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5-Mark Short Notes - Guyton & Hall Medical Physiology

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1. Oxygen-Hemoglobin Dissociation Curve

The Curve

100% |           ___________
     |          /
 75% |         /
     |        /  ← steep part
 50% |       /
     |      /
 25% |     /
      ____________________
  0   20  40  60  80  100 mmHg PO2

Key points:
- At PO2 = 100 mmHg (arterial): 97% saturated
- At PO2 = 40 mmHg (venous): 75% saturated
- P50 = PO2 at which Hb is 50% saturated = 26.5 mmHg (normal)

Labeled Points

• a (arterial end, alveoli): PO2 ~104 mmHg → 97% saturation
• v (venous end, tissues): PO2 ~40 mmHg → 75% saturation
• 5 mL O2 released per 100 mL blood under resting conditions

P50

Factors Affecting the Curve

1. Increased PCO2
2. Increased H+ (decreased pH) - Bohr Effect
3. Increased temperature
4. Increased 2,3-BPG
5. Increased DPG

1. Decreased PCO2
2. Decreased H+ (increased pH)
3. Decreased temperature
4. Fetal Hb (HbF)
5. Decreased 2,3-BPG
6. Carbon monoxide poisoning

Bohr Effect

• CO2 binds to Hb forming carbaminohemoglobin, releasing H+
• H+ binds to Hb, reducing its O2 affinity
• The curve shifts right, causing more O2 to be released to the actively metabolizing tissues
• This is the Bohr Effect - a physiologically important mechanism ensuring O2 delivery is greatest where metabolism is highest

Role of 2,3-BPG (2,3-Bisphosphoglycerate)

• Produced in RBCs via the Rapoport-Luebering shunt of glycolysis
• Binds to the beta chains of deoxygenated Hb, stabilizing the deoxy form
• Decreases Hb's affinity for O2 (rightward shift of curve)
• Increased 2,3-BPG occurs in: chronic hypoxia, high altitude, anemia, chronic lung disease
• Significance: ensures more O2 release in tissues under conditions of chronic O2 deprivation
• HbF has low affinity for 2,3-BPG → higher O2 affinity (useful for placental O2 transfer)

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2. Characteristics of Pulmonary Circulation

Special Characteristics

1. Low Pressure System:
   • Pulmonary arterial pressure: 25/8 mmHg (mean ~15 mmHg)
   • Compare to systemic: 120/80 mmHg
   • Enables the thin-walled RV to maintain this low-pressure circuit
2. Low Resistance:
   • Pulmonary vascular resistance = 1/6th of systemic
   • Vessels are short, wide, and highly distensible
   • Large cross-sectional area of capillary bed
3. High Compliance / Distensibility:
   • Pulmonary vessels can accommodate increased blood flow (exercise, etc.) without marked rise in pressure
   • Capillaries can recruit collapsed vessels at rest
4. Pulmonary Blood Volume:
   • ~500 mL in pulmonary circulation at rest (~10% total blood volume)
   • Acts as a blood reservoir - can expel blood into systemic circulation during hemorrhage
5. Capillary Exchange:
   • Capillary wall is extremely thin (0.5 micrometers)
   • Capillary PO2 rises from 40 to 104 mmHg across length of capillary
   • Transit time ~0.75 sec at rest (0.3 sec during exercise) - still adequate for full equilibration
6. Effect of Gravity (Zones of Lung - West's Zones):
   • Zone 1 (apex): PA > Pa > Pv - no flow if alveolar pressure exceeds arterial
   • Zone 2 (middle): Pa > PA > Pv - intermittent flow
   • Zone 3 (base): Pa > Pv > PA - continuous flow, greatest perfusion
7. Hypoxic Vasoconstriction:
   • Unlike systemic vessels, pulmonary arterioles constrict in response to LOW PO2
   • Diverts blood away from poorly ventilated alveoli (V/Q matching)
   • Opposite to systemic vessels
8. Pulmonary Edema:
   • Occurs when pulmonary capillary pressure rises above 28-30 mmHg (exceeds plasma oncotic pressure)
   • Normal capillary pressure ~7 mmHg (preventing edema)

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3. Acclimatization to High Altitude

Definition

Physiological Changes (Enumerate)

1. Increased ventilation - hypoxic stimulation of peripheral chemoreceptors (carotid/aortic bodies) → hyperventilation → PO2 rises, PCO2 falls
2. Respiratory alkalosis - due to CO2 washout; initially limits further ventilation

Significance of Changes

• Hyperventilation raises alveolar PO2 toward normal
• Polycythemia increases O2 carrying capacity
• Increased 2,3-BPG improves O2 delivery
• Increased vascularity reduces diffusion distances
• All changes collectively maintain adequate O2 delivery to tissues

Acute Mountain Sickness (AMS)

• Occurs within 6-12 hours of arrival above 2500m
• Due to cerebral edema from hypoxia-induced vasodilation
• Features: headache, nausea, fatigue, insomnia, dizziness
• Treatment: descent, supplemental O2, acetazolamide (promotes HCO3- excretion, stimulates ventilation), dexamethasone for severe cases

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4. Heat Stroke

Definition

Types

1. Classic (Non-exertional) Heat Stroke: seen in elderly, chronically ill, hot environment
2. Exertional Heat Stroke: during intense physical activity in hot/humid conditions

Pathophysiology

• Extreme heat overwhelms sweating mechanisms
• Hypothalamic thermostat "fails" - sweating ceases (anhydrosis)
• Core temperature rises rapidly
• Protein denaturation, enzyme failure, cellular dysfunction
• Cytokine storm and systemic inflammatory response

Clinical Features

• Core temperature >40°C
• Hot, dry skin (anhydrosis - sweating stops)
• CNS dysfunction: confusion, delirium, seizures, coma
• Tachycardia, hypotension
• Rhabdomyolysis, acute kidney injury
• Liver damage, DIC (disseminated intravascular coagulation)

Management

• Immediate rapid cooling is the cornerstone:
  • Cold water immersion (most effective)
  • Ice packs to neck, groin, axillae
  • Evaporative cooling with fans
• IV fluids, seizure control
• Monitor and treat organ dysfunction
• Target: reduce core temp below 39°C within 30 minutes

Distinction from Heat Exhaustion

• Heat exhaustion: temperature normal or mildly elevated, sweating present, no CNS dysfunction
• Heat stroke: temperature >40°C, CNS dysfunction, organ failure

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5. Heat Loss - Methods, Mechanisms, and Sweating

Methods of Heat Loss

1. Radiation (60% of total):
   • Transfer of heat as infrared electromagnetic waves from skin to cooler surroundings
   • Occurs without direct contact
   • Increased by vasodilation (blood to skin surface)
   • No heat loss if environment temperature > body temperature
2. Conduction (3%):
   • Direct transfer of heat from skin to cooler objects in contact
   • Small contribution under normal conditions
   • Enhanced when in contact with cool surfaces
3. Convection:
   • Transfer of heat to moving air or water currents adjacent to body
   • Wind chill effect accelerates convective heat loss
4. Evaporation (22% at rest, up to 90% during exercise):
   • Water evaporates from skin (insensible perspiration ~600 mL/day) and respiratory tract
   • Each gram of water evaporated removes ~0.58 kcal heat
   • The ONLY method effective when environmental temperature exceeds body temperature
   • Becomes ineffective in high humidity
5. Radiation from lungs (minor):
   • Through breathing warm air out

Mechanism of Sweating

• Controlled by hypothalamus (anterior/preoptic area)
• Signals via cholinergic sympathetic fibers to eccrine sweat glands
• Acetylcholine acts on muscarinic receptors of sweat glands
• Sweat gland has:
  • Secretory coil: produces isotonic primary secretion (similar to plasma)
  • Reabsorptive duct: reabsorbs NaCl, making sweat hypotonic
• At high rates, reabsorption is overwhelmed → sweat becomes more isotonic (more salt lost)
• Rate: up to 1.5 L/hour (short term), up to 2-3 L/hour after acclimatization
• Acclimatization to heat: increased sweating capacity, lower salt content in sweat (aldosterone effect)

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6. Peripheral Chemoreceptors

Location and Types

1. Carotid bodies - at bifurcation of common carotid arteries (most important)
2. Aortic bodies - in the aortic arch

Characteristics

• Highly vascular structures (highest blood flow per gram of any tissue in body)
• Contain two cell types:
  • Type I (Glomus) cells: chief cells, contain dopamine, oxygen sensors, synapse with nerve fibers
  • Type II (Sustentacular) cells: supportive, capsule-forming
• Send signals via Hering's nerve (branch of CN IX, glossopharyngeal) from carotid bodies and via CN X (vagus) from aortic bodies
• Both nerves travel to the nucleus tractus solitarius (NTS) and then to respiratory centers in medulla

