Lecture 2: Body Fluids Flashcards
Define the following major body fluid compartments and indicate the size of each as a percentage of body weight:
total body water (TBW) Intracellular fluid (ICF) Exctracellular fluid (ECF) Interstitial fluid (ISF) Plasma
TBW = 60%
ECF = 20% plasma = 5% ISF = 15%
ICF = 40%
Describe the use of the dilution principle to measure plasma volume, ECF, and TBW
Identify the marker substances used in such measurements
ICF = TBW - ECF
ISF = ECF - plasma
MARKER SUBSTANCES:
TBW = isotopes of H2O (D2O, HTO), antipyrine
ECF = isotopes of EC ions (Na, Cl), mannitol
Plasma = labeled albumin
Compare the ionic composition and osmolality of ECF and ICF
ECF = Na, Cl, HCO3 ICF = K, proteins, phosphates
Define
Osmolality
Iso-osmotic
Hyperosmotic
Hypo-osmotic
Osmolality = total concentration of solutes
Iso-osmotic = solution with osmolality that is equal to plasma osmolality
Hyper-osmotic = solution with osmolality that is greater than plasma osmolality
Hypo-osmotic = solution with osmolality that is less than plasma osmolality
Define the iso-osmolality principle, explaining why the body fluids are iso-osmotic
All body fluids have the same osmolality because:
Cell membranes and other barriers are freely permeable to water
Water channels, paracellular water movement
Identify the major solutes that contribute to Posm
Electrolytes: Na+ and accompanying anions
Glucose
Urea
Write an equation for the calculation of Posm that includes the contributions of the major solute contributors
Posm = 2 P(Na) + P(glucose) + Purea
= 2 P(Na) + P(glucose)/18 + BUN/2.8
P(glucose) = mg/dl BUN = mg/dl
Because of iso-osmolality principle, can approx. Posm = 2 P(Na)
Define tonicity, isotonic,, hypertonic, hypotonic, effective osmole
Identify the most important physiological example of an ineffective osmole
Tonicity = concentration of impermeant solutes (effective osmoles)
Effective osmoles = solute that can influence movement of water
Isotonic: cell will not swell/shrink
Hypertonic: cell will shrink
Hypotonic: cell will swell and possibly burst!
Important ineffective osmole = urea
Given the composition and osmolality of a fluid, identify it as isotonic, hypertonic, or hypotonic
Isotonic
= 290 mOsm/kg H20 = Posm
Hypertonic
= 580 mOsm/kg H2O = 2 Posm
Hypotonic
= water
Predict the fluid shifts that would occur if a RBC is placed in solutions of different compositions and tonicities
Isotonic:
No solute or net water flux
Hypertonic:
No significant solute flux
Water leaves RBC (drawn out by NaCl)
Hypotonic:
No significant solute flux
Water enters RBC (drawn in by impermeant IC solutes)
List important exceptions to the iso-osmolality principle
In kidney:
Epithelial cells in thin AL, thick AL, DCT always impermeable to water; water permeability of cells in CNT, CD regulated by ADH; medullary gradient of ISF osm becomes hyperosmotic moving into inner medulla so that CD can function as concentrating segment
Plasma osmolality is slightly greater than ISF osmolality due to proteins
What term is used to describe osmotic contributions of proteins (and extra ions)
Oncotic pressure
= Pi
Define effective circulating volume (ECV), explaining how it differs from ECF
ECV = part of the ECF that can activate mechanisms to result in changes in Na+ excretion
What is hyponatremia, its most common general cause, and its effect on cell volume?
Hyponatremia = too low plasma sodium
Cause: inc. TBW
(Inappropriate water retention)
Effect on cell volume: inc.
What is hypernatremia, its most common general cause, and its effect on cell volume?
Hypernatremia = too high plasma sodium
Cause: dec. TBW
(Lacking access to water or not responding to thirst signals)
Effect on cell volume: dec.
List the assumptions that are made when deriving the equations that commonly are used to predict the effects of fluid and electrolyte perturbations on PNa and Posm
- Iso-osmolality principle: all body fluid compartments are in osmotic equilibrium
- Pi can be ignored because it is only important when using Starling to describe fluid fluxes across capillary endothelium
- Na and K (and accompanying anions) are the only effective osmoles
Na = extracellular, EC osmoles
K = intracellular, IC osmoles
What are differences in ionic composition between plasma and ISF?
ISF = diffusible anions
Plasma = diffusible cations
When is electrochemical equilibrium achieved - or the so-called Gibbs-Donnan equilibrium?
When higher concentration of diffusible cations is in the plasma and a higher concentration of diffusible anions is in the ISF
In Gibbs-Donnan equilibrium, do the small diffusible ions have equal concentrations in the plasma and ISF?
No!
Its as if the cations are trapped in plasma by anionic proteins present, while anions are chased out to ISF
In the Gibbs-Donnan equilibrium, is the osmolality different in plasma vs. ISF?
Plasma osmolality slightly > ISF
What is the iso-osmolality principle?
States that all body fluids have the same osmolality - Posm can be used as a measure of the osmolality of all body fluids
Exception: higher Posm of plasma because plasma has proteins and extra diffusible ions
What are the major solutes that contribute to plasma osmolality P(osm)?
Electrolytes (sodium + other anions)
Glucose
Urea
What is an equation for calculation of Posm involving sodium, glucose and urea?
Posm = 2 P(Na) + P(glucose)/18 + BUN/2.8
Describe two situations when the approximation Posm = 2 P(Na) is NOT valid
(1) elevated plasma levels of osmotically active solutes (glucose, urea - DM, renal failure) - would underestimate!
(2) conditions like hyperproteinemia or hyperlipidemia
What is an “effective osmole”?
Effective osmole: impermeant solute = solute that can “hold” water in a body fluid compartment
What is the meaning of the subscript “e” in the equation
TB osmoles = 2 TB(e) (Na+ + K+)
“E” = exchangeable = osmotically active
Not all sodium or potassium ions in the body are osmotically active though!
Why is it almost always necessary to start with an equation that uses total body solute and total body water?
Because of iso-osmolality principle
