Physiology

Physiology Homeostasis

  • Definition: Homeostasis refers to the regulation of the internal environment within the body to maintain a stable and constant state, employing a negative feedback loop to manage fluctuations.

Major Body Fluid Compartments

  • Intracellular Compartment:
    • Contains fluid inside cells, including erythrocytes and leucocytes.
    • High concentrations of K+ and low concentrations of Na+.
  • Extracellular Compartment:
    • Comprises three components:
    1. Interstitial Fluid: Surrounds and nourishes cells.
    2. Transcellular Fluid: Includes fluids like aqueous humor, synovial fluid, cerebrospinal fluid, and urine in the bladder.
    3. Plasma: The liquid component of blood excluding blood cells.
    • High concentrations of Na+ and low levels of K+.
  • Osmolarity: The osmolarities of intracellular fluid, interstitial fluid, and plasma are iso-osmotic (equal), despite differing ion concentrations.

Cell Membranes and Junctions

  • Structure: Composed of phospholipid bilayers that block the passage of large, charged molecules or ions due to their non-polar nature.
  • Types of Cell Junctions:
    1. Tight Junctions: Control permeability and reduce leakage between cells.
    2. Adhering Junctions
    3. Gap Junctions
    4. Desmosomes

Blood Circulation – Bulk Flow

  • Definition: Bulk flow refers to the movement of fluid in the same direction at similar rates, primarily driven by hydrostatic pressure.
  • Capillary Exchange: Involves hydrostatic and osmotic pressures:
    • At the arteriole end, hydrostatic pressure > osmotic pressure (net filtration out).
    • Near the venule end, osmotic pressure > hydrostatic pressure (net reabsorption).
    • Approximately 20L of fluid enters the extracellular compartment, mostly returning to venous circulation; the remainder goes into the lymphatic system.

Diffusion

  • Definition: The movement of particles from high concentration to low concentration, described mathematically as:
    textext ext x2Dt ext{ } ext{~} ext{~} x^2 D
  • Osmosis: Specific type of diffusion where water moves from areas of low solute concentration to high solute concentration.
    • No energy is required (passive process).
  • Simple Diffusion: Non-polar substances (e.g., gases like O2, steroids) can passively diffuse across cell membranes.
  • Facilitated Diffusion: Charged particles or larger molecules require assistance from proteins:
    1. Carrier-mediated: Molecules are bound and transported by proteins.
    2. Ion channel mediated: Ions pass through channels that may be selective and gated.

Active Transport

  • Definition: Movement of molecules against their concentration gradient, necessitating energy.
  • Types:
    1. Primary Active Transport: Direct use of ATP, e.g., Na+/K+/ATPase Pump.
    2. Secondary Active Transport: Uses potential energy from gradients, coupling the movement of different ions or solutes.
  • Na+/K+/ATPase Pump:
    • Transports 2 K+ ions in and 3 Na+ ions out, crucial for cellular homeostasis.
    • Accounts for roughly 80% of cellular ATP use.

Electrical Signalling

  • Action Potential: Rapid changes in membrane potential essential for nerve and muscle cell functions, ranging from about -70 mV to +20 mV.
  • Membrane Potential: A result of differences in ion distribution between intracellular and extracellular environments.
  • Electrochemical Equilibrium:
    • Membrane Voltage (Vm) = Intracellular voltage - Extracellular voltage.
    • Influenced by chemical (concentration) and electric (charge distribution) forces.
    • Equilibrium occurs when the electrochemical forces balance.

Osmolarity and Tonicity

  • Osmolarity: Measures the solute particles per litre of solution.
    • Hyperosmotic: Higher concentration; Isosmotic: Same concentration; Hypoosmotic: Lower concentration.
  • Tonicity: Comparison based solely on non-penetrating solutes.
  • Implications for red blood cells (RBCs) in saline and glucose solutions are critical in understanding shifts in water movement and cell swelling or shrinking.

Endocrine System

  • Function: Coordinates physiology related to metabolism, growth, reproduction, and homeostasis, responding to external stimuli more slowly than the nervous system.
  • Common Features:
    • Grouped cell arrangements, a well-vascularized structure permits rapid hormone distribution directly into blood, fenestrated capillaries for swift hormone movement.
  • Primary Endocrine Organs: Hypothalamus, pituitary gland, thyroid, parathyroid glands, adrenal glands, pancreas, ovaries, and testes.
  • Secondary Endocrine Organs: Non-endocrine primary functions (e.g., heart, stomach, kidneys, liver, skin, placenta).
  • Hormones: Chemical messengers categorized by source (amines, peptides/proteins, steroids), water affinity (hydrophilic, hydrophobic), and action (autocrine, paracrine, endocrine, neuroendocrine).

Hormone Release

  • Triggers: Hormones are released in response to hormonal, humoral, or neuronal stimuli.
  • Regulation: Endocrine axes often drive negative feedback loops where hormone levels influence further release.

Action at Receptors

  • Receptor Specificity: Each hormone has a designated receptor that regulates its action.
    • Surface receptors (G-protein coupled, catalytic) trigger pathways leading to amplified responses.
    • Steroid and thyroid hormones bind to intracellular receptors influencing gene expression directly.