Physiology: Homeostasis and Body Fluid Dynamics

Introduction

  • The lecture continues from the previous discussion, focusing on key physiological concepts.

Homeostasis

  • Definition: A major term in physiology that refers to the stability of the internal environment.
  • Key Concept: Homeostasis involves the interaction between intracellular fluid (ICF) and extracellular fluid (ECF), which is affected by external environmental changes.

Intracellular and Extracellular Fluid

  • Components:
    • ICF:
    • Volume: Approximately 25 liters (30% of total body weight).
    • Ionic Constituents: Low sodium and chloride, high potassium concentration (considered less "salty").
    • ECF:
    • Volume: Approximately 15 liters (including plasma and interstitial fluid).
    • Components: Saltier with high concentrations of sodium and chloride and low concentration of potassium.
  • Total Body Fluid Volume: Approximately 40 liters (25 liters ICF + 15 liters ECF).

Barriers and Transport

  • Semi-permeable Barriers:
    • Capillaries are more permeable compared to cell membranes, impacting the exchange between ECF and ICF.
  • Cell Membrane:
    • Less permeable, conserves potassium while allowing sodium and chloride in ECF to fluctuate.

Interaction with External Environment

  • Systems for Exchange:
    • Skin: Acts as a barrier with selective permeability.
    • Alimentary Canal: Ingestion of food and fluids contributes to body fluid balance.
    • Lungs: Gas exchange is crucial for maintaining oxygen and CO2 levels in the body.
    • Kidneys: Regulations of water and electrolyte balance through filtration and urination.
  • Fluid Loss: Through urine, sweat, and gastrointestinal loss.

Balance and Regulation of Water

  • Sources of Water Ingestion:
    • Direct intake from liquids (recommended: ~2 liters/day).
    • Water content in solid foods.
    • Oxidative metabolism of carbohydrates also serves as a source of water.
  • Daily Water Loss:
    • Urination: Average ~1.5 liters.
    • Evaporation: Approximately 900 milliliters (insensible water loss).
    • Feces: About 200 milliliters.

Physiological Processes

  • Metabolism and Energy Production:
    • ATP (adenosine triphosphate): Fundamental energy currency of the cell.
    • Developed through:
    1. NADH Dehydrogenase
    2. Succinic Dehydrogenase
    3. Cytochrome c
    4. Cytochrome Oxidase
    5. ATP Synthase: Assembles ATP utilizing proton gradients.

Control Mechanisms and Feedback Loops

  • Negative Feedback Loop:
    • System responds to deviations from a set point (e.g., temperature, blood glucose levels).
    • Components:
    1. Sensors: Detect deviations.
    2. Integrative Center: Processes information.
    3. Effectors: Execute responses to restore balance.
  • Comparison and Comparator:
    • Compares current state to desired values (set points) and direct commands to effectors.

Examples of Feedback Mechanisms

Eating and Blood Glucose Regulation

  • Increase in blood glucose triggers insulin release, promoting glucose uptake by cells, lowering blood glucose levels.

Positive Feedback Loop

  • Definition: Amplifies and enhances responses that increase changes (e.g., blood clotting).
  • Examples:
    • Blood Clotting Cascade: Sequential activation of clotting factors leading to fibrin formation.
    • Labor: Oxytocin release during childbirth increases uterine contractions leading to birth.
    • Sodium Channel Activation: Calcium-induced calcium release during heartbeats.

Measurement of Body Fluid Volume and Composition

  • Methods to measure compartments include:
    1. Isotopic Substances: E.g., 131I-albumin for plasma volume.
    2. Dilution Methods: Evans blue for total ECF and deuterium oxide for total body water.

Homeostasis Index (HI)

  • Describes control and failure rates of homeostasis based on environment and physiological responses.
  • Examples:
    • HI for temperature is compared under extreme conditions to measure control effectiveness.

Conclusion

  • Homeostasis maintains a stable internal environment through complex interactions and feedback mechanisms in physiology.
  • The lecture emphasizes the importance of feedback systems in regulating physiological processes and how deviations are managed to ensure homeostasis.