Physiology
Medical physiology for MBBS: cell and body fluids, blood, nerve and muscle, and the cardiovascular, respiratory, gastrointestinal, renal, endocrine, reproductive and nervous systems, special senses and integrative physiology.
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Definition
The autonomic nervous system (ANS) is the involuntary division controlling visceral functions; it has sympathetic and parasympathetic divisions.
General Features
- Involuntary control of viscera, glands, vessels
- Two-neuron pathway (pre- + postganglionic)
- Sympathetic + parasympathetic (usually opposing)
Sympathetic (Thoracolumbar)
- Origin: T1–L2
- Short preganglionic, long postganglionic
- ‘Fight or flight’
Parasympathetic (Craniosacral)
- Origin: cranial nerves III, VII, IX, X + S2–S4
- Long preganglionic, short postganglionic
- ‘Rest and digest’
Two divisions with generally opposing actions balance visceral control. Feature Sympathetic Parasympathetic Origin T1–L2 Cranial + S2–S4 Response Fight/flight Rest/digest Ganglia Near cord Near organ Applied
- Autonomic dysfunction → postural hypotension
- Many drugs act on the ANS (e.g. atropine)
🔑KEY POINTS TO REMEMBER- ANS = involuntary; sympathetic (T1–L2) + parasympathetic (craniosacral).
- Two-neuron pathway; usually opposing actions.
- Sympathetic = fight/flight; parasympathetic = rest/digest.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
The ANS uses acetylcholine (cholinergic) and noradrenaline (adrenergic) as transmitters, acting on specific receptors.
Cholinergic (ACh)
- All preganglionic fibres (both divisions)
- All parasympathetic postganglionic fibres
- Sympathetic fibres to sweat glands
- Receptors: nicotinic (ganglia), muscarinic (organs)
Adrenergic (Noradrenaline)
- Most sympathetic postganglionic fibres
- Receptors: alpha (α₁, α₂), beta (β₁, β₂)
ACh acts at all ganglia; the organ receives ACh (parasympathetic) or noradrenaline (sympathetic). Receptor Transmitter Location Nicotinic ACh Ganglia, NMJ Muscarinic ACh Organs (parasympathetic) Alpha/Beta Noradrenaline Sympathetic targets Applied
- Atropine blocks muscarinic receptors
- Beta-blockers block β receptors
🔑KEY POINTS TO REMEMBER- Preganglionic + all parasympathetic postganglionic = cholinergic.
- Most sympathetic postganglionic = adrenergic.
- Receptors: nicotinic, muscarinic, α, β.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Thermoregulation keeps core body temperature ~37°C by balancing heat production and loss, controlled by the hypothalamus.
Heat Production
- Basal metabolism (BMR)
- Muscle activity and shivering
- Thyroxine and adrenaline (chemical thermogenesis)
Heat Loss
- Radiation, conduction, convection
- Evaporation (sweating)
- Skin blood flow
Hypothalamic Control
- Anterior hypothalamus → heat loss (cooling centre)
- Posterior hypothalamus → heat conservation (warming centre)
- Negative feedback around a set point
The hypothalamus balances heat loss and production to hold temperature at ~37°C. Stimulus Response ↑ Temperature Sweating, vasodilation ↓ Temperature Shivering, vasoconstriction Applied
- Fever — set point reset upward
- Heat stroke and hypothermia
🔑KEY POINTS TO REMEMBER- Core temp ~37°C, set by the hypothalamus.
- Heat loss: radiation, evaporation, skin blood flow.
- Anterior = cooling; posterior = warming.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Higher brain functions include the EEG (cortical electrical activity), sleep (a reversible unconscious state) and consciousness.