Stimuli (MOST sensitive to)

1. Decreased PO2 (hypoxia) - primary stimulus; threshold ~60 mmHg
2. Increased PCO2 (hypercapnia) - also stimulate, but less potent than central chemoreceptors for CO2
3. Decreased pH (acidosis)
4. Increased temperature
5. Decreased blood flow (ischemia)

Functions

1. Hypoxic ventilatory response:
   • When PaO2 drops below 60 mmHg, strong stimulation of ventilation occurs
   • At PaO2 60 mmHg ventilation doubles; at 30 mmHg it quadruples
2. CO2/pH sensing:
   • Contribute to about 20-30% of hypercapnic ventilatory response
   • More important for acute CO2 changes than central receptors
3. Important in:
   • Acclimatization to altitude
   • Ventilatory response in patients with chronic hypercapnia (e.g., COPD) - peripheral chemoreceptors become the primary drive

Key Difference from Central Chemoreceptors

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7. Factors Affecting Composition of Alveolar Air

Factors

1. Rate of Alveolar Ventilation:
   • Increased ventilation → PO2 rises, PCO2 falls
   • Decreased ventilation (hypoventilation) → PO2 falls, PCO2 rises
   • Alveolar ventilation = (Tidal volume - Dead space) × Rate
2. Rate of Gas Exchange (O2 consumption and CO2 production):
   • Increased metabolic rate (exercise) → more O2 consumed, more CO2 produced → tends to lower PO2 and raise PCO2 (balanced by increased ventilation)
   • Respiratory Quotient (RQ) = CO2 produced / O2 consumed (normally 0.8)
3. Composition of Inspired Air:
   • At high altitude: reduced barometric pressure → lower PO2 in inspired air → lower alveolar PO2
   • Breathing supplemental O2 → raises alveolar PO2
   • Atmospheric PO2 at sea level = 159 mmHg (21% of 760 mmHg)
4. Water Vapor:
   • Airways saturate all inspired air with water vapor at 37°C (PH2O = 47 mmHg)
   • This is constant regardless of altitude - reduces effective PO2 available
5. Dead Space (Anatomical + Alveolar):
   • Increased dead space reduces effective alveolar ventilation
   • Anatomical dead space ~150 mL (conducting airways)
   • Physiological dead space = anatomical + alveolar (in disease)
6. V/Q (Ventilation/Perfusion) Ratio:
   • Ideal V/Q = 0.8
   • Mismatching (shunt or dead space) alters alveolar gas composition regionally
7. Diffusion Capacity:
   • Thickened alveolar membrane (fibrosis, edema) impairs gas exchange but alveolar gas composition itself only changes significantly with severe impairment

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8. Oxygen Debt

Definition

Background

Components of O2 Debt

1. Alactic (Fast) Component (~3-4 L O2):
   • Replenishment of ATP and phosphocreatine (PCr) stores in muscles
   • Occurs within 2-3 minutes post-exercise
   • No lactate involved
2. Lactic (Slow) Component (~5-10 L O2):
   • Metabolism/removal of accumulated lactic acid
   • Lactate is: converted to glucose (Cori cycle in liver), oxidized in heart/slow muscle fibers, used to resynthesize glycogen
   • Takes 30-60 minutes

Additional Factors Contributing to EPOC

• Elevated body temperature (increases metabolic rate)
• Elevated catecholamines (persist post-exercise)
• Elevated thyroid hormones
• Increased cardiac and respiratory work
• Resaturation of myoglobin with O2

Significance

• Explains why heavy breathing continues after stopping exercise
• Important in planning athletic training
• Maximum O2 debt = ~10-11 L in highly trained athletes

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9. Hypoxia - Definition, Classification, Hypoxic Hypoxia

Definition

Classification (Barcroft's Classification - modified by Guyton)

Hypoxic Hypoxia (in detail)

1. High altitude (low barometric pressure → low PO2)
2. Hypoventilation (e.g., COPD, neuromuscular disease, opiate overdose)
3. Diffusion impairment (pulmonary fibrosis, pulmonary edema)
4. Ventilation-perfusion (V/Q) mismatch (commonest cause in clinical practice: asthma, pneumonia, PE)
5. Right-to-left shunt (congenital heart disease, hepatopulmonary syndrome)
6. Breathing low O2 mixtures

• PaO2 falls → SaO2 falls (especially below 60 mmHg due to steep part of curve)
• O2 delivery to tissues falls
• Anaerobic metabolism → lactic acidosis
• Cyanosis when deoxyhemoglobin >5 g/dL (central cyanosis)

• Hyperventilation (peripheral chemoreceptors stimulated)
• Increased cardiac output
• Polycythemia (if chronic)
• Increased 2,3-BPG
• Rightward shift of O2-Hb curve

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10. Ways of Heat Loss / Heat Stroke

Heat Loss Methods (Summary)

1. Radiation - 60% at rest; infrared waves; requires temperature gradient
2. Conduction - 3%; direct contact; enhanced by water (25x better conductor than air)
3. Convection - warm air layer carried away; wind increases this
4. Evaporation - 22-90%; only method when environment hotter than body; 0.58 kcal/g water
5. Respiration - minor; warm humid air exhaled

Heat Stroke (Key Points)

• Core temperature >40°C
• Failure of thermoregulatory mechanism
• Hot, DRY skin (sweating has ceased - this distinguishes it from heat exhaustion)
• CNS signs: confusion, seizures, coma
• Rapid cooling is life-saving (cold water immersion)
• Complications: rhabdomyolysis, AKI, DIC, multi-organ failure

Comparison

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11. Surfactant - Functions and Respiratory Distress Syndrome (RDS)

Surfactant

• 90% lipid (mainly dipalmitoylphosphatidylcholine, DPPC)
• 10% protein (SP-A, SP-B, SP-C, SP-D)
• Produced by Type II pneumocytes (alveolar epithelial cells)
• Production begins at 24 weeks gestation; mature levels by 35 weeks

Functions

1. Reduces surface tension:
   • Water molecules at air-liquid interface pull inward, tending to collapse alveoli
   • Surfactant molecules intercalate between water molecules, reducing surface tension from ~50 to <5 dynes/cm
2. Increases lung compliance:
   • By reducing surface tension, surfactant makes lungs easier to expand (less work of breathing)
3. Prevents alveolar collapse (atelectasis):
   • LaPlace's law: P = 2T/r (pressure needed to keep alveolus open)
   • Surfactant reduces T → smaller alveoli are stabilized (otherwise they would empty into larger ones)
4. Prevents alveolar edema:
   • Reduces surface tension forces that would otherwise draw fluid into alveoli
5. Maintains alveolar stability:
   • Works better in small alveoli (more compressed = more effective) → equalizes pressures across differently-sized alveoli

Respiratory Distress Syndrome (RDS) / Hyaline Membrane Disease

• Premature birth (<35 weeks) → insufficient surfactant
• Surface tension rises → alveoli collapse at end of expiration
• Each breath requires enormous effort (work of breathing greatly increased)
• Atelectasis → V/Q mismatch → hypoxia and hypercapnia
• Protein-rich fluid leaks into alveoli → hyaline membrane formation

• Premature neonate
• Respiratory distress at birth or within hours
• Tachypnea, grunting (to create auto-PEEP), nasal flaring, intercostal retractions
• Cyanosis
• CXR: "ground glass" appearance, air bronchograms

• Maternal antenatal corticosteroids (betamethasone - stimulates fetal lung maturity)
• Exogenous surfactant replacement therapy (via endotracheal tube at birth)
• CPAP/mechanical ventilation
• Oxygen therapy

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12. Transport of Carbon Dioxide and Haldane Effect

CO2 Transport - Three Forms

1. Dissolved in plasma (7%):
   • Small amount directly dissolved as CO2 in blood
   • PaCO2 = 40 mmHg (arterial), PvCO2 = 45 mmHg (venous)
2. As Bicarbonate (70% - most important):
   • CO2 enters RBC → reacts with H2O (catalyzed by carbonic anhydrase)
   • CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3-
   • HCO3- exits RBC in exchange for Cl- (chloride shift / Hamburger shift)
   • H+ buffered by hemoglobin (forming HHb)
   • At lungs: process reverses (diffusion gradient drives CO2 out)
3. As Carbaminohemoglobin (23%):
   • CO2 binds directly to amino groups of Hb (not heme iron)
   • CO2 + Hb-NH2 ⇌ Hb-NH-COOH
   • Deoxy-Hb has greater affinity for CO2 than oxy-Hb

Haldane Effect

• When O2 is released from Hb at tissues, deoxy-Hb forms
• Deoxy-Hb is a better buffer - it binds H+ more avidly than oxy-Hb
  • This drives the bicarbonate reaction to the right (more CO2 → HCO3-)
• Deoxy-Hb also binds CO2 more readily as carbaminohemoglobin

