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Discover the fascinating intricacies of human physiology, uncovering the secrets behind our body's functions and unlocking a deeper understanding of our own physicality.

USMLE Guide: Physiology


This USMLE guide aims to provide a concise overview of key concepts in Physiology to help medical students prepare for the United States Medical Licensing Examination (USMLE). Physiology is the study of how living organisms function, encompassing various systems and processes within the human body. This guide will cover the fundamental principles and important topics frequently tested on the USMLE.

Table of Contents

  • Cellular Physiology
    • Membrane Transport
    • Cellular Metabolism
    • Signal Transduction
  • Neurophysiology
    • Resting Membrane Potential
    • Action Potential
    • Synaptic Transmission
  • Cardiovascular Physiology
    • Cardiac Cycle
    • Blood Pressure Regulation
    • Hemodynamics
  • Respiratory Physiology
    • Gas Exchange
    • Lung Volumes and Capacities
    • Oxygen-Hemoglobin Dissociation Curve
  • Renal Physiology
    • Glomerular Filtration Rate (GFR)
    • Renal Blood Flow
    • Electrolyte and Acid-Base Balance
  • Gastrointestinal Physiology
    • Digestion and Absorption
    • Gastric Secretion
    • Intestinal Motility
  • Endocrine Physiology
    • Hypothalamic-Pituitary Axis
    • Thyroid and Parathyroid Hormones
    • Adrenal Glands and Cortisol
  • Reproductive Physiology
    • Male and Female Reproductive Systems
    • Menstrual Cycle
    • Fertilization and Implantation

Cellular Physiology

Membrane Transport

  • Active Transport: ATP-dependent movement of molecules across the cell membrane.
  • Passive Transport: Movement of molecules down their concentration gradient without the need for energy.
  • Facilitated Diffusion: Use of carrier proteins to transport molecules across the membrane.
  • Osmosis: Diffusion of water across a selectively permeable membrane.

Cellular Metabolism

  • Glycolysis: Conversion of glucose to pyruvate, generating ATP and NADH.
  • Krebs Cycle (Citric Acid Cycle): Oxidation of acetyl-CoA producing ATP, NADH, FADH2, and CO2.
  • Electron Transport Chain: Transfer of electrons from NADH and FADH2 to generate ATP through oxidative phosphorylation.

Signal Transduction

  • G-protein Coupled Receptors (GPCRs): Activation of intracellular signaling pathways through G-proteins.
  • Tyrosine Kinase Receptors: Phosphorylation of intracellular proteins upon ligand binding.
  • Second Messengers: Intracellular molecules that amplify and transmit signals from cell surface receptors.


Resting Membrane Potential

  • Na+/K+ ATPase: Maintains the concentration gradient of sodium (Na+) and potassium (K+) ions across the cell membrane.
  • Ion Channels: Selectively allow the passage of ions, contributing to the resting membrane potential.

Action Potential

  • Depolarization: Rapid influx of sodium ions through voltage-gated channels.
  • Repolarization: Potassium efflux through voltage-gated channels, restoring the negative resting membrane potential.
  • Hyperpolarization: Excessive efflux of potassium ions, briefly exceeding the resting membrane potential.

Synaptic Transmission

  • Neurotransmitters: Chemical messengers that transmit signals between neurons.
  • Excitatory Postsynaptic Potential (EPSP): Depolarization of the postsynaptic membrane.
  • Inhibitory Postsynaptic Potential (IPSP): Hyperpolarization of the postsynaptic membrane.

Cardiovascular Physiology

Cardiac Cycle

  • Systole: Contraction phase of the heart.
  • Diastole: Relaxation phase of the heart.
  • Cardiac Output: Volume of blood pumped by the heart per minute.
  • Stroke Volume: Volume of blood ejected by the heart with each contraction.

Blood Pressure Regulation

  • Baroreceptors: Pressure-sensitive receptors that regulate blood pressure.
  • Renin-Angiotensin-Aldosterone System (RAAS): Hormonal cascade that regulates blood volume and pressure.
  • Sympathetic Nervous System: Activation leads to vasoconstriction and increased heart rate.


  • Blood Flow: Movement of blood through the blood vessels.
  • Resistance: Opposition to blood flow.
  • Poiseuille's Law: Describes the relationship between blood flow, pressure, and resistance.

Respiratory Physiology

Gas Exchange

  • Diffusion: Movement of gases across the alveolar-capillary membrane.
  • Oxygen Transport: Binding of oxygen to hemoglobin in red blood cells.
  • Carbon Dioxide Transport: Dissolved in plasma, bound to hemoglobin, or as bicarbonate ions.

Lung Volumes and Capacities

  • Tidal Volume: Volume of air inspired or expired during normal breathing.
  • Vital Capacity: Maximum
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