercise

Parameter	Rest	Moderate Isotonic Exercise	Maximal Isotonic Exercise	Isometric Exercise
Heart rate (bpm)	70	120–140	180–200	100–120
Stroke volume (mL)	70	100–120	110–130 (plateaus)	Unchanged or ↓
Cardiac output (L/min)	5	12–15	20–35	8–10
Systolic BP (mmHg)	120	160–180	200–220	200–300+
Diastolic BP (mmHg)	80	75–80 (unchanged or slight ↓)	70–80 (↓)	120–150 (↑↑)
Mean arterial pressure	93	Modest ↑	Moderate ↑	Marked ↑
TPR	Normal	↓↓ (vasodilation in muscles)	↓↓↓	↑↑ (compression of intramuscular vessels)

Key differences — Isotonic vs Isometric:
- Isotonic exercise is a volume load on the heart: ↑ venous return → ↑ stroke volume (Frank-Starling) + ↑ HR → ↑↑ cardiac output; TPR falls (muscle vasodilation) → systolic BP rises but diastolic stays same or falls → widened pulse pressure
- Isometric exercise is a pressure load on the heart: sustained muscle contraction compresses intramuscular vessels → ↑ TPR → ↑↑ both systolic AND diastolic BP; cardiac output increases modestly (mainly via HR, not stroke volume)
- Clinical significance: Isometric exercise is dangerous in hypertensive patients and those with cardiac disease because of the dramatic BP rise. Isotonic exercise is recommended for cardiovascular conditioning.

Figure: Cardiovascular Responses to Exercise

Mechanisms of Cardiovascular Response:
1. Central command: motor cortex signals simultaneously activate cardiovascular centres in medulla
2. Exercise pressor reflex: mechanoreceptors and metaboreceptors (Group III and IV afferents) in working muscles detect mechanical deformation and metabolic byproducts (K⁺, H⁺, lactate, adenosine) → reflex sympathetic activation
3. Arterial baroreceptor resetting: the baroreflex operating point shifts upward, allowing higher BP during exercise
4. Skeletal muscle pump: rhythmic contraction squeezes venous blood back to heart → ↑ venous return
5. Respiratory pump: increased ventilatory excursions enhance venous return via intrathoracic pressure changes

Blood Flow Redistribution During Exercise:
- Skeletal muscle: 20% of CO at rest → 80–85% during maximal exercise (vasodilation via local metabolites: adenosine, K⁺, NO, H⁺, CO₂)
- Skin: increases initially (for heat dissipation) but decreases during maximal exercise (vasoconstriction to prioritise muscle perfusion — this is why athletes overheat)
- Splanchnic and renal: decrease by 50–80% (sympathetic vasoconstriction)
- Coronary: increase 4–5× (metabolic vasodilation, mainly adenosine)
- Cerebral: maintained (autoregulation)

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Respiratory Responses to Exercise

Parameter	Rest	Moderate Exercise	Maximal Exercise
Minute ventilation (V̇E)	6 L/min	40–60 L/min	100–150 L/min
Tidal volume	500 mL	2–2.5 L	2.5–3 L (plateaus)
Respiratory rate	12–15/min	20–30/min	40–50/min
O₂ consumption (V̇O₂)	250 mL/min	1–2 L/min	3–6 L/min (V̇O₂ max)
O₂ extraction	25%	50–60%	75–85%
A-V O₂ difference	5 mL/dL	10–12 mL/dL	15–17 mL/dL

V̇O₂ max (maximal oxygen consumption) is the gold standard measure of aerobic fitness. It represents the maximum rate at which the body can consume oxygen during maximal exercise. Typical values: untrained male ~35–40 mL/kg/min, trained endurance athlete ~70–85 mL/kg/min. Limited primarily by cardiac output (O₂ delivery) rather than lung capacity.

Figure: Respiratory Responses to Exercise

Anaerobic threshold (lactate threshold): the exercise intensity at which blood lactate begins to rise exponentially (typically ~60–70% of V̇O₂ max in untrained, ~80–85% in trained). Below this threshold, aerobic metabolism supplies most ATP. Above it, anaerobic glycolysis contributes significantly → lactate accumulation → metabolic acidosis → ventilatory compensation (hyperventilation to blow off CO₂).

Figure: Anaerobic threshold (lactate threshold)

Oxygen debt (EPOC — Excess Post-Exercise Oxygen Consumption): After exercise cessation, O₂ consumption remains elevated above resting levels for minutes to hours. This 'repays' the oxygen debt: (1) rapid phase — replenishment of ATP, phosphocreatine, and myoglobin O₂ stores; (2) slow phase — lactate metabolism (hepatic gluconeogenesis via Cori cycle), elevated body temperature, catecholamines, and repair processes.