EEG Waves
- Beta (>13 Hz) — alert, active
- Alpha (8–13 Hz) — awake, relaxed, eyes closed
- Theta (4–8 Hz) — drowsy, children
- Delta (<4 Hz) — deep sleep
Sleep Stages
- NREM — four stages, deepening (delta)
- REM — dreaming, rapid eye movements, atonia
- Cycles of ~90 minutes
Consciousness
- Maintained by the reticular activating system
- Levels: alert → drowsy → stupor → coma
As sleep deepens EEG slows to delta, then REM brings dreaming with an active EEG. Wave Frequency State Beta >13 Hz Alert Alpha 8–13 Hz Relaxed Theta 4–8 Hz Drowsy Delta <4 Hz Deep sleep Applied
- EEG for epilepsy and brain death
- Sleep disorders (insomnia, apnoea)
🔑KEY POINTS TO REMEMBER- EEG: beta (alert), alpha (relaxed), theta (drowsy), delta (deep sleep).
- Sleep: NREM (4 stages) + REM (dreaming, atonia), ~90-min cycles.
- Consciousness maintained by the RAS.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Integrative physiology studies how organ systems work together to maintain homeostasis — a stable internal environment.
Principles
- Multiple systems coordinate (nervous + endocrine)
- Negative feedback maintains set points
- Systems are interlinked (e.g. BP = heart + vessels + kidney)
Examples of Integration
- Blood pressure — heart, vessels, kidney, ANS
- Blood glucose — pancreas, liver
- Exercise — muscle, heart, lungs together
Homeostasis emerges from many systems responding together to a disturbance. Variable Systems involved Blood pressure Heart, vessels, kidney Glucose Pancreas, liver Temperature Skin, muscle, hypothalamus Applied
- Disease = failure of integration
- The stress response is highly coordinated
🔑KEY POINTS TO REMEMBER- Integrative physiology = systems working together for homeostasis.
- Coordinated by nervous + endocrine systems via negative feedback.
- E.g. BP, glucose, temperature each involve several systems.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
The sympathetic and parasympathetic divisions of the ANS have generally opposing effects on organs.
Sympathetic (fight/flight)
- ↑ Heart rate and BP
- Bronchodilation, pupil dilation
- ↓ Gut activity
- Thoracolumbar (T1–L2)
Parasympathetic (rest/digest)
- ↓ Heart rate
- Bronchoconstriction, pupil constriction
- ↑ Gut activity
- Craniosacral
The two divisions oppose each other to balance the body’s organs. Organ Sympathetic Parasympathetic Heart ↑ rate ↓ rate Pupil Dilate Constrict Gut ↓ ↑ Bronchi Dilate Constrict Applied
- Atropine — antimuscarinic (↑ HR)
- Fight-or-flight vs rest-and-digest
🔑KEY POINTS TO REMEMBER- Sympathetic = fight/flight (↑ HR, dilate pupil, ↓ gut).
- Parasympathetic = rest/digest (↓ HR, constrict pupil, ↑ gut).
- Generally opposing actions.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Adrenergic receptors are receptors for noradrenaline/adrenaline, classified into alpha (α) and beta (β) types.
Alpha Receptors
- α₁ — vasoconstriction, pupil dilation
- α₂ — ↓ noradrenaline release (feedback)
Beta Receptors
- β₁ — ↑ heart rate & force (heart)
- β₂ — bronchodilation, vasodilation (lungs, vessels)
The receptor type determines the response — α (constrict), β₁ (heart), β₂ (dilate). Receptor Main effect α₁ Vasoconstriction β₁ ↑ heart rate/force β₂ Bronchodilation Applied
- β-blockers (β₁) — hypertension, angina
- β₂ agonists (salbutamol) — asthma
🔑KEY POINTS TO REMEMBER- α₁ = vasoconstriction; α₂ = feedback.
- β₁ = heart (↑ rate); β₂ = bronchodilation.
- β-blockers and β₂ agonists are key drugs.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Fever (pyrexia) is a regulated rise in body temperature due to an upward reset of the hypothalamic set point by pyrogens.
