Body Fluid Compartments Flashcards
What are the three body fluid compartments?
plasma
interstitial fluid
intracellular fluid
In a person with 42 liters of water, how much is in the intracellular fluid? plasma?interstitial fluid?
22 L intracellular
17 L interstitial fluid
3 L plasma
What is the concentration of the solvent (water) in the body?
about 55,500 mM
What solute has the highest plasma concentration in the body?
Na+ at 140 mM
Second highest concentration solute?
Cl- at 103 mM
Third highest?
Bicarb at 27 mM
4th highest? It’s a tie….
Urea and weak organic acids at 6 mM
5th highest?
Glucose at 5 mM or 90 mg/dl
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?
with albumin - it’s an anion with 17 net negative charges
What is the osmolarity of plasma then?
about 300 mM
What are some things albumin is responsible for?
- carries substances like Ca2+, fatty acies, H+ and drugs
- opposes capillary leak because it’s BIG
- responsible for the anion gap
How do you calculate the anion gap?
[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
What is the main difference between the composition of the interstitial fluid and plasma? WHY?
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
How does albumin effectively keep the capillaries from being leaky?
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.
What is KATZ’S formula for net fluid movement across a membrane?
K (Pcap+ Nisf) - (Pisf + Ncap)
forces favoring filtration - forces favoring reabsorption
What are the concentrations of K+, Na+ and Cl- in the intracellualr fluid?
K+ = 140 mM
Cl - = 20 mM
Na+ = 10 mM
What’s the total solute concentration intracellularly?
about 300 mM again - same as the plasma!
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?
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
For medicine, a solution is isosmotic if it has an osmolality of what?
300 mOsms
What does isotonic means? What is required for a solution to be isotonic?
it means a solution will NOT change cell volume.
it means the solution must be 300 mOsms of IMPERMEABLE solutes
What does isoncotic mean? What is required for a solution to be isoncotic?
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)
If you drink water, to what fluid compartments does it distribute?
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)
To what fluid comaprtments does normal saline IV distribute? Why?
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.
Thus, what kind of solution is normal saline?
isotonic - doesn’t change cell volume
isosomotic - about 300 mOsms of solute (.9% NaCl)
WHat is the clinical correlate to a loss of normal saline?
secretory diarrhea
How does 1/2 normal saline distribute?
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
When do you give 1/2 normal saline?
when someone has become dehydrated due to sweat loss
How does isoncotic saline distribute to the body fluid compartments and why?
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
So when should you give isoncotic saline?
after blood loss - it will get the BP back up the fastest
What’s in 5% dextrose (D5W)?
5 grams of glucose per 100 ml of water
What is the ultimately distribution of D5W?
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
So if D5W distributes like water, why don’t we just give water?
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
In the case of hypovolemia, most solutions will distribute proportionally more within the plasma compartment - why?
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
