84. Physiology | Body Fluid Compartments

Amount vs concentration

Amount: Mole

Concentration: molarity (mol/L) or molality (mol/kg)

percent of material and how does it differ from molarity

grams/100 ml

• different amounts in percents because have to consider mass

for Molarity, equal to each other

Osmole

amount of particles in solution:
EX: NaCL - 2 particles dissociated

• Glucose does not dissociate

All particles regardless of size will produce osmotic pressure

Osmolarity vs osmolality

moles of dissociable solute particles/liter

osmolality: moles of dissociable solute /kg

Osmolarity of most body fluids

osmolarity ~ osmolarity 1 L ~ 1 kg

What determines osmolarity?

number of dissociable particles in solution

Compartments of body fluid

Intracellular: cytoplasm

Extracellular: outside of cell

• Plasma: (vascular): blood vessels (1/4)
• interstitial (extravascular): outside (3/4)

What is the 60-40-20 rule?

60% of body weight = total body water
40% = intracellular fluid
20% = extracellular fluid

What separates extra and intracellular fluid and also plasma from interstitial fluid?

extra and intracellular fluid: plasma membrane

Interstitial and plasma: capillary wall

Abnormal fluid distribution in interstitial (extravascular) region?

pitting edema

• interstitiual fluid high (liower limbs: ankles and feet)
  dehydration pinch test
• not enough interstitial fluid

Why does pitting edema happen?

hypertension due to increased blood volume

Increased vasc pressure - fluid forced into extravascular - pool in LE - gravity pulls fluid down to lowest point

How does blood volume increase?

decreased kidney function: can't excrete water and salt properly

2 driving forces for fluid movement between body fluid compartments

osmotic pressure
oncotic pressure: large macromolecules

Conditions in osmotic pressure:

when you have a semi-permeable membrane where only water can pass through

When you add salt to one side, since salt cannot travel, osmotic pressure created

• to relieve pressure, water moves from low solute to high solute

Normal conditions for extracellular and intracellular

Extracellular: Na: 140mM, K: 5 mM

intracellular: K: 120 mM, Na: 20 mM

How to calculate total osmolarity?

Osmolarity of effective cations (can't pass barrier) + osmolarity (effective anions) + osmolarity (uncharged compounds)

Rule of thumb for plasma

Total osmolarity = 2x Na plasma (anions) + D glucose + urea

• all concentrations in mM

Normal range for osmolarity

275 mOsm -295 mOsm

How to calculate rule of thumb for plasma if looking at mg/dl ?

2x Na plasma (anions) + D glucose/18 + BUN/2.8

What determines permeability across plasma membrane?

lipophilicity: lipid soluble
Transporters/pores: allows movement

Reflection coefficient

Assumption: all solutes cannot cross barriers
Reality: barriers have limited permeability to multiple solutes

coefficient: ability to cross cell membrane

• between 0 and 1
  0: permeable
  1: maximal impermeability

Reflection coefficient of 0 vs 1

coefficient of 1: impermeable: maximal osmotic pressure

• NaCL drives water movement across membrane

coefficeint of 0:

• no force to move water

permeability drives osmotic pressure

What happens if we have a semipermeable membrane that is freely permeable to both NaCL and water?

no net water movement becuase NaCL moves between both to equalize

• through diffusion

Diffusion

no osmotic pressure driving water movement because solute moves freely

• so coefficient is 0
• solute moves to equalize concentrations
• ions move through channels
• solutes move through trasnporters
• solutes move in and out of capillaries

Fick's first law of diffusion

J = -DA( C/x)
J: molecules moving/unit time
D: diffusion coefficient
A: area of diffusion
C: solute concentration gradient
x: distance diffusing

How does osmosis contribute to normal functioning of proximal tubule?

coefficients: 0 for D glucose and 1 for L glucose and Na+

Na and glucose will transport sodium adn glucose from lumen to between proximal tubule (intercellular space)

• increase Na+, glucose in proximal region

since glucose sigma is 0 , fluid moves to intercellular space

gradient created by sodium

What if you add L instead of D glucose to luminal fluid to proximal tubule?

