Body Fluid Compartments (B2: W5)

Question 1

What equation is used to estimate the volume of a body fluid compartment?

Answer 1

Volume = Quantity of tracer / Concentration of tracer

Divide that volume by body weight to figure out what percentage of body weight is water

Question 2

What equation is used to find the intracellular fluid volume?

Answer 2

Intracellular fluid volume = Total water in body - Extracellular fluid volume

Question 3

How do you find blood volume usine hematocrit?

Answer 3

Blood volume = Plasma volume / (1 - hct)

Women hct: 0.38 - 0.46

Men hct: 0.42 - 0.54

Question 4

Of the total body water (50-70% body weight), where is the distribution?

Answer 4

• Greater volume inside of cells than outside: ~40%
• Outside of cells:
  
  • Interstitial
  • Plasma
  • Transcellular (joint fluid, CSF, etc.)

Question 5

What assumptions do we make when considering disturbances of volume and osmolarity?

Answer 5

• Na is essentially an extracellular ion
• K is essentially an interacellular ion
• The intracelluar compartment behaves as a perfect osmometer

Question 6

What is the difference in osmolarity between the ECF and the ICF?

Answer 6

None

• Osmolarity is the same
• More volume in ICF
• Also more partilces in ICF

Question 7

What happens in the event of isotonic overhydration (careless over adminisration of saline)?

Answer 7

ECF compartment expands

• Water stays put: isotonic

Question 8

What happens in the event of hypotonic overhydration (compulsive water drinking)?

Answer 8

• Extracellular particles are diluted
  
  • Decrease in osomolarity
  • ECF compartment expands
• Shifts water inside cells
  
  • ICF compartment expands

Question 9

What happens in the event of hypertonic overhydration (drinking sea water)?

Answer 9

• There is an increase in extracellular osmolarity
  
  • Extra particles draw water outside of cells
• ECF compartment expands
• ICF compartment shrinks
  
  • leads to intense thirst

Question 10

What happens in the event of isotonic dehydration (hemorrhage)?

Answer 10

• Fluid loss contracts ECF compartment
• Osmolarity remains the same: isotonic

Question 11

What happens in the event of hypotonic dehydration (Addison’s disease - Adrenal cortical insufficiency/lack of aldosterone)?

Answer 11

• Drop salt from the body in excess in the urine
• Osmolarity of the ECF goes down
  
  • ECF compartment contracts
• Water goes towards more concentrated cells
  
  • Expands ICF compartment

Question 12

What happens in the event of hypertonic dehydration (lost in desert/lack of ADH)?

Answer 12

• Loosing water through sweating (more water than salt)
• Particles become more concentrated
• Water comes out of cells
  
  • ICF compartment contracts
• Osmolarity goes up, blood volume goes down
  
  • ECF also shrinking due to sweat

Question 13

What happens to sodium concentration in IV over administration? In hemorrhage?

Answer 13

Remains normal and constant with volume disturbance

Question 14

What happens to sodium concentration with dehydration? Overhydration?

Answer 14

• Sodium concentration can decrese with both
  
  • Water intoxication
  • Addison’s disease (lack of aldosterone)
• OR, [Na] can increases with both
  
  • Drinking sea water
  • Lost in the dessert

Question 15

Since you can’t distinguish if hypernatremia is a result of overhydration or dehydration, how can you tell them apart?

Answer 15

Look at plasma proteins

• Decrease with drinking sea water (overhydration)
• Increase with sweating (dehydration)

Question 16

What is the difference in a large amount of sweat vs. a small amount?

Answer 16

Sweat is always hypotonic

• With a small amount of sweat: more hypotonic
  
  • Time for reabsorption of Na from the tube
• With a lot of sweat: less hypotonic
  
  • Approaches isotonic
  • More salt

Question 17

What happens to a healthy individual who loses 2 liters of sweat and simultaneously drinks 2 liters of pure water?

Answer 17

This person loses salt!

• Plasma osmolarity will decrease
• Intracellular osmolarity will decrease
• Exctracellular volume will decrease
• Intracellular volume will increase
• Total solute content in the body will decrease

Question 18

Describe water turnover in a steady state

Answer 18

Water going in = water going out

• Intake can be from water in food, metabolic water, or water in beverage
• Water can leave in different ways
  
  • Insensible - lungs and skin
  • Sensible - urine, sweat, feces

Question 19

How is plasma osmolarity regulated by ADH and thrist?

Answer 19

• Osmoreceptors are in hypothalamus
• Activation stimulates nerves
• ADH released from posterior pituitary
  
  • Results in thirst
  • Regulates the kidney - water reabsorption

Question 20

What is the equation for osmolar clearance?

Answer 20

C_osm = (U_osm x V) / P_osm

Total number of particles cleared in urine

Question 21

How does osmolar clearance vary between isotonic urine, maximally dilute urine, and maximally concentrated urine?

Answer 21

Clearance cannot be determined by concentration

• Each condition is capable of clearing the same volumeof plasma per minute
• Consider flow

Question 22

What is the equation for urine flow related to osmotic clearance and free water clearance?

Answer 22

V = C_osm + C_H2O

• Dilute urine: free water clearance is high
  
  • Urine is hypotonic
• Concentrated urine: free water is reabsorbed
  
  • Urine is hypertonic
• Isotonic urine: clearance of free water is 0
  
  • V = C_osm

Question 23

Where is free water formed in the nephron?

Answer 23

Ascending limb of the loop of Henle

• Salt moves out
• Water stays in
• Hypotonic

Question 24

Where is free water reabsorbed back into the body?

Answer 24

Collecting tubule

• Interstitial concentration reabsorbes water
• Salt doesn’t move much
• Hypertonic urine

Question 25

How do diuretics affect free water clearance?

Answer 25

• We make free water in the ascending limb
• If we stop making it, we are going to reduce it and clear less
• Furosemide

Question 26

How do you determine if a diuretic is working in the ascending limb?

Answer 26

• Look at the clearance of free water
  
  • The drug will create and clear less free water
• Salty water is diuresed, but with particles

Example: Drug Y