Fluid balance, Intro to Renal Phys + GFR

• H+ is uncontrollably generated internally
• Input of substances is generally poorly/not controlled
• Fluid Balance

  • Maintenance of H2O and salt balance
• Internal pool

  The quantity of a substance in the ECF
• Total body input equals total body output

  • If input is greater than output, positive balance exists
  • If input is less than output, negative balance exists
• Acid-base balance


  • Maintenance of H+ balance
• Eating and drinking are variable
• If quantity is to remain stable, input must be balanced with output
• Compensatory adjustments in H2O, salt, and H+ usually occur on the output side through urinary excretion
• Water is the most abundant substance in the body
• Output of H2O, salt, and H+ can be lost to the external environment uncontrollably through vomiting and sweating
• A high body water content is associated with leanness
• Body water percentage varies among individuals
• A low body water content is associated with obesity
• The lowest percentage of body H2O would be in an obese, elderly woman
• Body water is distributed between two major fluid compartments: 2/3 in ICF and 1/3 in ECF
• ICF is 0.4 times body weight
• Total body water is calculated as 0.6 times body weight
• ECF is 0.2 times body weight
• Plasma is 0.04 times body weight or 0.2 times ECF
• Interstitial fluid is 0.16 times body weight or 0.8 times ECF
• Body fluid compartment barriers include capillary walls and cell membranes
• Capillary walls allow free exchange between plasma and interstitial fluid, except for plasma proteins
• Cell membranes are highly selective and have an unequal distribution of Na+ and K+
• Ion composition differs between ECF and ICF
• Two Factors Regulate Fluid Balance - ECF volume

  1. Imp't in (long-term) regulation of blood pressure

  2. How? By maintaining salt balance in plasma
• Transient changes to maintain BP: (baroreceptor reflex and fluid shifts between ECF & ICF) until ECF volume can be restored
• Ion Composition of the Major Body Fluid Compartments - ECF

  • ...
• What governs water movement vs capillary wall?

  • Hydrostatic pressure (Blood pressure)
  • Oncotic pressure (plasma proteins)
• Barrier between ECF and ICF

  • Cell Membrane
  • Highly selective
  • Presence of cell proteins that cannot leave cells
  • Unequal distribution of Na+ and K+ due to Na+, K+ pump
• Ion Composition of the Major Body Fluid Compartments - ICF

  • ...
• What governs water movement vs plasma membrane?

  Osmotic effects alone
• Plasma is the only fluid that can be directly acted on to control volume and composition
• Two Factors Regulate Fluid Balance - ECF osmolarity

  1. Prevents swelling or shrinking of cells

  2. Controlled by water balance in plasma
• Salt input must equal salt output
• Kidneys adjust Na+ via adjusting GFR and tubular reabsorption
• Normally, osmolarities of ECF = ICF
• Causes of dehydration: insufficient water intake, excessive water loss, diabetes insipidus
• Causes of overhydration: renal failure patients consuming more water than solutes, rapid water ingestion, inappropriate secretion of ADH
• If lose/gain water, changes in ECF osmolarity occur