Understanding Water Balance in the Body

Aug 30, 2024

Lecture Notes: Water Balance and Body Fluids

Importance of Water Balance

  • Human body is approximately 60% water.
  • Nurses must track water balance — input and output of fluids in patients.
    • Fluid administered (e.g., IV drip) must be recorded.
    • Urine output measured and recorded to maintain water balance.
  • Disbalance can lead to dehydration or water intoxication.

Calculating Total Body Water (TBW)

  • TBW is 60% of body weight in kilograms.
  • Conversion from pounds to kilograms:
    • Formula: 1 kg = 2.2 lbs.
    • Example: 154 lbs is approximately 70 kg.
  • Calculation: TBW = 0.6 × body weight (in kg).
    • Example: A person weighing 70 kg has 42 kg of water.

Metric System and Water

  • Metric system: 1 liter of fluid = 1 kilogram.
  • This allows direct conversion from body weight in kg to liters of water.

Reference Person for Measurements

  • Medical reference: 70 kg young adult male.
  • Historical data from army recruits and medical students.
  • Reference values used for drug dosage and other medical norms.

Fluid Compartments

  • Intracellular Fluid (ICF):
    • 2/3 of TBW.
    • Example: 28 liters in a 70 kg person.
  • Extracellular Fluid (ECF):
    • 1/3 of TBW.
    • Example: 14 liters in a 70 kg person.
  • Types of ECF:
    • Tissue Fluid (Interstitial Fluid): Surrounds tissue cells.
    • Blood Plasma: Fluid in blood vessels.
    • Transcellular Fluids: Localized in specific areas (e.g., cerebrospinal, synovial, intraocular).

Fluid Movement and Balance

  • Fluid moves between compartments.
  • Edema: Excess fluid moving from plasma to tissue fluid.
  • Dehydration: Fluid moves from cells to plasma.

Blood Volume Estimation

  • Total Blood Volume (TBV) is 8% of body weight (in kg).
    • Example: 5.6 liters in a 70 kg person.
  • Blood donation: 1 pint is about 1/10th of blood volume.

Electrolyte Distribution

  • Intracellular Fluid:
    • Major cations: Potassium (K+) and Magnesium (Mg2+).
    • Major anions: Phosphates and negatively charged proteins.
  • Extracellular Fluid (Tissue Fluid, Blood Plasma):
    • Major cation: Sodium (Na+).
    • Major anions: Chloride (Cl-) and Bicarbonate (HCO3-).
    • Blood plasma contains proteins; tissue fluid does not.
  • Protein production mainly by the liver.

Key Points

  • Importance of understanding fluid compartments and electrolytes.
  • Use of reference person for medical and physiological norms.
  • Awareness of fluid imbalances and implications in medical treatment.