General Physiology MBBS 1st Year: One Shot Overview

General Physiology MBBS One Shot gives you a clear and quick overview of how the human body functions at a basic level. It covers essential concepts like body fluid compartments, homeostasis, and membrane transport in a simple and structured way. This session helps you understand how different systems stay balanced and how substances move within the body. It is ideal for building strong fundamentals, making revision easier, and preparing effectively for MBBS exams and NEET-PG.

General Physiology 

General Physiology is a foundational subject in MBBS 1st year, looking into how the human body functions. This overview covers essential concepts like body fluid distribution, how the body maintains stability (homeostasis), and the mechanisms by which substances move across cell membranes, all crucial for understanding health and disease.

Body Fluid Compartments

The human body is largely composed of water. Understanding its distribution is key to physiology.

• Total Body Water (TBW): In a healthy 70 kg adult, TBW is 60% of body weight, totaling 42 liters.

• Distribution of TBW:

• Intracellular Fluid (ICF): 2/3 of TBW, or 40% of body weight (28 liters).

• Extracellular Fluid (ECF): 1/3 of TBW, or 20% of body weight (14 liters). (Memory Tip: Remember the 14 table: 14 liters (ECF), 28 liters (ICF), 42 liters (TBW)).

• Sub-compartments of ECF:

• Interstitial Fluid: 3/4 of ECF (11 liters).

• Plasma: 1/4 of ECF (3 liters).

• Blood Volume: Approximately 5 liters (8% of body weight) in a 70 kg adult, comprising 3 liters of Plasma and 2 liters of Blood Cells.

Measurement of Body Fluid Compartments (Indicator Dilution Method)

This method, also known as the Start-Hamilton Method, is used to measure fluid volumes.

• Principle: An indicator is injected into a specific compartment, allowed to disperse evenly, and then its concentration is measured.

• Core Idea: A larger compartment volume dilutes the indicator more, resulting in a lower concentration.

• Formula:

• V = I / C

• Where: V = Volume, I = Initial amount of indicator, C = Concentration after dispersion.

• Modified Formula (if indicator leaves the compartment):

• V = (I - A) / C

• Where: A = Amount of indicator that leaves the compartment.

Disturbances in Body Fluid Compartments

Disturbances primarily affect the Extracellular Fluid (ECF). Changes in ECF osmolarity trigger secondary water shifts between ICF and ECF.

Types of Dehydration

Dehydration is classified based on ECF tonicity:

1. Isotonic Dehydration:

• Description: Equal loss of water and sodium from ECF, maintaining isotonicity.

• Causes: Gastrointestinal fluid loss (vomiting, diarrhea), burns, hemorrhage.

• Changes: ECF volume decreases, ECF tonicity unchanged, ICF volume unchanged.

2. Hypertonic Dehydration:

• Description: Predominant water loss from ECF, making it hypertonic.

• Causes: Diabetes Mellitus, Diabetes Insipidus, chronic alcoholism, Lithium salts.

• Changes: ECF volume decreases, ECF tonicity increases, ICF volume decreases (due to water shifting from ICF to ECF).

3. Hypotonic Dehydration:

• Description: Disproportionately greater sodium loss than water from ECF, making it hypotonic.

• Causes: Primary Hypoaldosteronism.

• Changes: ECF volume decreases, ECF tonicity decreases, ICF volume increases (due to water shifting from ECF to ICF).

Types of Overhydration

Overhydration involves water gain into the ECF, classified by ECF tonicity:

1. Isotonic Overhydration:

• Description: Isotonic water gain in ECF.

• Causes: Administration of oral or intravenous isotonic saline.

• Changes: ECF volume increases, ECF tonicity unchanged, ICF volume unchanged.

2. Hypertonic Overhydration:

• Description: ECF expansion with hypertonic saline, making ECF hypertonic.

• Causes: Administration of hypertonic saline.

• Changes: ECF volume increases, ECF tonicity increases, ICF volume decreases (due to water shifting from ICF to ECF).

3. Hypotonic Overhydration:

• Description: Disproportionately greater water gain than sodium in ECF, making it hypotonic.

• Causes: Syndrome of Inappropriate ADH Secretion (SIADH).

• Changes: ECF volume increases, ECF tonicity decreases, ICF volume increases (due to water shifting from ECF to ICF).

Homeostasis

Homeostasis is the body's ability to maintain a stable internal environment, known as the Milieu Intérieur (specifically the ECF).

Homeostatic Regulatory Mechanisms

All homeostatic mechanisms involve:

1. Controlled Variable: The parameter being regulated (e.g., pH, temperature, O2 levels).

2. Sensor: Detects changes in the controlled variable.

3. Controller: Receives sensor information and has a set point.

4. Effector Organs: Act to correct deviations.

Types of Homeostatic Mechanisms: Feedforward and Feedback

1. Feedforward Mechanism: The controller anticipates a change and acts before it occurs, ensuring no time lag.

• Examples: Cephalic phase of gastric acid secretion, increased heart rate at the start of exercise.

2. Feedback Mechanism: A disturbance occurs, and this change is fed back to the controller, which then takes action. This involves a time lag.

Negative Feedback

• Prevalence: Most (99%) body homeostatic mechanisms.

• Mechanism: The controller performs the opposite action to the disturbance, negating the change and minimizing error.

• Efficiency: Measured by gain, where Gain = (Correction Applied) / (Residual Error). A higher gain indicates a more efficient system.

Positive Feedback

• Mechanism: The controller performs the same action as the disturbance, leading to error amplification or a "Vicious Cycle."

