Body fluids

Body Fluids Notes for PHSL2004 and PHSL2007

Introduction

Terms and Definitions:

  • Understanding the key terms related to body fluids and electrolytes is essential.

Transport Processes in Living Systems:

  • Osmosis: Movement of water across a semi-permeable membrane.

  • Passive Transport: Movement of substances without energy (down concentration gradient).

  • Active Transport: Movement of substances against the concentration gradient, requiring energy.

  • Tonicity vs Osmolarity:

    • Osmolarity: Total concentration of solute particles in a solution.

    • Tonicity: Effect of a solution on the volume of cells; involves only non-penetrating solutes.

Body Fluid Compartments:

  • Total Body Water: Comprises 60% of body weight; divided into:

    • Intracellular Fluid (ICF): Approximately 2/3 of total body water.

    • Extracellular Fluid (ECF): Approximately 1/3, subdivided into:

      • Plasma: Fluid part of blood.

      • Interstitial Fluid: Fluid surrounding cells.

      • Transcellular Fluid: Includes CSF and synovial fluid.

  • Movement of Fluid: Governed by Starling's forces.

Water Balance:

  • Intake vs Output: Water intake includes fluids consumed and metabolic water; output includes urine, sweat, and respiration.

  • Control and Regulation of Water Balance:

    • ADH (Antidiuretic Hormone): Increases water reabsorption in kidneys during dehydration.

    • Thirst Mechanism: Stimulated by high plasma osmolarity.

Special Fluids:

  • Cerebrospinal Fluid (CSF): Cushions the brain, provides buoyancy, and supplies nutrients.

  • Synovial Fluid: Lubricates joints, consists of hyaluronic acid and lubricin.

  • Sweat: Primarily for thermoregulation, is hypotonic compared to plasma.

Changes to Body Fluid Compartments:

  • Dehydration: Often results from loss of fluid without adequate replacement (e.g., diarrhea, vomiting).

  • Oral Rehydration Therapy: Essential for treating dehydration, particularly in children. Includes water, electrolytes, and glucose.

Important Formulae:

  • Osmolarity Calculation:

    • Formula: Osmolarity = 2[Na+] + [Glucose/18] + [Urea/6].

      • Explanation: The formula considers sodium (Na+), glucose, and urea concentrations in plasma to calculate osmolarity in mOsm/L. Each component is weighted appropriately for their contribution to osmotic pressure.

    • Clinical Significance: Understanding osmolarity is crucial in diagnosing and managing fluid and electrolyte imbalances, especially in conditions like dehydration or overhydration.

    • Normal Osmolarity Range: Typically, the normal plasma osmolarity falls within the range of 280-300 mOsm/L.

Movement of Substances in Body Fluids:

  • Diffusion: Movement of molecules from high to low concentration.

  • Transport Mechanisms:

    • Simple Diffusion: Lipid-soluble substances passively move across membranes.

    • Carrier-Mediated Transport: Requires transport proteins and can be passive or active.

    • Endocytosis & Exocytosis: Mechanisms for taking in (endocytosis) and expelling substances (exocytosis).

Osmosis and Water Movement:

  • Osmosis: Water moves towards areas of higher solute concentration.

  • Osmotic Pressure: Governed by the concentration of solutes (crystalloid versus colloidal pressure).

  • Tonicity Effects on Cells:

    • Isotonic: No change in cell volume.

    • Hypotonic: Cells swell due to water influx.

    • Hypertonic: Cells shrink due to water efflux.

Oedema:

  • Definition: Pathological accumulation of fluid in interstitial compartments.

  • Causes: Increased capillary hydrostatic pressure (e.g., heart failure). Decreased capillary oncotic pressure (e.g., low protein states). Lymphatic obstruction. Increased permeability due to inflammation.

Disturbances of Body Fluid Balance:

  • Dehydration and Overhydration: Balance between intake and output crucial for homeostasis.

  • Euhydration: Normal hydration state.

  • Clinical Relevance: Understanding how various conditions affect fluid balance is vital in patient care.

Summary Sections Recap:

The composition and movement of body fluids is crucial in physiology, with various transport mechanisms (diffusion, osmosis, active transport) regulating fluid balance. Pathologies such as dehydration and oedema underscore the importance of fluid management in clinical practice.