Figure: Oxygen debt (EPOC — Excess Post-Exercise Oxygen Consumption)

Acclimatization, Sedentary Lifestyle, and Metabolic Syndrome

Acclimatization

Figure: Acclimatization, Sedentary Lifestyle, and Metabolic Syndrome

Acclimatization is the physiological adaptation to a sustained change in environmental conditions. It takes days to weeks and is reversible.

Heat Acclimatization (7–14 days of heat exposure):
- ↑ Plasma volume (↑ aldosterone → Na⁺/H₂O retention) → ↑ stroke volume → ↑ cardiac output at a lower heart rate
- ↑ Sweat rate (earlier onset, higher maximum) with ↓ NaCl concentration in sweat (aldosterone-mediated Na⁺ reabsorption in sweat duct) → more efficient evaporative cooling with less electrolyte loss
- ↑ Cutaneous vasodilation capacity
- ↓ Core temperature threshold for sweating onset (sweating begins earlier)
- Subjective improvement in heat tolerance
- Clinical relevance: Athletes, military recruits, and outdoor labourers should acclimatize gradually. Sudden intense exertion in heat without acclimatization → exertional heat stroke. In India, the annual April–June heat wave period is when most heat casualties occur among unacclimatized populations.

Cold Acclimatization:
- ↑ Non-shivering thermogenesis (↑ BAT activity, ↑ UCP1 expression)
- ↑ Thyroid hormone secretion → ↑ BMR
- ↑ Peripheral vasomotor tone with periodic cold-induced vasodilation (CIVD or 'Lewis hunting reaction') — protects extremities from frostbite while minimizing core heat loss
- Behavioral adaptations predominate in humans (clothing, shelter, heating)

Figure: Cold Acclimatization

High Altitude Acclimatization (days to weeks at >2,500 m):
- Immediate (hours): ↑ ventilation (hypoxic ventilatory response — peripheral chemoreceptors in carotid body detect ↓ PaO₂) → respiratory alkalosis → ↑ renal HCO₃⁻ excretion (compensatory metabolic acidosis over 2–3 days)
- Short-term (days): ↑ erythropoietin (EPO) from renal peritubular cells → ↑ erythropoiesis → ↑ RBC count and haemoglobin (polycythaemia; Hb may reach 20–22 g/dL at extreme altitude)
- ↑ 2,3-DPG in RBCs → right-shift of O₂-Hb dissociation curve → ↑ O₂ unloading at tissues
- ↑ Capillary density in muscles and myocardium
- ↑ Myoglobin in muscles
- ↑ Mitochondrial density and oxidative enzymes
- Pulmonary vasoconstriction (hypoxic pulmonary vasoconstriction) → ↑ pulmonary artery pressure → can cause high altitude pulmonary oedema (HAPE)
- Failure of acclimatization → Acute Mountain Sickness (headache, nausea, insomnia), HAPE, or High Altitude Cerebral Oedema (HACE)

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Sedentary Lifestyle and Obesity (PY12.4)

Physical inactivity is the fourth leading risk factor for global mortality (WHO). India is experiencing an epidemic of sedentary behaviour driven by urbanisation, technology, and changing occupational patterns.

Figure: Sedentary Lifestyle and Obesity (PY12.4)

Body Composition Assessment:
- BMI (Body Mass Index) = Weight (kg) / Height² (m²)
- Normal: 18.5–22.9 (Asian criteria, lower than WHO 25 for Caucasians)
- Overweight: 23–24.9
- Obese: ≥25 (Asian criteria; WHO uses ≥30)
- Asian populations have higher body fat % and visceral adiposity at lower BMI values — hence lower thresholds
- Waist circumference: >90 cm (men) or >80 cm (women) in Asian Indians indicates central obesity
- Waist-hip ratio: >0.90 (men) or >0.85 (women)
- Body fat %: measured by DEXA, bioimpedance, skinfold thickness

Metabolic Syndrome (defined by presence of ≥3 of 5 criteria — modified ATP III for Asian Indians):
1. Central obesity: waist circumference >90 cm (men) / >80 cm (women)
2. Hypertriglyceridaemia: fasting TG ≥150 mg/dL
3. Low HDL cholesterol: <40 mg/dL (men) / <50 mg/dL (women)
4. Hypertension: BP ≥130/85 mmHg or on treatment
5. Hyperglycaemia: fasting glucose ≥100 mg/dL or on treatment

