FLUIDES AND ELECTROLYTES

• Fluid


  A solution of solvent and solute
• Solvent


  A liquid substance where particles can be dissolved
• Solute


  A substance, either dissolved or suspended in a solution
• Solution


  A homogeneous mixture of 2 or more substances of dissimilar molecular structure, usually applied to solids in liquids but applies equally to gasses in liquids
• Functions of body fluids

  • Transporter of nutrients, wastes, hormones, proteins
  • Medium or milieu for metabolic processes
  • Body temperature regulation
  • Lubricant of musculoskeletal joints
  • Insulator and shock absorber
• Body fluid compartments


  • Intracellular
  • Extracellular
  • Transcellular
• Intracellular fluid (ICF)


  Fluid within the cells themselves, 2/3 of body fluid, located primarily in skeletal muscle mass, provides nutrients for metabolism
• Extracellular fluid (ECF)


  1/3 of body fluid, comprised of intravascular, interstitial and transcellular components, provides nutrients for cell functioning
• ECF components


  • Intravascular
  • Interstitial
  • Transcellular
• Intravascular


  Fluid within blood vessels, plasma makes up 7.5% of total body water
• Interstitial


  Fluid surrounding the cells, makes up 20% of total body water or 2/3 of ECF
• Transcellular


  Fluid over or across the cells, less than 1L in an adult, located in CSF, pericardial, synovial, intraocular, pleural fluids, sweat digestive secretions
• Normal values for total body water (TBW) and ECF/ICF percentages vary with age and gender
• Tonicity


  The concentration of particles in a solution, normal tonicity or osmolality of body fluids is 250-300 mOsm/L
• Types of tonicity


  • Isotonic
  • Hypotonic
  • Hypertonic
• Osmole


  The weight in grams of a substance producing an osmotic pressure of 22.4 atm when dissolved in 1.0 liter of solution, calculated as (gram molecular weight) / (no. of freely moving particles per molecule)
• Osmolality


  The number of osmoles of solute per kilogram of solvent
• Osmolarity


  The number of osmoles of solute per liter of solution
• Mole


  The number of molecules contained in 0.012 kg, the molecular weight of a substance in grams = Avogadro's number = 6.023 x 10^23
• Molality


  The number of moles of solute per kilogram of solvent
• Molarity


  The number of moles of solute per liter of solution
• Methods of solute/fluid movement across membranes

  • Osmosis
  • Diffusion
  • Filtration
  • Active transport
• Osmosis


  The movement of water/liquid/solvent across a semi-permeable membrane from a lesser concentration to a higher concentration
• Diffusion


  The movement of particles/solutes/molecules from an area of higher concentration to an area of lower concentration, affected by size of molecules, concentration difference, and temperature
• Facilitated diffusion


  A type of diffusion that uses a carrier but does not require energy, e.g. fructose and amino acid transport in intestinal cells
• Filtration


  The movement of both solute and solvent together across a membrane from an area of higher pressure to an area of lower pressure
• Active transport


  The movement of substances/solutes from an area of lower concentration to an area of higher concentration, utilizing energy, often with a carrier or enzyme
• Types of active transport

  • Primary active transport (uses ATP)
  • Secondary active transport (uses ion gradients)
• To maintain homeostasis, many body systems interact to ensure a balance of fluid intake and output
• Normally, fluid intake equals fluid output in adults, ranging from 1500-3500 mL per day with an average of 2500 mL
• Changes in ECF volume

  Alterations in sodium balance
• Change in sodium/water ratio


  Leads to hypoosmolarity or hyperosmolarity
• Fluid excess or deficit


  Loss of fluid balance
• The same pathophysiologic change can have different significance for different people
• Systemic regulators of body fluid balance

  • Renal regulation (renin-angiotensin system)
  • Endocrine regulation (thirst, ADH, aldosterone, ANF)
  • Gastrointestinal regulation
• Renal regulation


  The renin-angiotensin system regulates sodium and water balance in the ECF, with urine formation as the main mechanism
• Endocrine regulation


  The thirst mechanism, ADH, aldosterone, and ANF regulate water intake, water reabsorption, and sodium/water balance
• Gastrointestinal regulation


  Digestion and absorption of electrolytes, water and solutions maintain fluid balance
• Fluid intake sources


  • Dietary intake (90%)
  • Metabolic processes (10%)
  • IV fluids, TPN, blood products, colloids
• Fluid output routes


  • Urine
  • Feces
  • Insensible losses through skin and lungs
  • Sweat