both have coefficient of 1 which is not permeable so no h20 absorption
L glucose is not a substrate for tranporter

no osmotic driving force for water reabsorption: decreased water reabsorption = increased excretion: dieurtic

• not absorbing water from urine = releasing it

oncotic pressure

osmotic pressure produced by large macromolecules such as proteins
doesn't follow equation
instead follows Gibbs Donnan equilibrium

How is oncotic pressure different from regular?

excluded volume effect:

• proteins large enough to occupy a large volume

Why is oncotic pressure important?

constant force of moving fluid in and out of capillaries (plasma) using pressure generated by plasma

not case for interstitial fluid

Important points for fluid movement between body compartments

• fluid moves in response to osmotic pressure
• no osmotic pressure if solute can move between
• all volume changes start with changes in extracellular fluid

Tonicity vs osmolarity

tonicity:

• osmolarity compared to normal intercellular conditions: 290
  osmolarity: solute particle concentration

Darrow Yannet diagrams

• show how manipulation will affect fluid movements between body compartments

What happens if you add 1 liter of hypertonic solution to body using IV infusion?

only adding to extracellular side

• more solute in ECF so want to increase volume
  water moves from ICF - ECF
  so ICF: decrease volume, increase solute

What happens if you add 1 liter of hypotonic solution to body using IV infusion?

water moves from ECF - ICF because decreasing solute in ECF
diabetes insipdus, alcohol

What happens if you add 1 liter of isotonic solution to body using IV infusion?

since no gradient, no change in osmolarity just increase in volume
hemmorhage

Volume contraction vs expansion

osmolarity of remaining fluid not what is lost

A patient receives intravenous infusion of a hypertonic saline solution. Using the Darrow-Yannet diagram, which of the following best describes the immediate fluid shift?

A. Intracellular volume expands
B. Intracellular osmolarity decreases
C. Extracellular osmolarity initially increases
D. Extracellular fluid volume initially decreases
E. No immediate change in either compartment

C

When hypertonic saline is administered intravenously, it first increases the extracellular osmolarity, pulling water from the intracellular to the extracellular compartment. Thus, intracellular fluid volume shrinks, and extracellular fluid expands.

• A: Incorrect. Intracellular volume decreases, not expands.

• B: Incorrect. Intracellular osmolarity initially increases due to water loss.

• C: Correct. Extracellular osmolarity initially increases.

• D: Incorrect. Extracellular fluid volume initially expands.

• E: Incorrect. Immediate osmolarity and volume changes occur.

A 60-year-old patient has a severe hypoalbuminemia secondary to chronic liver disease. Which fluid compartment change would you predict based on oncotic pressure principles?

A. Increased intracellular fluid volume
B. Increased interstitial fluid volume
C. Increased plasma volume
D. Reduced extracellular fluid osmolarity
E. No net fluid shifts occur

B

Reduced plasma albumin lowers plasma oncotic pressure, allowing fluid to shift from plasma into the interstitial compartment, causing edema.

• A: Incorrect. Albumin affects extracellular compartments, not intracellular directly.

• B: Correct. Reduced oncotic pressure increases interstitial fluid volume, causing edema.

• C: Incorrect. Plasma volume decreases, not increases.

• D: Incorrect. Extracellular fluid osmolarity isn't significantly altered by albumin changes.

• E: Incorrect. Fluid shifts occur due to oncotic pressure changes.

A researcher notes that glucose transporters in renal proximal tubules are defective in a specific genetic disorder. Considering osmotic gradients, what is the direct consequence of impaired glucose transport on water reabsorption?

A. Enhanced water reabsorption
B. Decreased luminal fluid osmolarity
C. Increased sodium reabsorption
D. Reduced osmotic driving force for water reabsorption
E. Increased intracellular osmotic pressure

D

Impaired glucose transport prevents sodium-glucose cotransport into intercellular spaces, failing to increase intercellular osmolarity. Thus, osmotic gradient decreases, reducing water reabsorption.

• A: Incorrect. Less solute transport means reduced water reabsorption.

• B: Incorrect. Luminal fluid osmolarity remains elevated if glucose remains unabsorbed.

• C: Incorrect. Sodium reabsorption decreases because it's cotransported with glucose.

