Questions
3-4 questions per major university exam
Difficulty
Medium
Importance
Core foundational topic for MBBS and BPT degree examinations
Overview
General Physiology establishes the foundation for understanding how the human body maintains homeostasis through cellular and systemic processes. Mastering cell transport and bioelectric potentials is critical for clinical exams as these principles govern nerve conduction, muscle contraction, and fluid balance.
Cell Membrane Transport
The cell membrane acts as a selective barrier regulating substance exchange via passive and active transport mechanisms. Understanding these pathways is essential for grasping drug delivery, renal filtration, and cellular nutrient uptake.
- Simple diffusion: Passive movement of lipid-soluble substances down concentration gradients
- Facilitated diffusion: Protein-mediated movement requiring no ATP
- Primary active transport: Direct utilization of ATP via pumps like Na+/K+-ATPase
- Secondary active transport: Utilizing energy from established electrochemical gradients
- Osmosis: Movement of water across a semipermeable membrane
- Exocytosis/Endocytosis: Bulk transport involving vesicular trafficking
Body Fluid Compartments
Total body water is distributed into intracellular and extracellular fluid (ECF) compartments, with the latter divided into interstitial fluid and plasma. Maintaining the ionic balance between these compartments is vital for cardiovascular and renal homeostasis.
- Total Body Water (TBW) is approximately 60% of body weight in adults
- Intracellular Fluid (ICF) constitutes roughly 40% of body weight
- Extracellular Fluid (ECF) constitutes roughly 20% of body weight
- The sodium-potassium ratio is higher in ECF than in ICF
- Gibbs-Donnan equilibrium governs the distribution of ions across membranes
Resting and Action Potentials
Bioelectric signals are generated by transient changes in membrane permeability to ions. These potentials are the fundamental basis for all neurological and muscular signaling, described mathematically by the Nernst and Goldman-Hodgkin-Katz equations.
- Resting Membrane Potential (RMP) is typically -70mV to -90mV
- Nernst Equation: Calculates the equilibrium potential for a specific ion
- Goldman-Hodgkin-Katz Equation: Determines membrane potential based on multiple ions
- Depolarization: Rapid influx of sodium ions through voltage-gated channels
- Repolarization: Delayed efflux of potassium ions restoring polarity
- All-or-none principle: Action potentials occur fully or not at all once threshold is reached
Formula Sheet
Nernst Equation: E = (RT/zF) * ln(Cout/Cin)
Goldman-Hodgkin-Katz Equation: Vm = (RT/F) * ln(Pk[Kout] + Pna[Naout] + Pcl[Clin]) / (Pk[Kin] + Pna[Nain] + Pcl[Clout])
Exam Tip
Always draw a labeled diagram of the action potential cycle with distinct phases (Depolarization, Repolarization, Hyperpolarization) to guarantee maximum marks in descriptive questions.
Common Mistakes
- Confusing the Nernst equation (single ion) with the Goldman-Hodgkin-Katz equation (multiple ions permeability).
- Neglecting the role of the Na+/K+ pump in maintaining the resting membrane potential vs. merely generating the action potential.
- Miscalculating ECF distribution as part of total fluid instead of distinguishing between interstitial and plasma components.
More Revision Notes
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