• In tissues: O2 released → Hb becomes deoxy → picks up more CO2 (facilitates loading)
• In lungs: O2 binds Hb → oxy-Hb releases H+ and CO2 (facilitates unloading)
• The Haldane Effect accounts for approximately 50% of CO2 exchange between tissues and lungs
• Works in conjunction with Bohr Effect (which enhances O2 delivery in tissues)

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13. Response to Extreme Cold, Hypothermia, and Induced Hypothermia

Response to Extreme Cold

• Sympathetic adrenergic stimulation → arteriolar constriction in skin and extremities
• Reduces heat loss by radiation/conduction
• Hunting reaction: periodic vasodilation in extremities (prevents frostbite)

1. Shivering - most important acute response
   • Hypothalamus activates primary motor center for shivering (dorsomedial hypothalamus)
   • Intense skeletal muscle activity → heat production increases 4-5x above basal
2. Non-shivering thermogenesis (NST):
   • Brown adipose tissue (BAT) - important in infants, less so in adults
   • Norepinephrine → uncoupling protein (UCP-1/thermogenin) in brown fat → uncouples oxidative phosphorylation → heat instead of ATP
3. Increased metabolic rate - thyroid hormones (longer-term)
4. Piloerection - traps air; effective in animals, minor in humans

Hypothermia

• Mild (32-35°C): shivering, vasoconstriction, tachycardia
• Moderate (28-32°C): shivering stops, bradycardia, atrial fibrillation, CNS depression
• Severe (<28°C): ventricular fibrillation risk, loss of consciousness, no reflexes
• Critical (<20°C): cardiac standstill

Induced (Therapeutic) Hypothermia

1. Post-cardiac arrest: Target temperature management (32-36°C for 24 hours) - reduces brain injury
2. Neonatal hypoxic-ischemic encephalopathy (HIE): 33.5°C for 72 hours
3. Cardiac surgery (deep hypothermic circulatory arrest): 18-20°C allows bloodless field
4. Severe traumatic brain injury (selected cases)
5. Spinal cord protection during aortic surgery

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14. Effect of Exercise on Respiration and VO2max

Ventilatory Response to Exercise

1. Immediate increase at onset:
   • Neurogenic: motor cortex activates respiratory center simultaneously with muscles
   • Proprioceptors from joints/muscles signal respiratory centers
   • Even before blood gas changes occur
2. Sustained increase:
   • Increased CO2 production and O2 consumption
   • Elevated temperature
   • Increased lactic acid (metabolic acidosis) → stimulates ventilation
   • Sympathetic stimulation
3. Quantitative changes:
   • Resting: ~5 L/min ventilation, 250 mL/min O2 consumption
   • Moderate exercise: ventilation rises proportionally with VO2
   • Heavy exercise: ventilation rises disproportionately (anaerobic threshold crossed) - ventilatory threshold
   • Maximum: ventilation up to 100-150 L/min; VO2 up to 3-5 L/min
4. Other respiratory changes:
   • Increased tidal volume (up to 3L)
   • Increased respiratory rate (up to 40-50/min)
   • Arterial PO2, PCO2, and pH remain near normal during moderate exercise

VO2max (Maximal Oxygen Consumption)

• Cardiac output (most important limiting factor - "the pump")
• O2 diffusion capacity of lungs
• Hb concentration and O2-carrying capacity
• Skeletal muscle oxidative capacity

• Average untrained male: ~35-40 mL O2/kg/min
• Average trained male: ~60-70 mL O2/kg/min
• Elite athletes (cyclists, cross-country skiers): >80 mL O2/kg/min

• Declines with age (~1% per year after 25)
• Reduced in heart failure, anemia, lung disease
• Used to assess surgical risk and fitness
• Increases with aerobic training (by 15-20%)

• At VO2max, cardiac output is maximal and O2 extraction is maximal

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15. Work of Breathing, Applied Importance, Muscles of Respiration

Work of Breathing

1. Compliance Work (elastic work) - ~65%:
   • Work needed to expand the lung and chest wall against elastic recoil forces
   • W = ½ × P × V = area under pressure-volume curve
   • Increased in: pulmonary fibrosis, pulmonary edema (decreased compliance)
2. Airway Resistance Work (viscous work) - ~28%:
   • Work needed to overcome airway resistance during airflow
   • Proportional to flow rate and airway resistance
   • Increased in: asthma, COPD, bronchospasm
3. Tissue Viscous Resistance - ~7%:
   • Work needed to overcome viscosity of lung and chest wall tissues

• In severe asthma or COPD: work of breathing can rise to 30-50% of total energy expenditure
• Respiratory muscle fatigue can cause ventilatory failure
• Work of breathing guides decisions on mechanical ventilation
• Increased work of breathing is a sign of impending respiratory failure
• CPAP/BiPAP/mechanical ventilation reduce the patient's work of breathing
• Surfactant deficiency markedly increases compliance work (as in RDS)
• Optimal tidal volume (~500 mL) minimizes combined elastic and resistive work

Muscles of Respiration

1. Diaphragm (most important - contributes 70-80% of tidal volume)
   • Innervated by phrenic nerve (C3, C4, C5)
   • Descends ~1.5 cm in quiet breathing, up to 10 cm in deep breathing
2. External intercostals - elevate ribs, increase A-P diameter

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16. Functional Residual Capacity (FRC) and Physiological Significance

Definition

Measurement

• Cannot be measured by spirometry (because RV cannot be expelled)
• Measured by:
  1. Helium dilution method (closed circuit)
  2. Nitrogen washout method
  3. Body plethysmography (most accurate; measures all gas including trapped air)

Physiological Significance

1. Acts as O2 reservoir:
   • Large volume of O2 in FRC buffers against rapid changes in alveolar PO2 between breaths
   • Prevents wide swings in alveolar PO2 during the respiratory cycle
2. Prevents alveolar collapse:
   • Maintains airway patency throughout the respiratory cycle
   • Surfactant + FRC together prevent atelectasis
3. Maintains V/Q ratio:
   • Adequate FRC ensures continuous gas exchange even during expiration
4. Represents equilibrium point:
   • FRC = point where outward chest wall recoil = inward lung recoil
   • At FRC, respiratory muscles are fully relaxed
5. Clinical Importance:
   • Reduced FRC in: obesity, pregnancy, supine position, ARDS, pulmonary edema, atelectasis
   • Reduced FRC → airway closure → V/Q mismatch → hypoxemia
   • PEEP (Positive End-Expiratory Pressure) during mechanical ventilation effectively increases FRC
   • Pre-oxygenation before anesthesia extends apnea tolerance by maximizing FRC O2 store

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17. Acclimatization to High Altitude - Various Changes and Acute Mountain Sickness

Physiological Changes at High Altitude

Acute Mountain Sickness (AMS)

• Rapid ascent above 2500m without acclimatization
• Hypoxia → cerebral vasodilation → increased cerebral blood flow
• Disruption of blood-brain barrier → cerebral edema
• In severe cases: High Altitude Cerebral Edema (HACE) and High Altitude Pulmonary Edema (HAPE)

• Ataxia, confusion, altered consciousness
• Emergency: requires immediate descent + dexamethasone + O2

• Most common cause of death at high altitude
• Non-cardiogenic pulmonary edema from uneven hypoxic vasoconstriction
• Symptoms: dyspnea, cough, pink frothy sputum
• Treatment: descent, nifedipine (reduces pulmonary arterial pressure), O2

• Gradual ascent (300m/day above 3000m)
• Acetazolamide (carbonic anhydrase inhibitor): promotes HCO3- excretion → metabolic acidosis → stimulates breathing → pre-acclimatizes

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18. Haldane Effect

Definition

Mechanism

1. Buffering of H+:
   • Deoxy-Hb (HHb) is a better buffer than oxy-Hb (HbO2)
   • Deoxy-Hb has a higher pKa for its histidine residues → binds H+ more avidly
   • More H+ bound → carbonic anhydrase reaction driven rightward: CO2 + H2O → H2CO3 → H+ + HCO3-
   • Net: more HCO3- generated from CO2 at tissue level
2. Carbaminohemoglobin formation:
   • Deoxy-Hb reacts more readily with CO2 to form carbamino compounds
   • Contributes to increased CO2 carriage in venous blood

In Tissues (CO2 loading):

• O2 released from Hb → Hb becomes deoxy
• Deoxy-Hb picks up H+ from bicarbonate reaction and CO2 as carbamino
• Result: tissues can "load" more CO2 onto blood

In Lungs (CO2 unloading):

• O2 binds to Hb → Hb becomes oxy
• Oxy-Hb is less able to buffer H+, so H+ is released → drives HCO3- → CO2
• CO2 released from carbamino compounds
• Net CO2 is expelled from lungs