Pathophysiology of Metabolic Syndrome:
- Central adiposity → visceral adipose tissue is metabolically active → secretes pro-inflammatory adipokines (TNF-α, IL-6, resistin) and ↓ anti-inflammatory adiponectin
- Free fatty acid (FFA) flux from visceral fat → liver → hepatic insulin resistance → ↑ gluconeogenesis, ↑ VLDL production, ↑ hepatic steatosis (NAFLD)
- Insulin resistance is the central pathogenic mechanism: tissues (liver, muscle, adipose) require higher insulin levels to maintain glucose homeostasis → compensatory hyperinsulinaemia → eventually β-cell exhaustion → Type 2 DM
- Insulin resistance → ↑ Na⁺ reabsorption (kidneys), ↑ sympathetic activity, ↑ vascular smooth muscle proliferation → hypertension
- ↑ PAI-1 (from adipose tissue) → prothrombotic state
- Indian population has a genetic predisposition to insulin resistance (thrifty genotype hypothesis) — metabolic syndrome affects ~30–35% of urban Indian adults

Consequences: Type 2 DM, cardiovascular disease (MI, stroke), NAFLD progressing to cirrhosis, obstructive sleep apnoea, polycystic ovary syndrome, certain cancers (breast, colon, endometrial), osteoarthritis, depression.

Benefits of Regular Exercise (30 minutes of moderate isotonic exercise, 5 days/week):
- ↑ Insulin sensitivity (↑ GLUT4 translocation in skeletal muscle, independent of insulin)
- ↓ Visceral fat, ↓ inflammatory markers (CRP, IL-6)
- ↑ HDL, ↓ TG, improved lipid profile
- ↓ Resting BP (5–7 mmHg reduction with regular exercise)
- ↑ V̇O₂ max (cardiovascular fitness)
- Improved endothelial function (↑ NO bioavailability)
- ↓ All-cause mortality by 30–40%

Growth, Aging, Brain Death, and Yoga Physiology

Physiology of Infancy and Growth (PY12.5)

Figure: Growth, Aging, Brain Death, and Yoga Physiology

Growth is the most sensitive indicator of a child's health. Assessment uses standardised growth charts (WHO Child Growth Standards, 2006 — used globally including India; IAP growth charts for Indian-specific reference).

Key growth parameters:
- Weight: Birth weight ~3 kg (Indian average ~2.8 kg); doubles by 5 months, triples by 12 months, quadruples by 2 years. Weight gain: ~30 g/day (first 3 months), ~20 g/day (3–6 months), ~10 g/day (6–12 months).
- Length/Height: Birth length ~50 cm; 75 cm at 1 year, 87 cm at 2 years. Adult height is approximately double the height at 2 years.
- Head circumference: Birth ~35 cm; 47 cm at 1 year (increases ~12 cm in first year, then ~2 cm/year). Reflects brain growth — rapid in first 2 years.
- Anterior fontanelle: Closes by 12–18 months. Bulging = raised ICP; depressed = dehydration.

Growth chart interpretation:
- Plot weight-for-age, length/height-for-age, weight-for-length/height, BMI-for-age on WHO z-score charts
- Normal: -2 to +2 SD (z-scores)
- Underweight: weight-for-age < -2 SD
- Stunting: height-for-age < -2 SD (chronic malnutrition)
- Wasting: weight-for-height < -2 SD (acute malnutrition)
- The trend (trajectory) is more important than a single measurement — crossing two centile lines warrants investigation
- India context: 35.5% children under 5 are stunted, 19.3% wasted (NFHS-5, 2019–21)

Anthropometric assessment (PY12.5):
- Mid-upper arm circumference (MUAC): <11.5 cm (6–59 months) = severe acute malnutrition; 11.5–12.5 cm = moderate acute malnutrition
- Skinfold thickness (triceps, subscapular) — measures subcutaneous fat
- BMI-for-age: increasingly used from 2 years onward; >+2 SD = overweight, >+3 SD = obese

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Physiology of Aging (PY12.6)

Aging is the progressive decline in physiological function that increases vulnerability to disease and death. Key age-related changes:

Figure: Physiology of Aging (PY12.6)

Cardiovascular: ↑ arterial stiffness (↓ elastin, ↑ collagen, calcification) → ↑ systolic BP, ↑ pulse wave velocity; ↓ maximum heart rate (220 – age); ↓ cardiac output; ↓ baroreceptor sensitivity → orthostatic hypotension

Respiratory: ↓ elastic recoil → ↑ FRC, ↑ residual volume; ↓ FEV₁ (~30 mL/year after 30); ↓ chest wall compliance (costal cartilage calcification); ↓ PaO₂ (expected PaO₂ = 104 – 0.27 × age)

Renal: ↓ GFR (~1 mL/min/year after 40); ↓ number of nephrons; ↓ concentrating ability; ↓ Na⁺ conservation; ↓ drug clearance

Neurological: ↓ brain mass (~5% per decade after 40); ↓ neurotransmitter levels (dopamine, acetylcholine, serotonin); ↓ processing speed; ↓ short-term memory; ↓ sleep quality (↓ slow-wave sleep)

