Fluids and Electrolytes: Water Balance

Overview of Water in the Human Body

• The average adult human body is approximately 60% water.
• Water is distributed in two main compartments:
  • Intracellular Compartment:
    • Contains about 65% of total body water.
    • Located within cells.
  • Extracellular Compartment:
    • Contains about 35% of total body water.
    • Located outside of cells.
    • Includes several subcompartments:
      • Tissue fluid
      • Blood plasma
      • Lymph
      • Transcellular fluid (cerebrospinal, synovial, peritoneal, pleural, pericardial fluids, vitreous and aqueous humors of the eye, digestive fluids, bile, and fluids in the urinary and respiratory tracts)

Movement and Balance of Water

• Water moves between compartments through semi-permeable membranes.
• Water Balance: Gains and losses of water must be equal in the body.

Sources of Water Gain

• Metabolism (e.g., aerobic respiration): ~200 mL/day
• Food: ~700 mL/day
• Drink: ~1,600 mL/day

Sources of Water Loss

• Feces: ~200 mL/day
• Respiration: ~300 mL/day
• Evaporation: ~400 mL/day
• Sweat: ~100 mL/day
• Urine: ~1,500 mL/day
• Total daily water gain and loss: ~2,500 mL/day

Regulation of Water Balance

• Mechanisms of Regulation:
  • During exercise, osmoreceptors in the hypothalamus sense changes in blood concentration.
  • Messages are sent to the posterior pituitary gland to secrete Antidiuretic Hormone (ADH), which targets kidneys to retain water.
  • The hypothalamus signals the cerebral cortex to induce thirst.
  • Only a 2-3% increase in osmolarity can induce thirst.
  • A 10-15% loss of blood volume can also trigger thirst.
  • Thirst satiation occurs with decreased blood osmolarity, cooling and moistening of the mouth, and stomach distension.

Disorders of Water Balance

• Dehydration (Hypovolemia):
  • Occurs when water output exceeds input.
  • Increases osmolarity of extracellular fluid, drawing water from cells.
  • Severe dehydration affects many body cells, including neurons, and can lead to shock.
• Water Intoxication:
  • Occurs when water input exceeds output.
  • Extracellular fluid becomes hypotonic, causing water to enter cells.
  • Can lead to pulmonary and brain edema.