Quantitative Importance

• The Haldane Effect accounts for approximately 50% of CO2 transport between tissues and lungs
• The combination of Bohr Effect (O2 delivery) and Haldane Effect (CO2 removal) creates a powerful physiological tandem

• Bohr Effect: CO2/H+ → affects O2 transport
• Haldane Effect: O2 → affects CO2 transport
• They are reciprocal and complementary

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19. Role of Hypothalamus in Temperature Regulation

The Hypothalamus as Thermostat

Temperature Sensors

1. Central Thermoreceptors:
   • Warm-sensitive neurons (most numerous) in preoptic/anterior hypothalamus
   • Cold-sensitive neurons in preoptic/anterior hypothalamus
   • Also in spinal cord, abdominal viscera, and deep tissues
2. Peripheral Thermoreceptors:
   • Cold receptors (Krause end bulbs) > Warm receptors (Ruffini corpuscles) in skin
   • Send signals via spinothalamic tract to hypothalamus

Set Point

• Normal core temperature setpoint ~37.1°C
• Like a thermostat - deviations above or below trigger corrective responses

Responses to HEAT (Anterior Hypothalamus)

1. Sweating - via cholinergic sympathetic fibers to eccrine glands
2. Cutaneous vasodilation - sympathetic inhibition → blood brought to skin surface
3. Behavioral responses - seeking cool environment, removing clothing
4. Inhibition of heat production - reduced muscle tone, inhibit shivering, reduced brown fat thermogenesis

Responses to COLD (Posterior Hypothalamus)

1. Cutaneous vasoconstriction - sympathetic adrenergic activation
2. Shivering - via dorsomedial hypothalamus and motor pathways
3. Piloerection - sympathetic
4. Non-shivering thermogenesis - sympathetic → norepinephrine → BAT (UCP-1)
5. Behavioral - huddling, seeking warmth, adding clothing
6. Hormonal - increased TRH → TSH → thyroid hormones (chronic cold) → increased metabolic rate
7. Increased epinephrine/norepinephrine from adrenal medulla

Fever

• Pyrogens (bacterial LPS, cytokines: IL-1, IL-6, TNF) → COX enzyme → prostaglandin E2 (PGE2) → acts on preoptic hypothalamus → raises the set point
• Body activates cold responses (shivering, vasoconstriction) to reach new higher set point
• Antipyretics (aspirin, paracetamol) inhibit COX → reduce PGE2 → reset thermostat downward

Clinical Relevance

• Hypothalamic lesions (tumor, trauma, stroke): can cause poikilothermia (body temperature = environment)
• Heat stroke: "failure" of hypothalamic thermostat (though mechanism is primarily sweating failure)
• Malignant hyperthermia and neuroleptic malignant syndrome involve abnormal thermoregulation

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Index of All 19 Short Notes

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• Each note is structured to give ~400-500 words of examination content
• For diagram-based questions (Q1), practice drawing the sigmoidal curve with labelled axes
• Q1 and Q18 (Bohr + Haldane) are often examined together - know both effects
• Q3 and Q17 are the same topic - the detailed version in Q17 includes the table with timing which is high-yield
• Q4 and Q10 overlap - the hot/dry skin (anhydrosis) differentiating heat stroke from heat exhaustion is a classic exam point

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COMPLETE 5-MARK SHORT NOTES

From: Guyton and Hall Textbook of Medical Physiology (14th Edition / 4th South Asia Edition)

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Q1. Oxygen-Hemoglobin Dissociation Curve

The Curve - Draw and Label

% Hb
Sat.
100 |              •————————————
 97 |           •/   ← arterial (PO2=100)
    |          /
 75 |        •  ← venous (PO2=40)
    |       /
 50 |      • ← P50 (PO2=26.5 mmHg)
    |     /
 25 |    /
    |   /
  0 |__/________________________
    0  20  40  60  80  100  PO2 (mmHg)

• PO2 = 100 mmHg (arterial blood): 97% saturated
• PO2 = 40 mmHg (mixed venous blood): 75% saturated
• 5 mL O2 released per 100 mL blood per pass through tissues at rest
• The flat upper part = "plateau" → Hb is protected from desaturation even with moderate PO2 fall
• The steep middle part = small PO2 fall → large O2 release to tissues

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P50

• It is an index of Hb's affinity for O2
• Higher P50 = lower affinity (rightward shift)
• Lower P50 = higher affinity (leftward shift)

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Bohr Effect

• CO2 enters RBCs → forms H2CO3 → dissociates to H+ + HCO3-
• H+ binds to Hb, changing its conformation
• This reduces Hb's affinity for O2 → O2-Hb dissociation curve shifts RIGHT
• More O2 is released to the metabolically active tissues

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Factors Shifting Curve to the RIGHT (decreased O2 affinity, more O2 release)

1. Increased PCO2
2. Increased H+ (decreased pH / acidosis) - Bohr Effect
3. Increased temperature
4. Increased 2,3-BPG
5. Anemia (increased 2,3-BPG as compensation)
6. High altitude (chronic - increased 2,3-BPG)
7. Thyroid hormones, growth hormone, androgens

1. Decreased PCO2
2. Decreased H+ (alkalosis)
3. Decreased temperature (hypothermia)
4. Decreased 2,3-BPG
5. Fetal hemoglobin (HbF)
6. Carbon monoxide (CO) poisoning
7. Methemoglobin

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Role of 2,3-BPG (2,3-Bisphosphoglycerate)

• Produced in RBCs by Rapoport-Luebering shunt (side path of glycolysis)
• Binds to the beta chains of deoxygenated Hb, stabilizing the T (deoxy/taut) state
• Reduces Hb's O2 affinity → rightward shift of the curve
• Increased 2,3-BPG in: chronic hypoxia, high altitude, anemia, chronic lung disease, alkalosis
• Significance: In chronic hypoxia, 2,3-BPG rises → curve shifts right → more O2 unloaded to tissues per unit PO2 drop → compensatory mechanism
• HbF: Has γ-chains instead of β-chains; binds 2,3-BPG poorly → higher O2 affinity → allows fetus to extract O2 from maternal blood across placenta

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Q2. Characteristics of Pulmonary Circulation

1. Low Pressure System

• Pulmonary artery pressure: 25/8 mmHg, mean ~15 mmHg
• Systemic pressure: 120/80 mmHg
• Low pressure protects the delicate alveolar capillaries and prevents pulmonary edema
• Allows the thin-walled RV to do the work

2. Low Resistance

• Pulmonary vascular resistance = ~1/6th of systemic resistance
• Vessels are short, wide, thin-walled, and highly distensible
• Total cross-sectional area of capillary bed is enormous

3. High Distensibility and Recruitability

• Pulmonary vessels can greatly distend with increased blood flow
• Collapsed capillaries at rest are recruited during exercise
• This means pulmonary arterial pressure rises very little even when cardiac output doubles (exercise)

4. Pulmonary Blood Volume

• ~450-500 mL in pulmonary circulation at rest (~9% of total blood volume)
• Acts as a blood reservoir - can shift blood to/from systemic circulation
• Right heart failure → blood dams up → pulmonary congestion

5. Capillary Exchange

• Capillary wall is extremely thin (~0.5 μm) for gas diffusion
• Transit time through capillary = 0.75 sec at rest; ~0.3 sec during exercise
• Both times sufficient for complete equilibration of O2 and CO2
• Surface area of capillaries = ~70 m²

6. Zones of the Lung (West's Zones) - Effect of Gravity

• Zone 1 (apex): PA > Pa > Pv → alveolar pressure exceeds arterial → no flow (doesn't occur normally)
• Zone 2 (middle): Pa > PA > Pv → intermittent flow; PA acts as "sluice gate"
• Zone 3 (base): Pa > Pv > PA → continuous flow; greatest perfusion

7. Hypoxic Pulmonary Vasoconstriction (HPV)

• Unique feature: Pulmonary arterioles constrict in response to low alveolar PO2
• Opposite of systemic vessels (which dilate in hypoxia)
• Purpose: diverts blood from poorly ventilated alveoli to better-ventilated ones → maintains V/Q matching
• Generalized hypoxia (high altitude) → generalized HPV → pulmonary hypertension

8. Pulmonary Edema

• Occurs when pulmonary capillary pressure rises above 28-30 mmHg (exceeds plasma oncotic pressure ~28 mmHg)
• Normal capillary pressure = ~7 mmHg (safety margin)
• Causes: left heart failure, mitral stenosis, fluid overload

9. No Autoregulation

• Unlike systemic vessels, pulmonary vessels do not autoregulate blood flow
• Flow is largely passive, determined by upstream and downstream pressures

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Q3. Acclimatization to High Altitude

Definition

Physiological Changes (Enumerated)

