Master Physiology
for FMGE

Core Concepts

Physiology is the study of how the human body functions, emphasizing homeostatic mechanisms and their integration. Understanding these core principles is vital for clinical problem-solving.

General Principles

• Homeostasis: Maintenance of a stable internal environment. Achieved via dynamic equilibrium.
• Feedback Loops:
  • Negative Feedback (Most Common): Opposes initial change, stabilizing the system (e.g., blood glucose, BP regulation).
  • Positive Feedback (Rare, Amplifying): Enhances initial change, leading to rapid completion (e.g., childbirth, action potential generation, coagulation cascade).
• Cell Physiology:
  • Membrane Transport: Diffusion, facilitated diffusion, active transport (Na+/K+ ATPase is crucial for maintaining resting membrane potential and cell volume).
  • Action Potentials: Rapid changes in membrane potential (depolarization via Na+ influx, repolarization via K+ efflux). All-or-none principle. Essential for nerve and muscle signaling.

Systems Physiology

• Cardiovascular:
  • Cardiac Cycle: Systole (contraction, ejection) & Diastole (relaxation, filling). S1 (AV valve closure), S2 (Semilunar valve closure).
  • Cardiac Output (CO): Heart Rate (HR) x Stroke Volume (SV). Factors affecting SV: Preload (EDV, Frank-Starling law), Afterload (MAP), Contractility.
  • BP Regulation: Baroreceptors (short-term), RAAS, ADH, ANP (long-term).
  • ECG: P wave (atrial depolarization), QRS complex (ventricular depolarization), T wave (ventricular repolarization).
• Respiratory:
  • Lung Volumes & Capacities: Tidal Volume, FRC, Vital Capacity, TLC.
  • Gas Exchange: Occurs in alveoli by diffusion, driven by partial pressure gradients (Fick's Law).
  • O2-Hb Dissociation Curve: Right shift (CADET: CO2, Acid, 2,3-DPG, Exercise, Temperature) decreases Hb's O2 affinity, enhancing O2 release to tissues.
  • Control of Breathing: Medullary rhythmicity centers, central (PCO2/pH) and peripheral (PO2, PCO2, pH) chemoreceptors. PCO2 is the most potent stimulus.
• Renal:
  • GFR: Glomerular Filtration Rate, primary measure of kidney function. Regulated by afferent/efferent arteriolar tone.
  • Tubular Functions: PCT (bulk reabsorption of Na+, glucose, AAs, HCO3-), Loop of Henle (countercurrent multiplier, concentrating urine), DCT/Collecting Duct (fine-tuning, ADH for water reabsorption, Aldosterone for Na+/K+ balance).
  • Acid-Base Balance: Buffers (bicarbonate, phosphate, proteins), respiratory regulation (CO2 excretion), renal regulation (HCO3- reabsorption/generation, H+ excretion).
• Gastrointestinal:
  • Motility: Peristalsis, segmentation. Regulated by ENS, ANS, and hormones.
  • Digestion & Absorption: Majority occurs in small intestine. Pancreatic enzymes, bile salts, brush border enzymes are key.
  • Key Hormones: Gastrin (acid), Secretin (HCO3-), CCK (bile, enzymes), GIP (insulin release).
• Endocrine:
  • Hypothalamic-Pituitary Axis: Central control. Anterior pituitary hormones (GH, TSH, ACTH, FSH, LH, Prolactin). Posterior pituitary (ADH, Oxytocin).
  • Major Hormones: Insulin/Glucagon (glucose), Thyroid hormones (metabolism), Cortisol (stress, glucose), Aldosterone (Na+/K+), PTH/Calcitonin (Ca2+).
• Neurophysiology:
  • Synaptic Transmission: Neurotransmitter release (ACh, NE, GABA, Glutamate etc.), receptor binding, EPSP/IPSP.
  • Autonomic Nervous System: Sympathetic ("fight/flight") vs. Parasympathetic ("rest/digest"). Distinct receptor types (adrenergic, cholinergic).
• Blood:
  • Components: Plasma, RBCs (O2 transport via Hb), WBCs (immunity), Platelets (hemostasis).
  • Hemostasis: Vascular spasm, platelet plug formation, coagulation cascade (intrinsic/extrinsic pathways leading to fibrin formation).

Clinical Presentation

• Fluid & Electrolyte Imbalance: Edema, dehydration, arrhythmias (K+), seizures (Na+, Ca2+), muscle weakness.
• Acid-Base Disorders: Dyspnea, Kussmaul breathing (severe acidosis), altered mental status, cardiac arrhythmias.
• Cardiovascular Dysfunction: Hypotension/hypertension, dyspnea, chest pain, palpitations, syncope.
• Respiratory Compromise: Hypoxemia, hypercapnia, dyspnea, cyanosis, altered breathing patterns.
• Endocrine Imbalances: Weight changes, fatigue, polyuria/polydipsia, heat/cold intolerance, tremors, mood changes.
• Neurological Deficits: Weakness, sensory loss, altered consciousness, paralysis, abnormal reflexes.

Diagnosis (Gold Standard)

Evaluation of physiological function:

• Cardiovascular: ECG (rhythm, ischemia), Blood Pressure measurement, Echocardiography (cardiac output, structure), Cardiac Catheterization (pressures).
• Respiratory: Spirometry (lung volumes/flow), Arterial Blood Gas (ABG: pH, PCO2, PO2, HCO3-), Pulse Oximetry (O2 saturation).
• Renal: Serum Creatinine & eGFR (kidney function), Electrolytes, Urine analysis, Fractional Excretion of Sodium (FENa).
• Endocrine: Serum hormone levels (basal & dynamic tests), Glucose monitoring (fasting, post-prandial, HbA1c).
• Neurological: Nerve Conduction Studies, Electromyography (EMG), Electroencephalography (EEG).
• Blood: Complete Blood Count (CBC), Coagulation profile (PT, aPTT, INR, D-dimer).

Management (First Line)

Physiological interventions aim to restore or support normal body functions:

• Fluid & Electrolyte: IV fluids (isotonic for volume, hypotonic for free water deficit), targeted electrolyte replacement (e.g., KCl for hypokalemia).
• Acid-Base: Address underlying cause, ventilatory support (respiratory acidosis/alkalosis), IV sodium bicarbonate (severe metabolic acidosis).
• Cardiovascular: Oxygen, IV fluids, vasopressors (hypotension), diuretics (volume overload), antiarrhythmics, blood pressure control medications.
• Respiratory: Oxygen therapy, bronchodilators, non-invasive/invasive mechanical ventilation.
• Endocrine: Hormone replacement therapy (e.g., insulin for diabetes, thyroid hormone for hypothyroidism, corticosteroids for adrenal insufficiency).
• Hemostasis: Anticoagulants, antiplatelets, blood product transfusions.

Exam Red Flags