Physiology · MU Sofia

65. Volume and composition of body fluids. Water and electrolyte balance in the body.

Explains how body water is divided between intracellular fluid, about two-thirds of total water, and extracellular fluid, about one-third, with the extracellular part split into plasma and interstitial fluid. Covers the main cations and anions in each compartment, the units used for solute concentrations, daily water intake and losses through urine, faeces and insensible routes, and how thirst and ADH regulate balance. Ends with recap questions, including one on oedema.

Inside: 8 sections · 4 labelled pictures · 14 flashcards · 4 min

Topic 64 of 104 in the MU Sofia syllabus for Physiology (Dental Medicine, Year 2). The free AceMed note covers it in 8 sections, about 492 words (4 min of reading), with 14 flashcards.

In this note

  1. Volume and composition of body fluids. Water and electrolyte balance in the body.
  2. Body Fluid Compartments
  3. Composition of Body Fluid Compartments
  4. Units for Measuring Solute Concentrations
  5. Water and Electrolyte Balance
  6. Water Intake
  7. Water Losses
  8. Regulation of Water and Electrolyte Balance

Body Fluid Compartments

  • 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.

Composition of Body Fluid Compartments

  • 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.

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