1. Hyperventilation:
   • Low PO2 → stimulates carotid and aortic body peripheral chemoreceptors
   • Increases rate and depth of breathing
   • Raises alveolar PO2; lowers PCO2
   • Partially offset by resulting respiratory alkalosis (CO2 washout inhibits further ventilation)
2. Increased cardiac output:
   • Sympathetic stimulation → increased heart rate and stroke volume
   • Increases O2 delivery to tissues

1. Renal compensation for alkalosis:
   • Kidneys excrete HCO3- in urine, reducing blood pH back toward normal
   • Removes the alkalotic "brake" on ventilation → sustained hyperventilation
2. Increased 2,3-BPG in RBCs:
   • Alkalosis stimulates 2,3-BPG production
   • Rightward shift of O2-Hb curve → more O2 delivered to tissues at a given PO2

1. Increased erythropoietin (EPO):
   • Kidney (peritubular cells) senses hypoxia via HIF-1α → secretes EPO within 24-48 hrs
   • EPO stimulates bone marrow → increased RBC production
2. Polycythemia (increased RBC mass):
   • Hb rises from 15 g/dL to 18-20 g/dL after full acclimatization
   • Increases O2 carrying capacity per unit blood volume
3. Increased vascularity of tissues:
   • VEGF (vascular endothelial growth factor) → new capillary formation
   • Reduces O2 diffusion distances to cells
4. Increased mitochondrial density:
   • Cells develop more mitochondria → improved oxidative phosphorylation capacity
5. Increased myoglobin:
   • Muscle myoglobin increases → better O2 storage and transport within muscle cells
6. Pulmonary hypertension and RV hypertrophy:
   • Generalized hypoxic pulmonary vasoconstriction → raised pulmonary artery pressure
   • RV hypertrophies to handle increased afterload

Significance of Each Change

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Q4. Heat Stroke

Definition

Types

1. Classic (non-exertional) heat stroke: elderly or chronically ill people during environmental heat waves; gradual onset
2. Exertional heat stroke: young healthy individuals during intense physical activity in hot/humid conditions; rapid onset

Pathophysiology

1. Excess heat production or reduced heat dissipation overwhelms the hypothalamic thermostat
2. Core temperature rises → at extreme temperatures, sweating ceases (mechanism unclear - possibly exhaustion of sweat glands)
3. A vicious cycle: hyperthermia → cellular protein denaturation → enzyme failure → further heat production
4. Systemic inflammatory response similar to sepsis
5. Direct thermal injury to cells → multi-organ failure

Clinical Features

• Core temperature >40°C (rectal or esophageal probe)
• Hot, DRY skin (anhydrosis - sweating has stopped - hallmark finding)
• CNS dysfunction: confusion, disorientation, delirium, seizures, coma
• Tachycardia, hypotension
• Tachypnea, hyperventilation

Complications

• Rhabdomyolysis → myoglobinuria → acute kidney injury
• Hepatocellular damage → elevated liver enzymes
• DIC (disseminated intravascular coagulation)
• ARDS (acute respiratory distress syndrome)
• Multi-organ failure → death

Comparison: Heat Exhaustion vs Heat Stroke

Treatment

1. Immediate rapid cooling (most critical step):
   • Cold water immersion (most effective - target <39°C within 30 min)
   • Ice packs to neck, axillae, groin
   • Evaporative cooling with fans and mist
2. IV fluid resuscitation
3. Monitor and treat complications (AKI, DIC, seizures)
4. Benzodiazepines for seizures/shivering (shivering re-generates heat)
5. ICU monitoring

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Q5. Heat Loss - Methods, Mechanisms, and Sweating

Total Body Heat Production and Loss

Methods of Heat Loss

• Transfer of heat energy as infrared electromagnetic waves from skin surface to cooler surrounding objects/environment
• Occurs without direct contact between bodies
• Requires a temperature gradient (skin must be warmer than surroundings)
• Increased by: cutaneous vasodilation (brings warm blood to skin surface)
• Not effective when environmental temperature > body temperature

• Direct transfer of heat from skin to cooler objects in physical contact
• Water conducts heat ~25x better than air → immersion in water causes rapid heat loss
• Normally minor contribution (air is a poor conductor)

• Warm air layer next to skin is carried away by air currents; cooler air replaces it
• "Wind chill" effect greatly accelerates convective loss
• Works together with conduction

• Water evaporates from skin surface and respiratory mucosa
• Each gram of water evaporated removes 0.58 kcal of heat
• Two forms:
  • Insensible perspiration: ~600 mL/day; continuous, unconscious water diffusion through skin
  • Sensible sweating: controlled by hypothalamus; major heat-loss mechanism during heat stress
• The ONLY effective method when environmental temperature exceeds body temperature
• Ineffective in high humidity (water vapor pressure of air approaches saturated value)

• Warm, humid air exhaled; cooler dry air inhaled
• Contributes to heat and water loss from respiratory mucosa

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Mechanism of Sweating (Detail)

• Anterior hypothalamus (preoptic area) detects rise in core temperature
• Signals via cholinergic sympathetic nerve fibers to eccrine (merocrine) sweat glands

• Eccrine sweat gland has two parts:
  1. Deep secretory coil: secretes primary fluid that is isotonic to plasma (Na+, Cl-, K+, water)
  2. Reabsorptive duct: reabsorbs NaCl (but not water), making sweat hypotonic

• Basal rate: 0 to slow seepage
• Maximum rate: 1.5-2 L/hour (trained and acclimatized: up to 3 L/hour)
• At low rates: very hypotonic sweat (efficient NaCl reabsorption)
• At high rates: approaches isotonic (duct reabsorption overwhelmed) → significant salt loss

• Aldosterone secretion increases → upregulates NaCl reabsorption in duct
• Salt content of sweat falls → less electrolyte depletion
• Sweat glands enlarge → sweating begins earlier and at higher volume

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Q6. Peripheral Chemoreceptors

Definition and Location

1. Carotid bodies - located at the bifurcation of the common carotid arteries (most physiologically important)
2. Aortic bodies - scattered in the aortic arch region

Structure and Characteristics

• Highly vascular structures with the highest blood flow per gram of tissue in the entire body (~2000 mL/100g/min vs. brain ~54 mL/100g/min) → allows them to monitor arterial blood, not tissue gases

• Type I (Glomus) cells: the primary O2 sensors; contain dopamine, acetylcholine, and other neurotransmitters; communicate with afferent nerve fibers
• Type II (Sustentacular/Sheath) cells: supportive glial-like cells surrounding Type I cells

• Carotid bodies → Hering's nerve → Glossopharyngeal nerve (CN IX) → Nucleus Tractus Solitarius (NTS) → Respiratory centers
• Aortic bodies → Vagus nerve (CN X) → NTS → Respiratory centers

Stimuli (in order of importance)

1. Decreased PaO2 (hypoxia) - PRIMARY stimulus; threshold ~60-70 mmHg
2. Increased PaCO2 (hypercapnia) - also stimulate, but less potent than central chemoreceptors
3. Decreased pH (acidosis)
4. Decreased blood pressure / blood flow (ischemia to chemoreceptor itself)
5. Increased temperature
6. Certain drugs (nicotine stimulates; cyanide prevents O2 utilization → strong stimulus)

Functions

1. Hypoxic ventilatory response:
   • PaO2 must fall to ~60 mmHg before significant stimulation of ventilation
   • Below 60 mmHg: steep increase in ventilation (corresponds to steep part of O2-Hb curve)
   • At PaO2 = 30 mmHg: ventilation nearly quadruples
2. Contribute ~20-30% of hypercapnic ventilatory response
3. Critical in patients with chronic hypercapnia (COPD): central chemoreceptors become desensitized to CO2 → peripheral chemoreceptors become the primary drive to breathe ("hypoxic drive")
4. Essential for acclimatization to altitude
5. Respond to acute changes faster than central chemoreceptors

Comparison: Peripheral vs Central Chemoreceptors

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Q7. Factors Affecting Composition of Alveolar Air

Normal Alveolar Air Composition

• PO2 = 104 mmHg
• PCO2 = 40 mmHg
• PH2O = 47 mmHg (constant at 37°C)
• PN2 = 569 mmHg
• Total = 760 mmHg

Factors

• Increased ventilation (hyperventilation): PO2 rises, PCO2 falls
• Decreased ventilation (hypoventilation): PO2 falls, PCO2 rises
• Alveolar ventilation (VA) = (Tidal Volume - Dead Space) × Respiratory Rate
• Normal VA = (500 - 150) × 12 = ~4200 mL/min

• Increased metabolism (exercise, fever, hyperthyroidism): more O2 consumed, more CO2 produced
• Tends to lower alveolar PO2 and raise PCO2
• Normally balanced by proportional increase in ventilation
• Respiratory Quotient (RQ) = VCO2/VO2 = 0.8 (mixed diet)