• D: Correct. Reduced glucose and sodium transport lowers osmotic gradient for water reabsorption.

• E: Incorrect. Intracellular osmotic pressure isn't directly increased by impaired glucose transport.

A dialysis patient experiences significant loss of plasma proteins during treatment. Which change best explains the resulting edema using the concept of osmotic pressures?

A. Increased plasma oncotic pressure
B. Decreased plasma oncotic pressure
C. Increased intracellular osmolarity
D. Increased interstitial hydrostatic pressure
E. Decreased extracellular fluid volume

B

Loss of plasma proteins (e.g., albumin) reduces plasma oncotic pressure, causing fluid to leak from the vascular space into interstitial spaces, leading to edema.

• A: Incorrect. Plasma oncotic pressure decreases, not increases.

• B: Correct. Reduced plasma oncotic pressure is exactly the reason for edema.

• C: Incorrect. Intracellular osmolarity isn't directly impacted.

• D: Incorrect. Interstitial hydrostatic pressure changes are secondary, not primary, drivers of edema.

• E: Incorrect. Extracellular fluid volume expands due to fluid leaking into interstitial spaces.

A patient develops severe diarrhea due to lactulose administration. Lactulose acts as an osmotic agent in the gut lumen. Based on principles of osmolarity, what would be the expected change in body fluid compartments?

A. Hypertonic contraction: extracellular compartment contracts; intracellular compartment contracts
B. Hypotonic contraction: extracellular compartment contracts; intracellular compartment expands
C. Isotonic contraction: extracellular compartment contracts; intracellular compartment unchanged
D. Hypertonic expansion: extracellular compartment expands; intracellular compartment contracts
E. Hypotonic expansion: extracellular compartment expands; intracellular compartment expands

A

Osmotic diarrhea due to lactulose (an effective osmolyte) leads to loss of hypotonic fluid (more water than solute). The extracellular compartment becomes hypertonic, causing water to shift out of cells, contracting both extracellular and intracellular compartments.

• A: Correct. Losing hypotonic fluid results in hypertonic contraction.

• B: Incorrect. Hypotonic contraction would result from losing hypertonic fluid.

• C: Incorrect. Isotonic contraction would not cause intracellular fluid shifts.

• D: Incorrect. This scenario describes an infusion of hypertonic saline, not fluid loss.

• E: Incorrect. Hypotonic expansion would occur with hypotonic fluid gain, not loss.

A 40-year-old patient presents with significant pitting edema due to kidney disease impairing sodium excretion. Based on fluid compartment physiology, what primary factor directly contributes to this edema?

Answer choices:

• A. Reduced intracellular osmolarity

• B. Increased vascular hydrostatic pressure

• C. Increased intracellular oncotic pressure

• D. Decreased extracellular osmolarity

• E. Decreased vascular hydrostatic pressure

B

• A. Incorrect – Intracellular osmolarity changes do not directly cause pitting edema.

• B. Correct – Retention of sodium and water increases blood volume, raising vascular hydrostatic pressure and pushing fluid into the interstitial space.

• C. Incorrect – Oncotic pressure changes in the intracellular compartment aren't relevant here.

• D. Incorrect – Extracellular osmolarity typically increases or remains stable in sodium retention.

• E. Incorrect – Hydrostatic pressure increases, not decreases, during edema formation.

A researcher compares two solutes, Marker X (small molecule) and Marker Y (large protein). Marker X crosses capillary walls freely; Marker Y does not. If both markers are injected intravenously, in which compartment(s) would Marker Y predominantly remain?

Answer choices:

• A. Evenly distributed in intracellular and extracellular fluid

• B. Intracellular fluid compartment only

• C. Interstitial fluid compartment only

• D. Plasma compartment only

• E. Evenly distributed between plasma and interstitial fluid

D

• A. Incorrect – Marker Y cannot cross cell membranes or capillaries; it won’t be in all compartments.

• B. Incorrect – Marker Y is too large to enter cells.

• C. Incorrect – Marker Y cannot cross capillary walls, so it will not accumulate in interstitial space.

• D. Correct – Large proteins like albumin are confined to the plasma due to capillary size-exclusion.

• E. Incorrect – Marker Y cannot equilibrate between plasma and interstitium.