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Renal > Lecture 2: Body Fluids > Flashcards

Lecture 2: Body Fluids Flashcards

1
Q

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
A

TBW = 60%

ECF = 20%
plasma = 5%
ISF = 15%

ICF = 40%

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2
Q

Describe the use of the dilution principle to measure plasma volume, ECF, and TBW

Identify the marker substances used in such measurements

A

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

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3
Q

Compare the ionic composition and osmolality of ECF and ICF

A 
ECF = Na, Cl, HCO3
ICF = K, proteins, phosphates
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4
Q

Define

Osmolality
Iso-osmotic
Hyperosmotic
Hypo-osmotic

A

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

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5
Q

Define the iso-osmolality principle, explaining why the body fluids are iso-osmotic

A

All body fluids have the same osmolality because:

Cell membranes and other barriers are freely permeable to water
Water channels, paracellular water movement

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6
Q

Identify the major solutes that contribute to Posm

A

Electrolytes: Na+ and accompanying anions

Glucose

Urea

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7
Q

Write an equation for the calculation of Posm that includes the contributions of the major solute contributors

A 
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)

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8
Q

Define tonicity, isotonic,, hypertonic, hypotonic, effective osmole

Identify the most important physiological example of an ineffective osmole

A

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

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9
Q

Given the composition and osmolality of a fluid, identify it as isotonic, hypertonic, or hypotonic

A

Isotonic
= 290 mOsm/kg H20 = Posm

Hypertonic
= 580 mOsm/kg H2O = 2 Posm

Hypotonic
= water

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10
Q

Predict the fluid shifts that would occur if a RBC is placed in solutions of different compositions and tonicities

A

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)

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11
Q

List important exceptions to the iso-osmolality principle

A

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

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12
Q

What term is used to describe osmotic contributions of proteins (and extra ions)

A

Oncotic pressure

= Pi

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13
Q

Define effective circulating volume (ECV), explaining how it differs from ECF

A

ECV = part of the ECF that can activate mechanisms to result in changes in Na+ excretion

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14
Q

What is hyponatremia, its most common general cause, and its effect on cell volume?

A

Hyponatremia = too low plasma sodium

Cause: inc. TBW
(Inappropriate water retention)

Effect on cell volume: inc.

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15
Q

What is hypernatremia, its most common general cause, and its effect on cell volume?

A

Hypernatremia = too high plasma sodium

Cause: dec. TBW
(Lacking access to water or not responding to thirst signals)

Effect on cell volume: dec.

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16
Q

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

A
  1. Iso-osmolality principle: all body fluid compartments are in osmotic equilibrium
  2. Pi can be ignored because it is only important when using Starling to describe fluid fluxes across capillary endothelium
  3. Na and K (and accompanying anions) are the only effective osmoles
    Na = extracellular, EC osmoles
    K = intracellular, IC osmoles
17
Q

What are differences in ionic composition between plasma and ISF?

A

ISF = diffusible anions

Plasma = diffusible cations

18
Q

When is electrochemical equilibrium achieved - or the so-called Gibbs-Donnan equilibrium?

A

When higher concentration of diffusible cations is in the plasma and a higher concentration of diffusible anions is in the ISF

19
Q

In Gibbs-Donnan equilibrium, do the small diffusible ions have equal concentrations in the plasma and ISF?

A

No!

Its as if the cations are trapped in plasma by anionic proteins present, while anions are chased out to ISF

20
Q

In the Gibbs-Donnan equilibrium, is the osmolality different in plasma vs. ISF?

A

Plasma osmolality slightly > ISF

21
Q

What is the iso-osmolality principle?

A

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

22
Q

What are the major solutes that contribute to plasma osmolality P(osm)?

A

Electrolytes (sodium + other anions)
Glucose
Urea

23
Q

What is an equation for calculation of Posm involving sodium, glucose and urea?

A 
Posm =
2 P(Na) + P(glucose)/18 + BUN/2.8
24
Q

Describe two situations when the approximation Posm = 2 P(Na) is NOT valid

A

(1) elevated plasma levels of osmotically active solutes (glucose, urea - DM, renal failure) - would underestimate!
(2) conditions like hyperproteinemia or hyperlipidemia

25
Q

What is an “effective osmole”?

A

Effective osmole: impermeant solute = solute that can “hold” water in a body fluid compartment

26
Q

What is the meaning of the subscript “e” in the equation

TB osmoles = 2 TB(e) (Na+ + K+)

A

“E” = exchangeable = osmotically active

Not all sodium or potassium ions in the body are osmotically active though!

27
Q

Why is it almost always necessary to start with an equation that uses total body solute and total body water?

A

Because of iso-osmolality principle

Renal (17 decks)

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