• At high altitude: barometric pressure falls → inspired PO2 falls → alveolar PO2 falls
  • At 19,200 ft (5800m): barometric pressure = 380 mmHg; inspired PO2 = 73 mmHg
• Breathing supplemental O2: raises inspired PO2 → raises alveolar PO2
• Atmospheric PO2 = FiO2 × (PB - PH2O) = 0.21 × (760 - 47) = 150 mmHg

• Airways saturate inspired air with water vapor at body temperature (37°C)
• PH2O = 47 mmHg at 37°C, always, regardless of altitude
• This is a constant "dilution" factor that reduces available PO2

• Anatomical dead space (~150 mL): conducting airways where no gas exchange occurs
• Alveolar dead space: alveoli ventilated but not perfused (V/Q > normal)
• Increased dead space reduces effective alveolar ventilation
• Physiological dead space = anatomical + alveolar dead space

• Normal ratio = 0.8
• High V/Q (dead space effect): alveolar gas approaches inspired air composition (PO2 high, PCO2 low)
• Low V/Q (shunt effect): alveolar gas approaches mixed venous blood composition (PO2 low, PCO2 high)
• Regional variation: apex V/Q = 3.3 (high); base V/Q = 0.63 (low)

• In disease (fibrosis, pulmonary edema): diffusion impairment limits equilibration
• Mainly affects O2 (lower solubility) more than CO2

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Q8. Oxygen Debt

Definition

Background

• O2 demand of muscles exceeds O2 supply (especially at the start of exercise, "O2 deficit")
• Energy generated anaerobically via:
  1. ATP-phosphocreatine (PCr) breakdown
  2. Anaerobic glycolysis → lactic acid production
• When exercise stops, elevated O2 consumption persists until metabolic homeostasis is restored

Components of O2 Debt

• Replenishment of ATP and phosphocreatine (PCr) stores in muscle fibers
• Requires O2 for oxidative phosphorylation in mitochondria
• Completed within 2-3 minutes after exercise
• No lactate is involved

• Removal and metabolism of accumulated lactic acid
• Lactic acid is:
  • Oxidized back to pyruvate → enters Krebs cycle (heart muscle, slow-twitch fibers)
  • Converted to glucose in liver via Cori cycle (gluconeogenesis)
  • Converted to glycogen in liver and muscle
• Takes 30-60 minutes after exercise

Additional Contributors to EPOC

• Elevated body temperature: metabolic rate rises ~13% per 1°C → extra O2 consumed
• Elevated catecholamines (epinephrine, norepinephrine) persist post-exercise → increased metabolic rate
• Elevated thyroid hormones and glucocorticoids
• Increased cardiac and respiratory work immediately post-exercise
• Resaturation of myoglobin with O2

Quantitative Values

• Maximum O2 debt in untrained person: ~10 L
• Maximum O2 debt in highly trained athlete: up to 15-20 L

Significance

• Explains continued heavy breathing after stopping exercise
• Limits the maximum intensity of sustained exercise
• Training reduces O2 debt for the same workload (greater aerobic capacity)

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Q9. Hypoxia - Definition, Classification, Describe Hypoxic Hypoxia

Definition

Classification (4 Types - Barcroft/Guyton)

Hypoxic Hypoxia (in detail)

1. High altitude - reduced barometric pressure → reduced PO2 in inspired air
2. Hypoventilation - COPD, neuromuscular disease (GBS, MG), opiate/sedative overdose, obesity hypoventilation
3. Diffusion impairment - pulmonary fibrosis, pulmonary edema, interstitial lung disease
4. V/Q mismatch (commonest clinical cause) - asthma, pneumonia, pulmonary embolism, COPD
5. Right-to-left shunt - congenital heart disease (Tetralogy of Fallot), hepatopulmonary syndrome, intrapulmonary shunts
6. Low inspired O2 - breathing O2-depleted mixtures (mine gas, fire)

• PaO2 falls → SaO2 falls (particularly below 60 mmHg - steep part of dissociation curve)
• O2 delivery to tissues falls
• Cells switch to anaerobic metabolism → lactic acid → metabolic acidosis
• Cyanosis appears when deoxyhemoglobin >5 g/dL in capillaries (central cyanosis - lips, tongue)

• Peripheral chemoreceptors stimulated → hyperventilation
• Increased cardiac output (sympathetic)
• Polycythemia (if chronic hypoxia - EPO release)
• Increased 2,3-BPG in RBCs → rightward shift of curve
• Hypoxic pulmonary vasoconstriction (V/Q matching)

• PaO2 and SaO2 are LOW
• Responds to supplemental O2 (except in right-to-left shunts)
• This distinguishes it from other types where PaO2 is normal

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Q10. Ways of Heat Loss and Heat Stroke

Ways of Heat Loss

• Transfer of infrared electromagnetic waves from skin to cooler surroundings
• No medium required; occurs across a vacuum
• Requires temperature gradient (body > environment)
• Increased by vasodilation; impaired if environmental T > body T

• Direct heat transfer from skin to objects in contact
• Water conducts 25× better than air (water immersion = rapid heat loss)
• Normally small contribution

• Warm air carried away from body surface; replaced by cooler air
• Wind chill dramatically increases convective loss
• Works synergistically with conduction

• 0.58 kcal lost per gram of water evaporated
• Insensible loss: continuous (~600 mL/day from skin + lungs)
• Sensible sweating: hypothalamus-controlled; crucial in hot environment
• Only effective mechanism when environmental T > body T
• Blocked by high humidity

• Expired warm, humid air removes heat
• Small contribution under normal conditions

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Heat Stroke

• Heat overwhelms cooling mechanisms → sweating ceases (anhydrosis)
• Hypothalamic "thermostat" overwhelmed → runaway hyperthermia
• Protein denaturation, enzyme failure, SIRS (systemic inflammatory response)

• Temperature >40°C
• Hot, DRY skin (no sweating - hallmark)
• CNS: confusion, delirium, seizures, coma
• Tachycardia, hypotension, tachypnea
• Multiorgan failure: rhabdomyolysis, AKI, hepatic injury, DIC, ARDS

1. Immediate rapid cooling (cold water immersion - most effective)
2. Ice packs (neck, axillae, groin)
3. IV fluids, seizure control
4. ICU admission, organ monitoring
5. Target: core temp <39°C within 30 minutes

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Q11. Surfactant - Functions and Respiratory Distress Syndrome (RDS)

Surfactant - Composition

• 90% lipids: mainly dipalmitoylphosphatidylcholine (DPPC) - the active component
• 10% proteins: SP-A (host defense), SP-B and SP-C (surface tension reduction), SP-D (host defense)
• Produced by Type II alveolar pneumocytes
• Stored in lamellar bodies and secreted into alveolar lining fluid
• Development: synthesis begins ~24 weeks gestation; functionally mature levels by ~35-36 weeks

Physics Behind Surfactant - LaPlace's Law

• Where T = surface tension, r = radius
• Small alveoli have high pressure → tend to collapse into large alveoli
• Surfactant reduces T → reduces collapsing pressure → stabilizes all alveolar sizes

Functions of Surfactant

• Water lining creates surface tension ~50 dynes/cm
• Surfactant (DPPC molecules at interface) reduces this to <5 dynes/cm
• Prevents alveolar collapse at end of expiration

• Reduced surface tension = lungs expand more easily
• Reduces work of breathing enormously

• Stabilizes alveoli of all sizes (especially small ones)
• Works more efficiently when alveoli are small (compression increases surfactant density)

• Surface tension forces would otherwise "suck" fluid from capillaries into alveoli
• Surfactant prevents this fluid accumulation

• Without surfactant, small alveoli (high pressure) would empty into large ones (unequal ventilation)
• Surfactant makes pressure equal regardless of alveolar size

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Respiratory Distress Syndrome (RDS) / Hyaline Membrane Disease

• Premature birth → insufficient surfactant → surface tension remains high
• Alveoli collapse at end of each expiration
• Each breath requires enormous muscular effort → rapid fatigue
• Atelectasis → V/Q mismatch → hypoxia and CO2 retention
• Capillary leak → protein-rich fluid floods alveoli → forms hyaline membranes (eosinophilic lining visible on histology)

• Premature neonate
• Respiratory distress within 4-6 hours of birth
• Grunting (to generate auto-PEEP), nasal flaring, intercostal/subcostal retractions
• Cyanosis
• CXR: bilateral "ground glass" haziness, air bronchograms, low lung volumes

1. Antenatal corticosteroids (betamethasone IM to mother 48h before preterm delivery) → stimulates fetal surfactant production
2. Exogenous surfactant replacement (beractant, poractant - given via endotracheal tube immediately after birth)
3. CPAP (continuous positive airway pressure) - keeps alveoli open
4. Mechanical ventilation if CPAP inadequate
5. Supplemental O2 with careful titration

