Body Fluid Compartments

Question 1

What are the three body fluid compartments?

Answer 1

plasma
interstitial fluid
intracellular fluid

Question 2

In a person with 42 liters of water, how much is in the intracellular fluid? plasma?interstitial fluid?

Answer 2

22 L intracellular
17 L interstitial fluid
3 L plasma

Question 3

What is the concentration of the solvent (water) in the body?

Answer 3

about 55,500 mM

Question 4

What solute has the highest plasma concentration in the body?

Answer 4

Na+ at 140 mM

Question 5

Second highest concentration solute?

Answer 5

Cl- at 103 mM

Question 6

Third highest?

Answer 6

Bicarb at 27 mM

Question 7

4th highest? It’s a tie….

Answer 7

Urea and weak organic acids at 6 mM

Question 8

5th highest?

Answer 8

Glucose at 5 mM or 90 mg/dl

Question 9

If you add up the charges of the solutes, you get a total of 154 positive charges and 137 negative charges. But the cell has to be in balance? How does it accomplish this?

Answer 9

with albumin - it’s an anion with 17 net negative charges

Question 10

What is the osmolarity of plasma then?

Answer 10

about 300 mM

Question 11

What are some things albumin is responsible for?

Answer 11

1. carries substances like Ca2+, fatty acies, H+ and drugs
2. opposes capillary leak because it’s BIG
3. responsible for the anion gap

Question 12

How do you calculate the anion gap?

Answer 12

[Na+] - ([Cl-] + [HCO3-])

basically you approximate is by subtracting the two biggest anions from the major cation - it’s the charge differential made up by albumin

Question 13

What is the main difference between the composition of the interstitial fluid and plasma? WHY?

Answer 13

interstitial fluid is just plasma without the albumin

the interstitial fluid and plasma are just separated by capillary walls, which are leaky. But albumin is too big to get through

Question 14

How does albumin effectively keep the capillaries from being leaky?

Answer 14

albumin can’t leave the plasma, so it maintains the oncotic pressure within the capillary. This oncotic pressure opposes the hydrostatic pressure, hence, capillaries don’t leak as expected.

Question 15

What is KATZ’S formula for net fluid movement across a membrane?

Answer 15

K (Pcap+ Nisf) - (Pisf + Ncap)

forces favoring filtration - forces favoring reabsorption

Question 16

What are the concentrations of K+, Na+ and Cl- in the intracellualr fluid?

Answer 16

K+ = 140 mM
Cl - = 20 mM
Na+ = 10 mM

Question 17

What’s the total solute concentration intracellularly?

Answer 17

about 300 mM again - same as the plasma!

Question 18

Based on the different solute concentration in and outside the cell, Na+ and Cl- will both want to flow IN. But this would make water follow and the cell would swell and lyse! What stops it?

Answer 18

the Na/K ATPase pump, which pumps the Na back out in exchange for K going in.

This means less Cl- will follow the positive charge and you won’t have the huge influx of water

Question 19

For medicine, a solution is isosmotic if it has an osmolality of what?

Answer 19

300 mOsms

Question 20

What does isotonic means? What is required for a solution to be isotonic?

Answer 20

it means a solution will NOT change cell volume.

it means the solution must be 300 mOsms of IMPERMEABLE solutes

Question 21

What does isoncotic mean? What is required for a solution to be isoncotic?

Answer 21

It’s a solution that has 1mM of plasma albumin (about 4.5 g albumin per 100 mL), or any solution with a protein osmotic pressure (oncotic pressure) of about 25 mmHg (because the oncotic pressure of plasma is about 25 mmHg)

Question 22

If you drink water, to what fluid compartments does it distribute?

Answer 22

to all three compartments - the cell, ISF and plasma in accordance to their respective volumes (22/42L goes to the cell, 17/42 L goes to ISF, 3/42L goes to plasma)

Question 23

To what fluid comaprtments does normal saline IV distribute? Why?

Answer 23

To plasma and ISF (but not to the cell!) (so 17/20 to the ISF and 3/20 to the plasma)

Because the cell membrane has Na/K pumps to maintain cell volume, the Na+ from the IV saline gets pumped back into the ISF and the water follows.

Question 24

Thus, what kind of solution is normal saline?

Answer 24

isotonic - doesn’t change cell volume

isosomotic - about 300 mOsms of solute (.9% NaCl)

Question 25

WHat is the clinical correlate to a loss of normal saline?

Answer 25

secretory diarrhea

Question 26

How does 1/2 normal saline distribute?

Answer 26

half of it distributes like water and half of it distributes like normal saline

so if you calculate it out…
.26L to the cell, .63L to the ISF ,and .11L to the plasma

Question 27

When do you give 1/2 normal saline?

Answer 27

when someone has become dehydrated due to sweat loss

Question 28

How does isoncotic saline distribute to the body fluid compartments and why?

Answer 28

Only to the plasma compartment - because it has the same oncotic pressure as the plasma, it doesn’t change the oncotic pressure of the plasma when it’s transfused. So even though the hydrostatic pressure of the capillary increases, whatever fluid is filtered out of the capillaries just gets pulled right back in my the albumin

Question 29

So when should you give isoncotic saline?

Answer 29

after blood loss - it will get the BP back up the fastest

Question 30

What’s in 5% dextrose (D5W)?

Answer 30

5 grams of glucose per 100 ml of water

Question 31

What is the ultimately distribution of D5W?

Answer 31

is ultimately distributes in all three compartments in proportion to the volume of the compartments just like water - this is because the glucose is completely removed by the cell via oxidation, glycogen storage or fat synthesis

Question 32

So if D5W distributes like water, why don’t we just give water?

Answer 32

Water can’t be given IV because it would just make the blood cells lyse.

Destrose is isosmotic, so it won’t do that and you can give it IV to people who are unconscious

Question 33

In the case of hypovolemia, most solutions will distribute proportionally more within the plasma compartment - why?

Answer 33

when you have low volume, the hydrostatic pressure in the capillary is lower than normal to begin with

when you infuse something, the Pc doesn’t go that much higher than normal, so there isn’t tons of filtration - more of it stays in the plasma compartment than you’d expect based on compartment volumes alone