Body Fluid Spaces

Question 1

What is effective vascular volume?

3 factors that contribute to it?

Answer 1

The ability to load the arteries appropriately - “a gestalt.”
CO, SVR, and plasma volume contribute to it.

Question 2

What’s a significant determinant of plasma volume?

Answer 2

Extracellular fluid (ECF) volume.

Question 3

Approximately what percent of total body water (TBW) is ECF? How about intracellular fluid (ICF)?

Answer 3

1/3 ECF

2/3 ICF

Question 4

What ion predominates in the ECF?

Answer 4

Na+

Question 5

What ion predominates in the ICF?

Answer 5

K+

Question 6

What fraction of the ECF is plasma volume?

Answer 6

About 1/4.

Remaining 3/4 is interstitial fluid.

Question 7

Does all of your blood volume count as ECF?

Answer 7

Nope, about 40% (the hematocrit) is intracellular.

Question 8

What’s the difference between osmolality and tonicity?

Answer 8

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)

Question 9

Formula for calculated tonicity?

Answer 9

2 x [Na+] + Glu/18

this is like calculated osmolality… but doesn’t include urea, because urea crosses membranes

Question 10

2 bad things that can happen if the ECF if hypotonic?

Answer 10

RBC lysis (but this really would only happen if you gave somebody IV water).
Brain swelling - important - kills people who are hyponatremic.

Question 11

What’s the main way effective vascular volume (via ECF) is regulated?

Answer 11

By changing total Na+ content.

Question 12

What’s the main way to regulate tonicity?

Answer 12

By changing water levels.

Question 13

Why is water intake a “lousy” way to increase ECF? When does the body use this method?

Answer 13

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.

Question 14

If the body has to choose been effective vascular volume and tonicity, which will it choose?

Answer 14

It will always choose effective vascular volume.

Question 15

Does plasma [Na+] determine ECF volume?

Answer 15

No! It’s the total body Na+ content that counts.

Question 16

How do you evaluate effective vascular volume?

Answer 16

Through physical exam - JVP, listening for pulmonary edema, etc.

Question 17

What do volume overload and volume depletion refer to?

Answer 17

Volume overload: too much total body Na+
Volume depletion: too little total body Na+
(the “volume” part more refers to ECF volume)

Question 18

What’s the difference between dehydration and volume depletion?

Answer 18

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+.

Question 19

What are the terms used for perturbations in tonicity?

Answer 19

Hyponatremia (should be called “hyperaquemia”)

Hypernatremia (eg. “dehydration”, should be “hypoaquemia”)

Question 20

If somebody’s hypovolemic, what do you want to give them: Na+ or water? In what form is it given?

Answer 20

You give them Na+ in the form of isotonic saline (NaCl),

Question 21

If somebody’s hypernatremic, what do you give them: Na+ or water? In what form do you give it?

Answer 21

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.

Question 22

How is serum [Na+] changed by giving water, saline, and dry NaCl?
How will each of these affect ECF?

Answer 22

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)

Question 23

How does giving K+ affect serum [Na+]?

Answer 23

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)