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Q12. Transport of Carbon Dioxide and Haldane Effect

CO2 Production

• Normal CO2 production at rest: ~200 mL/min
• PCO2 in tissues = ~45 mmHg; in arterial blood = 40 mmHg
• 5 mmHg gradient drives CO2 from tissues into blood

Three Forms of CO2 Transport in Blood

• Directly dissolved in plasma as CO2 gas
• Small amount, but clinically important (determines PCO2)
• PaCO2 = 40 mmHg; PvCO2 = 45 mmHg

• CO2 enters RBCs → reacts with H2O, catalyzed by carbonic anhydrase (CA):
  CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3-
• HCO3- leaves RBC and enters plasma in exchange for Cl-: Chloride shift (Hamburger phenomenon)
• H+ is buffered by hemoglobin (oxyhemoglobin releases O2 → deoxyhemoglobin, better buffer)
• In lungs: reversed - H+ + HCO3- → CO2 + H2O → CO2 exhaled

• CO2 binds directly to free amino (-NH2) groups of Hb (not to heme iron):
  CO2 + Hb-NH2 ⇌ Hb-NHCOOH (carbaminohemoglobin)
• Deoxy-Hb binds CO2 more readily than oxy-Hb
• In lungs: O2 binds → oxy-Hb releases CO2 from carbamino sites → exhaled

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Haldane Effect

• O2 is released from Hb → Hb becomes deoxygenated
• Deoxy-Hb is a better H+ buffer (higher pKa of histidine residues in deoxy configuration)
• Deoxy-Hb binds H+ → drives the equation: CO2 + H2O → HCO3- (more bicarbonate formed)
• Deoxy-Hb also forms carbamino compounds more readily
• Net result: More CO2 loaded onto blood

• O2 binds Hb → Hb becomes oxygenated
• Oxy-Hb is a weaker H+ buffer → releases H+
• Released H+ drives: H+ + HCO3- → CO2 → exhaled
• Oxy-Hb also releases CO2 from carbamino sites
• Net result: More CO2 unloaded from blood

• The Haldane Effect accounts for ~50% of CO2 exchange between tissues and lungs

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Q13. Response to Extreme Cold, Hypothermia, and Induced Hypothermia

Body's Response to Extreme Cold

• Sympathetic adrenergic stimulation → skin and extremity arteriolar constriction
• Reduces blood flow to skin surface → reduces heat loss by radiation/conduction
• Core temperature preserved at expense of peripheral temperature
• Hunting reaction (Lewis reaction): periodic vasodilation in fingers/toes to prevent frostbite

• Posterior hypothalamus activates "primary motor center for shivering" (dorsomedial posterior hypothalamus)
• Involuntary, oscillating skeletal muscle contractions (~10-18/sec)
• Increases heat production 4-5 times above basal
• No external work done → all energy appears as heat

• Brown adipose tissue (BAT): rich in mitochondria; contains uncoupling protein-1 (UCP-1/thermogenin)
• Norepinephrine → β3-adrenergic receptors → UCP-1 activation → uncouples oxidative phosphorylation → generates heat instead of ATP
• Important in newborns (significant BAT around neck, shoulders, kidneys)
• Less significant in adults

• Arrector pili muscles contract → "goosebumps"
• Traps air next to skin (effective insulation in animals with fur; minimal in humans)

• Hypothalamus → TRH → anterior pituitary → TSH → thyroid hormones (T3/T4)
• Thyroid hormones increase basal metabolic rate → sustained heat production
• Increased adrenal medullary catecholamines (epinephrine) → increased metabolic rate

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Hypothermia

• Every 1°C drop reduces cerebral O2 consumption by ~7%
• Reduced enzyme activity → slowed all metabolic processes
• "Cold heart" is prone to ventricular fibrillation (VF)
• "No one is dead until warm and dead" - patient may be resuscitated even after prolonged hypothermia

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Induced (Therapeutic) Hypothermia

1. Post-cardiac arrest (Target Temperature Management - TTM):
   • 32-36°C for 24 hours after ROSC (Return of Spontaneous Circulation)
   • Reduces neurological damage from global cerebral ischemia
2. Neonatal Hypoxic-Ischemic Encephalopathy (HIE):
   • 33.5°C for 72 hours in term neonates with perinatal asphyxia
   • Significantly improves neurological outcome
3. Cardiac surgery (Deep Hypothermic Circulatory Arrest - DHCA):
   • 18-20°C allows complete circulatory arrest for ~30-60 minutes
   • Cerebral protection during complex aortic arch surgery
4. Traumatic brain injury (selected cases):
   • Reduces intracranial pressure and cerebral metabolic demand

• Reduced O2 demand (7% per °C drop)
• Reduced excitatory neurotransmitter release (glutamate)
• Reduced free radical production
• Reduced cerebral edema and inflammation
• Reduced apoptosis

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Q14. Effect of Exercise on Respiration and VO2max

Changes in Respiration During Exercise

• Before/at onset of exercise: motor cortex sends collateral signals to respiratory centers simultaneously with muscle activation
• Proprioceptors from limb joints and muscles signal NTS and respiratory centers
• Ventilation increases immediately (within first breath), before any blood gas changes

• Increased CO2 production and O2 consumption
• Lactic acid production (anaerobic threshold) → metabolic acidosis → strong ventilatory stimulus
• Elevated core temperature → stimulates respiratory centers
• Increased K+ from muscle → additional ventilatory drive

• At moderate exercise: ventilation rises linearly with VO2 (proportional)
• At ~50-60% VO2max: anaerobic threshold crossed → lactic acid accumulates → ventilation rises disproportionately (ventilatory threshold)

• Arterial PO2, PCO2, and pH remain near normal (ventilation increases proportionally)
• Venous PO2 falls (more extraction), venous PCO2 rises

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VO2max (Maximal Oxygen Uptake)

• At VO2max: CO is maximum AND O2 extraction is maximum

1. Cardiac output (the most important limiting factor - "the pump")
2. O2 diffusion capacity of lungs
3. Hemoglobin concentration and O2-carrying capacity
4. Skeletal muscle mitochondrial density and oxidative enzyme capacity

• Training increases it by 15-20%
• Declines ~1% per year after age 25
• Reduced in: heart failure, anemia, pulmonary disease, deconditioning
• Altitude (acute): reduces VO2max

• Pre-surgical cardiopulmonary exercise testing (CPET)
• Assessment in heart failure, COPD, pulmonary hypertension
• Sports medicine and athletic training

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Q15. Work of Breathing, Applied Importance, Muscles of Respiration

Work of Breathing

• Work done against elastic recoil of lungs + chest wall
• Energy stored as elastic potential energy during inspiration, released passively during expiration
• Work = ½ × P × V (area under pressure-volume loop)
• Increased when compliance is REDUCED: pulmonary fibrosis, pulmonary edema, ARDS, surfactant deficiency
• Optimal (compliance) at large tidal volumes with slow rate

• Work done to overcome viscous drag of lung and chest wall tissues
• Relatively constant; proportional to flow velocity

• Work done to move air through conducting airways against frictional resistance
• Resistance = Pressure gradient / Flow rate
• Increased when: asthma (bronchospasm), COPD, foreign body, tumor, secretions
• Optimal (resistance work) at small tidal volumes with fast rate

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Applied Importance

1. In severe asthma or COPD: WOB rises to 30-50% of total body O2 consumption → respiratory muscle fatigue → ventilatory failure
2. Respiratory failure: high WOB is a clinical warning sign → guides mechanical ventilation decisions
3. Mechanical ventilation (CPAP/BiPAP/MV): takes over WOB → rests fatigued respiratory muscles
4. In ARDS: reduced compliance → high elastic WOB; PEEP helps by recruiting alveoli and improving compliance
5. Surfactant deficiency (RDS): dramatically increases elastic WOB
6. Monitoring: work of breathing measured by esophageal pressure × volume loops in ICU
7. Obesity/Pregnancy: reduced FRC → reduced compliance → increased WOB

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Muscles of Respiration

1. Diaphragm (chief muscle of respiration - 70-80% of tidal volume)
   • Innervation: Phrenic nerve (C3, C4, C5 - "C3,4,5 keep the diaphragm alive")
   • Descends ~1.5 cm in quiet breathing; up to 10 cm in deep breathing
   • Increases thoracic volume craniocaudally
2. External intercostals
   • Elevate ribs → increase anteroposterior and transverse diameter (pump handle + bucket handle)
   • Innervation: Intercostal nerves (T1-T11)

• Rectus abdominis
• External oblique
• Internal oblique
• Transversus abdominis
• Contract → increase intra-abdominal pressure → push diaphragm up

1. Internal intercostals - depress ribs, reduce thoracic volume

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Q16. Functional Residual Capacity (FRC) - Physiological Significance

