Body Fluid Spaces Flashcards
What is effective vascular volume?
3 factors that contribute to it?
The ability to load the arteries appropriately - “a gestalt.”
CO, SVR, and plasma volume contribute to it.
What’s a significant determinant of plasma volume?
Extracellular fluid (ECF) volume.
Approximately what percent of total body water (TBW) is ECF? How about intracellular fluid (ICF)?
1/3 ECF
2/3 ICF
What ion predominates in the ECF?
Na+
What ion predominates in the ICF?
K+
What fraction of the ECF is plasma volume?
About 1/4.
Remaining 3/4 is interstitial fluid.
Does all of your blood volume count as ECF?
Nope, about 40% (the hematocrit) is intracellular.
What’s the difference between osmolality and tonicity?
Osmolality is number of particles in a given volume.
Tonicity refers to particles restricted to one side of the membrane, and that thus drive the movement of water.
(osmolality can be measured, tonicity can’t be measured directly)
Formula for calculated tonicity?
2 x [Na+] + Glu/18
this is like calculated osmolality… but doesn’t include urea, because urea crosses membranes
2 bad things that can happen if the ECF if hypotonic?
RBC lysis (but this really would only happen if you gave somebody IV water). Brain swelling - important - kills people who are hyponatremic.
What’s the main way effective vascular volume (via ECF) is regulated?
By changing total Na+ content.
What’s the main way to regulate tonicity?
By changing water levels.
Why is water intake a “lousy” way to increase ECF? When does the body use this method?
2/3 of it will go into cells, and only 1/3 will contribute to ECF volume.
When there’s severe effective volume depletion (e.g. hemorrhage), body will increase water intake if it has to.
If the body has to choose been effective vascular volume and tonicity, which will it choose?
It will always choose effective vascular volume.
Does plasma [Na+] determine ECF volume?
No! It’s the total body Na+ content that counts.
How do you evaluate effective vascular volume?
Through physical exam - JVP, listening for pulmonary edema, etc.
What do volume overload and volume depletion refer to?
Volume overload: too much total body Na+
Volume depletion: too little total body Na+
(the “volume” part more refers to ECF volume)
What’s the difference between dehydration and volume depletion?
Dehydration is increased tonicity - too little water for the amount of solute in the body.
Volume depletion had unaltered tonicity, but too little total little Na+.
What are the terms used for perturbations in tonicity?
Hyponatremia (should be called “hyperaquemia”)
Hypernatremia (eg. “dehydration”, should be “hypoaquemia”)
If somebody’s hypovolemic, what do you want to give them: Na+ or water? In what form is it given?
You give them Na+ in the form of isotonic saline (NaCl),
If somebody’s hypernatremic, what do you give them: Na+ or water? In what form do you give it?
You give them water in the form of 5mg/dL dextrose (D5W). The dextrose (i.e. glucose) makes the solution iso-isosmolar initially, but then is metabolized and doesn't contribute to tonicity.
How is serum [Na+] changed by giving water, saline, and dry NaCl?
How will each of these affect ECF?
Water will decrease [Na+]. Saline won't change [Na+]. Dry NaCl will increase [Na+]. All of these will increase ECF. (this is why [Na+] =/= ECF)
How does giving K+ affect serum [Na+]?
Giving K+ will increase [Na+] as if one gave the same about of Na+.
This happens either by activating Na+/K+ ATPases or by drawing water into cells.
(this isn’t the focus of this lecture - just an extra note at the end)
