Overview
Water and salt balance depends on the distribution of body fluids and on hormones that adjust water excretion, sodium handling and the plasma calcium level. This topic covers body fluid compartments, ADH, the natriuretic peptides, and parathyroid hormone, calcitonin and vitamin D in calcium-phosphate balance.
Body fluids and water and electrolyte balance
- Water is a major component of the body, distributed among two major compartments: intracellular fluid (ICF) and extracellular fluid (ECF).
- ICF is contained within cells and comprises two-thirds of total body water.
- ECF is outside the cells and comprises one-third of total body water.
- ECF is further divided into two subcompartments: plasma and interstitial fluid.
- Plasma is the fluid circulating in blood vessels.
- Interstitial fluid bathes the cells.
- Total body water correlates inversely with body fat.
- Individuals with lower body fat have a higher percentage of total body water.
- Females, having a higher percentage of adipose tissue, tend to have less body water than males.
- The transcellular compartment, minor compartment, includes cerebrospinal, pleural, peritoneal, and digestive fluids.
- ICF and ECF have vastly different concentrations of various solutes.
- The major cation in ECF is sodium (Na+), with chloride (Cl-) and bicarbonate (HCO3-) as the balancing anions.
- The major cations in ICF are potassium (K+) and magnesium (Mg2+), balanced by proteins and organic phosphates as anions.
- Plasma and interstitial fluid also differ in solute concentrations due to the exclusion of proteins from interstitial fluid.
- Amounts of solute are typically expressed in moles, equivalents, or osmoles.
- Solute concentrations are expressed in moles per liter (mol/L), equivalents per liter (Eq/L), or osmoles per liter (Osm/L).
- Concentrations of solutes in biological solutions, being usually quite low, are expressed in millimoles per liter (mmol/L), milliequivalents per liter (mEq/L), or milliosmoles per liter (mOsm/L).
- Water and electrolytes are major components of the body and are required in large amounts for normal function.
- Electrolyte levels can be too high or too low, often due to changes in water levels (e.g., dehydration or overhydration).
- The body maintains water balance through intake and output.
- Water intake should be at least 1 liter daily.
- Sources of water intake include:
- Oral fluids: Beverages and water-rich foods.
- Food content: Many foods contain water.
- Metabolism: Oxidation of food produces metabolic water (around 300ml).
- Water is lost from the body through various routes:
- Urine: The kidneys regulate water excretion in urine, typically around 1500 ml/day (with a minimum of 500 ml/day needed to excrete metabolic wastes).