Mechanism
- Infection → pyrogens (cytokines, IL-1)
- ↑ Prostaglandin E2 in the hypothalamus
- Set point raised → body ‘feels cold’
- Shivering, vasoconstriction → temperature rises
Phases
- Onset — chills, shivering
- Plateau — hot, flushed
- Defervescence — sweating
Pyrogens raise the hypothalamic set point, so the body heats up to reach it. Feature Detail Cause Pyrogens (IL-1) Mediator Prostaglandin E2 Antipyretics Block PGE2 (paracetamol) Applied
- Antipyretics (paracetamol, NSAIDs) lower the set point
- Hyperpyrexia is dangerous
🔑KEY POINTS TO REMEMBER- Fever = raised hypothalamic set point (pyrogens → PGE2).
- Phases: chills → plateau → sweating.
- Antipyretics block PGE2.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
The electroencephalogram (EEG) records cortical electrical activity; waves are classified by frequency, reflecting arousal.
Types
- Beta (>13 Hz) — alert, active
- Alpha (8–13 Hz) — awake, relaxed, eyes closed
- Theta (4–8 Hz) — drowsy, sleep, children
- Delta (<4 Hz) — deep sleep
As arousal falls, EEG frequency slows from beta down to delta. Wave Frequency State Beta >13 Hz Alert Alpha 8–13 Hz Relaxed Theta 4–8 Hz Drowsy Delta <4 Hz Deep sleep Applied
- Diagnose epilepsy and sleep disorders
- Helps confirm brain death
🔑KEY POINTS TO REMEMBER- EEG waves: beta (alert), alpha (relaxed), theta (drowsy), delta (deep sleep).
- Frequency falls as arousal decreases.
- Used for epilepsy, sleep, brain death.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
REM (rapid eye movement) sleep is the sleep stage of dreaming, marked by rapid eye movements, muscle atonia and an active EEG.
Features
- Rapid eye movements
- Dreaming
- Muscle atonia (temporary paralysis)
- EEG resembles the awake state (paradoxical sleep)
- Irregular ↑ HR, BP, respiration
Cycle
- About 20–25% of total sleep
- Recurs roughly every 90 minutes
- ↑ in the later cycles of the night
Sleep alternates NREM and REM; REM brings dreaming with an awake-like EEG. Feature REM NREM Dreams Yes Rare Muscle tone Atonia Present EEG Active Slow Applied
- Important for memory consolidation
- REM rebound after deprivation
🔑KEY POINTS TO REMEMBER- REM = dreaming, rapid eye movements, atonia, active EEG (paradoxical).
- ~20–25% of sleep, every ~90 min.
- Aids memory consolidation.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Homeostasis is the maintenance of a stable internal environment, achieved mainly by negative feedback.
Feedback Types
- Negative feedback — opposes change (most systems)
- Positive feedback — amplifies change (childbirth, clotting)
Components
- Receptor → control centre → effector
- Maintains a set point
- Coordinated by nervous + endocrine systems
A negative-feedback loop detects a change and acts to reverse it. Type Effect Example Negative Opposes change Temperature, BP Positive Amplifies Childbirth, clotting Applied
- Failure of homeostasis → disease
- Most body systems use negative feedback
🔑KEY POINTS TO REMEMBER- Homeostasis = stable internal environment.
- Negative feedback opposes change; positive amplifies it.
- Loop: receptor → control centre → effector.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Exercise triggers integrated cardiovascular, respiratory and metabolic changes to meet the increased demand of working muscles.
Cardiovascular
- ↑ Heart rate and stroke volume → ↑ cardiac output
- ↑ Blood flow to muscles (vasodilation)
- ↑ Systolic blood pressure
Respiratory
- ↑ Rate and depth of breathing
- ↑ O₂ delivery and CO₂ removal
Metabolic
- ↑ Glucose and fatty-acid use
- Anaerobic metabolism → lactate
- ↑ Temperature → sweating
The heart, lungs and metabolism ramp up together to supply working muscle. System Change Heart ↑ output Lungs ↑ ventilation Muscle ↑ blood flow Applied
- Training → ↑ efficiency, ↓ resting heart rate
- Oxygen debt after exercise
🔑KEY POINTS TO REMEMBER- Exercise: ↑ cardiac output, ↑ ventilation, ↑ metabolism together.
- Blood diverted to muscle; lactate from anaerobic metabolism.
- Training lowers resting HR.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).