Definition

• FRC represents the equilibrium point where:
  • Outward recoil of chest wall (trying to spring outward) = Inward recoil of lungs (trying to collapse)
  • Net force on respiratory system = 0 at FRC
  • All respiratory muscles are fully relaxed at FRC

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Measurement

1. Helium dilution (closed circuit): patient breathes known concentration of He in a closed circuit; He dilutes to equilibrium; FRC calculated from dilution
2. Nitrogen washout (open circuit): patient breathes 100% O2; total N2 washed out measured; FRC = vol N2 washed / 0.79
3. Body plethysmography (most accurate): measures all intrathoracic gas including trapped air (useful in COPD with air trapping)

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Physiological Significance

• ~450 mL O2 stored in FRC at alveolar PO2 of 104 mmHg
• Buffers against rapid swings in alveolar PO2 between breaths
• Pre-oxygenation before anesthesia replaces N2 in FRC with O2 → extends safe apnea time ("apneic oxygenation")

• Keeps airways and alveoli open at end-expiration
• Works with surfactant to prevent collapse of small alveoli

• Even during expiration, FRC ensures alveolar PO2/PCO2 remain stable for ongoing gas exchange
• Without FRC: alveoli would collapse → no gas exchange during expiration

• FRC keeps lungs on the mid-range of the compliance curve (most compliant portion)
• Breathing away from FRC (too full or too empty) requires more work

• Adequate FRC ensures ventilated alveoli remain perfused throughout respiratory cycle

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Clinical Importance

• PEEP (Positive End-Expiratory Pressure) during MV: effectively increases FRC → recruits collapsed alveoli → improves oxygenation
• CPAP: same effect non-invasively
• Semi-recumbent positioning (30-45°): increases FRC vs. supine

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Q17. Acclimatization to High Altitude - Various Changes and Acute Mountain Sickness

Physiological Changes - Timed Sequence

Result of Full Acclimatization

• Alveolar PO2 at 4500m: ~50 mmHg (without acclimatization) → ~67 mmHg (with)
• VO2max reduced by ~25% even after full acclimatization (vs. ~40% without)

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Acute Mountain Sickness (AMS)

• Hypoxia → cerebral vasodilation (HIF-mediated, NO release)
• Disruption of blood-brain barrier integrity
• Cerebral edema → ↑ intracranial pressure → symptoms

• Headache (cardinal symptom) + at least one of:
  • Fatigue/weakness
  • Nausea/vomiting
  • Dizziness
  • Sleep disturbance (periodic breathing - Cheyne-Stokes)

• Ataxia, altered consciousness, severe headache
• Emergency - requires immediate descent
• Treatment: descent + O2 + dexamethasone (reduces cerebral edema)

• Non-cardiogenic pulmonary edema
• Mechanism: uneven hypoxic pulmonary vasoconstriction → overperfusion of some capillaries → capillary leak
• Symptoms: dyspnea at rest, dry then wet cough, pink frothy sputum, cyanosis
• Treatment: immediate descent, O2, nifedipine (reduces pulmonary vasoconstriction), sildenafil, portable hyperbaric chamber

• Gradual ascent: no more than 300-500m/day above 3000m
• Acetazolamide (Diamox): carbonic anhydrase inhibitor → HCO3- excretion → metabolic acidosis → stimulates breathing → pre-acclimatizes (dose: 125-250 mg BD starting 1-2 days before ascent)
• Stay well hydrated, avoid alcohol

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Q18. Haldane Effect

Definition

Mechanism in Detail

• The histidine residues of globin chains act as buffers
• In deoxy-Hb: histidine residues have a higher pKa → more protonated → better buffer
• Deoxy-Hb binds more H+, driving the reaction: CO2 + H2O → H2CO3 → H+ + HCO3- rightward
• Result: More CO2 converted to bicarbonate (HCO3-) for transport

• Deoxy-Hb has more freely available -NH2 terminal groups on globin chains
• CO2 binds to these: CO2 + Hb-NH2 → Hb-NHCOO- + H+
• Deoxy-Hb forms carbaminohemoglobin more readily than oxy-Hb

Tissue Level (CO2 Loading - Deoxygenation occurs)

1. O2 diffuses from capillaries to tissue
2. Hb releases O2 → becomes deoxy-Hb
3. Deoxy-Hb binds more H+ (buffers HCO3- formation) and more CO2 as carbamino
4. More CO2 is "loaded" onto venous blood for transport to lungs Net: Deoxygenation facilitates CO2 pickup

Lung Level (CO2 Unloading - Oxygenation occurs)

1. O2 diffuses from alveoli into blood
2. Hb binds O2 → becomes oxy-Hb
3. Oxy-Hb releases H+ (weaker buffer) → H+ + HCO3- → CO2 + H2O
4. Oxy-Hb releases CO2 from carbamino compounds
5. CO2 diffuses into alveoli → exhaled Net: Oxygenation facilitates CO2 unloading

Quantitative Significance

• Accounts for approximately 50% of CO2 transport between tissues and lungs
• The other 50% is from simple gradient-driven bicarbonate equilibration

Haldane Effect vs Bohr Effect - Key Comparison

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Q19. Role of the Hypothalamus in Temperature Regulation

Hypothalamus as the Thermostat

• Anterior hypothalamus / Preoptic area: contains thermosensitive neurons; detects temperature changes; controls heat-dissipating responses (sweating, vasodilation)
• Posterior hypothalamus: integrates input from all temperature sensors; activates heat-conserving and heat-producing responses

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Temperature Sensors

• Warm-sensitive neurons (the majority) in preoptic/anterior hypothalamus - fire faster as temperature rises
• Cold-sensitive neurons in preoptic/anterior hypothalamus and posterior hypothalamus
• Also in spinal cord, abdominal viscera, other deep body structures

• Cold receptors (free nerve endings) - more numerous; signal via Aδ fibers
• Warm receptors - signal via C fibers
• Located in skin; signals ascend via spinothalamic tract to hypothalamus and cortex

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Responses to HEAT (anterior hypothalamus activated)

1. Sweating:
   • Anterior hypothalamus → cholinergic sympathetic fibers → eccrine sweat glands
   • Evaporation of sweat → major heat loss
2. Cutaneous vasodilation:
   • Inhibition of sympathetic vasoconstrictor tone → cutaneous blood vessels dilate
   • Warm blood brought from core to skin surface → radiation and conduction heat loss
3. Inhibition of heat production:
   • Reduced skeletal muscle tone
   • Shivering inhibited
   • Brown fat thermogenesis reduced
4. Behavioral responses:
   • Seeks cool environment, fans, removes clothing, drinks cold fluid
   • Adopts sprawled posture to maximize surface area

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Responses to COLD (posterior hypothalamus activated)

1. Cutaneous vasoconstriction:
   • Sympathetic adrenergic activation → skin arterioles constrict → reduces heat loss
2. Shivering:
   • Posterior hypothalamus → primary motor center for shivering → motor neurons → oscillatory muscle activity
   • Increases heat production 4-5× above basal
3. Non-Shivering Thermogenesis:
   • Sympathetic → norepinephrine → β3 receptors on brown adipose tissue
   • UCP-1 (thermogenin) uncouples oxidative phosphorylation → heat production
4. Piloerection:
   • Sympathetic → arrector pili → hair standing up ("goosebumps")
   • Minimal insulation in humans; important in animals
5. Hormonal responses (chronic cold):
   • Hypothalamus → TRH → TSH → thyroid hormones → raised basal metabolic rate
   • Adrenal medulla → epinephrine → increased metabolic rate
6. Behavioral: seeks warm environment, adds clothing, huddling

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Set Point and Fever

• Pyrogens (exogenous: bacterial LPS; endogenous: IL-1, IL-6, TNF-α, PGE2)
• Pyrogens → COX-2 enzyme in hypothalamic blood vessels → PGE2 synthesis
• PGE2 acts on preoptic area → raises the set point (e.g., to 39°C)
• Body activates cold responses (shivering, vasoconstriction) to reach new higher set point → person feels cold/chills
• Once set point reached → feels hot → sweating begins
• Antipyretics (aspirin, paracetamol/acetaminophen) inhibit COX → reduce PGE2 → reset set point downward → sweating → fever breaks

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Clinical Relevance

• Hypothalamic lesion (tumor, hemorrhage, trauma): poikilothermia (body temperature varies with environment)
• Malignant hyperthermia (anesthetic complication): calcium dysregulation in muscle → runaway heat production
• Neuroleptic malignant syndrome: dopamine blockade in hypothalamus → thermoregulatory failure
• Heat stroke: sweating mechanism fails → thermostat cannot compensate
• Neonates: hypothalamic thermoregulation immature → cannot maintain temperature → require warm environment

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