Fluid and Electrolyte Balance

Normal Balance and Movement of Water and Solutes The balance and movement of water and solutes between the intracellular fluid (ICF) and extracellular fluid (ECF) compartments are crucial for maintaining isovolemia, which refers to a state where the volume of body fluids is stable and properly regulated. The regulation of water balance involves several systems, including the hypothalamic, pituitary, adrenal cortical, renal, cardiac, and gastrointestinal systems, as well as insensible water loss (e.g., through the skin and respiratory tract).

Abnormal Cellular Changes Changes in solute levels between the ICF and ECF can disrupt this balance. For example, hypovolemia occurs when the ECF has a lower osmolality compared to the ICF. This imbalance causes water to move into the cells, leading to cellular swelling and a decrease in plasma volume.

Immune Structure and Function – Body Fluid Compartments Body fluids are distributed within two main functional compartments: the intracellular fluid (ICF) compartment, which makes up about 40% of body weight, and the extracellular fluid (ECF) compartment, accounting for about 20% of body weight. The movement of fluids between these compartments occurs via osmosis. The ECF includes intravascular fluid (blood plasma, <5% of body weight) and interstitial fluid (fluid between cells and outside blood vessels, <15% of body weight). Other fluids in the body include lymph, synovial fluid, cerebrospinal fluid (CSF), intestinal fluids, sweat, urine, intraocular fluids, and fluids in body cavities. Collectively, these fluids are referred to as total body water (TBW). They play roles in body temperature regulation and lubrication of tissues (such as in the synovium, pleura, and pericardium).

Fluid States Fluids in the body can be categorized as normal, contracted, or expanded.

Electrolyte Balance Electrolytes help maintain isotonic, hypertonic, and hypotonic states within the body. Normal values for key electrolytes include:

• Sodium: 135-145 mEq/L
• Potassium: 3.5-5 mEq/L
• Chloride: 96-106 mEq/L
• Calcium: 9-11 mg/dL
• Blood Urea Nitrogen (BUN): 10-20 mg/dL
• Creatinine: 0.7-1.2 mg/dL
• CO2 (Bicarbonate): 22-26 mEq/L
• Magnesium: 1.5-2.5 mg/dL

Dehydration and Fluid Loss Dehydration occurs when there is an excessive loss of water and electrolytes, often exacerbated by the excretion of ketone bodies. This condition can be particularly dangerous in infants, where fluid loss from diarrhea can represent a significant proportion of their body weight. Because renal mechanisms that regulate fluid and electrolyte balance may not be fully developed in infants, they are at a higher risk of rapid dehydration.