• "Good" Examples: Oxytocin in parturition, platelet plug formation, action potential depolarization, LH surge leading to ovulation.

• Key Principles: All positive feedback eventually ends with negative feedback and are part of a larger negative feedback process.

Cell Membrane Structure

The cell membrane, based on the Singer-Nicolson Model, is a lipid bilayer with embedded proteins.

• Composition:

• Proteins: 55%

• Lipids: 42%

• Carbohydrates: 3% (provide asymmetry and are involved in immune reactions).

Transport Proteins in the Membrane

Four main types facilitate substance movement across the membrane:

1. Pores: Generally for water movement (e.g., Aquaporins).

2. Channels: For simple diffusion of water-soluble substances and ions.

• Types: Leak Channels (always open) and Gated Channels (Voltage-Gated, Ligand-Gated, Cyclic Nucleotide-Gated, Time-Gated, Mechanically-Gated).

• Channel Poisons: Tetrodotoxin, Saxitoxin (sodium channels), Tetraethylammonium (TEA) (potassium channels).

3. Carriers: Involved in facilitated diffusion and secondary active transport.

• Types: Uniport (single substance, one direction), Cotransport/Symport (two substances, same direction), Antiport/Exchange (two substances, opposite directions).

4. Pumps (ATPases): Involved in primary active transport (directly consume ATP).

• Examples: Sodium-Potassium Pump, H+/K+ ATPase.

Transport Processes

Substances move across membranes via:

1. Vesicular Transports (Cytopempsis): For large molecules.

• Endocytosis: Substances enter cells.

• Phagocytosis (cell eating), Pinocytosis (cell drinking), Clathrin-mediated endocytosis.

• Exocytosis: Substances are released from cells.

• Constitutive (continuous), Regulated (signal-dependent).

2. Transport Through the Membrane: Osmosis, Diffusion, Active Transport.

Osmosis

• Definition: Movement of water across a semipermeable membrane from low solute to high solute concentration. (Memory Tip: Where high sodium is, water follows.)

• Osmolarity: Solute concentration per liter of solution.

• Osmolality: Solute concentration per kilogram of solvent.

• Normal Serum Osmolality: 286 to 300 mOsm/L.

• Tonicity vs. Osmolality: Urea is considered for osmolality but not for tonicity as it's an ineffective osmole.

Diffusion

A passive transport process that does not require ATP. Net diffusion is always from high to low concentration.

Types of Diffusion:

1. Simple Diffusion:

• Occurs due to kinetic energy of molecules.

• Through lipid bilayer: For lipid-soluble substances (e.g., O2, CO2).

• Through channels: For water-soluble substances (e.g., Na+, K+).

Factors Influencing the Rate of Simple Diffusion (Fick's First Law: J = -D ⋅ A ⋅ (Δc / Δx)):

• Directly Proportional:

• Lipid Solubility: (CO2 is 20x more soluble than O2).

• Number of Channels Available: (Membranes are more permeable to K+ than Na+).

• Temperature: (Higher temperature, faster diffusion; explains tachycardia in fever).

• Surface Area: (Reduced in emphysema decreases gas diffusion).

• Concentration Gradient.

• Pressure Gradient (for gases; relevant in hyperbaric oxygen therapy and decompression sickness).

• Electrochemical Gradient.

• Inversely Proportional:

• Thickness of the Membrane (Diffusion Distance): (Increased thickness in pulmonary edema decreases O2 diffusion).

• Size of the Diffusing Substance: (Larger molecules diffuse slower; hydrated K+ is smaller than hydrated Na+, thus K+ diffuses faster).

1. Facilitated Diffusion:

• Passive (no ATP), downhill net transport, carrier-mediated.

• Principle: High concentration equals high affinity, low concentration equals low affinity.

• Characteristics:

• Specificity: Requires a specific carrier.

• Saturability: Rate plateaus at Vmax when carriers are saturated.

• Inhibition: Can be inhibited (e.g., Phlorizin inhibits glucose transport).

GLUT Transporters (Glucose Transporters):

• GLUT1: Ubiquitous (RBCs).

• GLUT2: Pancreatic Beta Cells, liver, muscle (for glucose exit).

• GLUT3: Brain Neurons.

• GLUT4: Insulin-dependent (fat cells, muscle cells).

• GLUT5: Transports fructose (Memory Tip: Fructose Five (FF)).

Active Transport

• Active process (requires ATP), uphill transport (low to high concentration).

• Types:

1. Primary Active Transport: ATP is used directly by pumps/ATPases.

2. Secondary Active Transport: ATP is used indirectly by carriers.

• Relies on a gradient established by primary active transport (e.g., Na+ gradient created by Na+/K+ pump).

• Cotransport (Symport): Both substances move in the same direction (e.g., SGLT).

• Counter-transport (Antiport/Exchange): Substances move in opposite directions (e.g., NCX).

• Crucial Implication: If primary active transport fails, all secondary active transport will also fail.

Sodium-Potassium Pump (Na+/K+ Pump)

• Discovered by Jens Skou.

• Located in all cell membranes, continuously active.

• Structure: Alpha, Beta, Gamma subunits (Alpha does most work).

• Activity: Pumps three Na+ ions out and two K+ ions in per cycle, both against their concentration gradients.

Functions of the Sodium-Potassium Pump:

1. Opposes Equilibrium: Prevents cell swelling.

2. Regulates Cell Volume.

3. Drives Secondary Active Transport.

4. Recharges Excitable Cells.

5. Contributes to Basal Metabolic Rate (BMR): 40% of body's total BMR, up to 70% in neurons.

6. Electrogenic Nature: Contributes about -4 mV to the resting membrane potential.