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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.
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