Musculoskeletal: Sarcopenia (↓ muscle mass and strength — ~1–2% per year after 50); ↓ bone mineral density (osteopenia/osteoporosis — accelerated in postmenopausal women due to oestrogen withdrawal)

Figure: Musculoskeletal

Endocrine: ↓ GH/IGF-1 axis; menopause (ovarian failure, mean age 51); ↓ testosterone in men (andropause — gradual); ↓ thyroid function; ↑ insulin resistance

Theories of Aging:
1. Free radical theory (Harman, 1956): Cumulative oxidative damage from mitochondrial ROS → DNA mutations, protein oxidation, lipid peroxidation → cellular senescence. Supported by: caloric restriction (↓ metabolic rate → ↓ ROS) extends lifespan in animal models; SOD overexpression extends lifespan in Drosophila. Contested by: antioxidant supplementation does NOT extend human lifespan.
2. Telomere shortening: Telomeres shorten with each cell division; when critically short → senescence (Hayflick limit ~50–60 divisions). Telomerase (active in stem cells and cancer cells) maintains telomere length.
3. Programmed senescence: Genetic programs determine lifespan (e.g., p53, Rb, senescence-associated secretory phenotype — SASP).
4. Immunosenescence: ↓ T-cell function, thymic involution, ↓ vaccine responsiveness, ↑ autoimmunity.
5. Hormonal theory: Decline in anabolic hormones (GH, sex steroids, DHEA) with preservation of catabolic hormones (cortisol).

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Brain Death (PY12.7)

Brain death is the irreversible cessation of ALL functions of the entire brain, including the brainstem. It is legal death in India (Transplantation of Human Organs Act, 1994, amended 2011).

Figure: Brain Death (PY12.7)

Diagnostic criteria (India — based on UK criteria):
Prerequisites:
- Known cause of irreversible brain damage (trauma, haemorrhage, anoxia)
- Exclusion of reversible causes: hypothermia (<35°C), drug intoxication (sedatives, muscle relaxants), metabolic/endocrine derangement (severe electrolyte imbalance, hepatic/renal failure)

Clinical tests (performed by a panel of 4 doctors, including the treating doctor and an independent specialist; repeated after 6 hours):
1. No pupillary response: pupils fixed and dilated, no reaction to bright light
2. No corneal reflex: no blink on touching cornea
3. No vestibulo-ocular reflex (caloric test): no eye deviation on irrigating ear with 50 mL ice-cold water
4. No motor response in cranial nerve distribution: no grimace to supraorbital pressure
5. No gag reflex: no response to pharyngeal or tracheal stimulation
6. No cough reflex: no response to tracheal suctioning
7. Apnoea test: disconnect ventilator; pre-oxygenate with 100% O₂; allow PaCO₂ to rise to ≥60 mmHg (or 20 mmHg above baseline); observe for any respiratory effort for 8–10 minutes. No breathing = positive test.

Spinal reflexes (limb withdrawal, plantar flexion) may persist — they do NOT negate brain death.

Implications: Legal death for organ transplantation (kidneys, liver, heart, lungs, corneas); allows ventilator withdrawal; essential for the deceased donor organ transplantation programme in India.

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Physiology of Yoga and Meditation (PY12.8)

Yoga (particularly pranayama, asanas, and dhyana/meditation) produces measurable physiological effects:

Figure: Physiology of Yoga and Meditation (PY12.8)

• Cardiovascular: ↓ resting heart rate, ↓ blood pressure, improved baroreflex sensitivity, ↑ heart rate variability (↑ parasympathetic/vagal tone)
• Respiratory: ↓ respiratory rate, ↑ tidal volume, improved lung function (↑ FVC, ↑ FEV₁), improved respiratory muscle endurance
• Autonomic: Shift from sympathetic dominance to parasympathetic dominance; ↓ plasma cortisol, ↓ catecholamines
• Metabolic: ↓ BMR (during deep meditation), ↓ O₂ consumption, ↓ blood lactate, improved glucose tolerance, ↓ oxidative stress markers (↓ MDA, ↑ SOD, ↑ GSH)
• Neurological: ↑ alpha and theta wave activity on EEG (relaxation/meditative states); ↑ grey matter volume in hippocampus and prefrontal cortex (neuroplasticity with long-term practice)
• Endocrine: ↑ melatonin, ↑ DHEA, ↓ cortisol, normalisation of thyroid function
• Immune: ↑ NK cell activity, ↑ IgA levels, improved immune surveillance

Evidence base: Multiple RCTs demonstrate benefit in hypertension, Type 2 DM, anxiety/depression, chronic pain, and COPD. AIIMS and several Indian medical institutions have departments of integrative medicine incorporating yoga. NMC includes yoga physiology in the MBBS curriculum, reflecting India's unique contribution to this field.