Applied Physiology · Body fluids & blood

Body Fluids

Water, as % of body weight (adult)

Intracellular (ICF)40%
Interstitial15%
Plasma5%

Total body water

≈ 60% of body weight
ICF ⅔ · ECF ⅓ (interstitial + plasma)

Blood

Plasma55%
Red cells44%
White cells & platelets1%

IV fluids by tonicity

Why fluids move

Water crosses cell membranes by osmosis: from the side with fewer dissolved particles to the side with more, until both are equal. So an IV fluid's tonicity, its concentration compared with plasma, decides whether cells stay the same, swell or shrink.

Isotonic

0.9% NaCl (normal saline) High yield
Ringer's lactate
stays in the ECF: replaces volume

Hypotonic

0.45% NaCl
water moves into cells: they swell

Hypertonic

3% NaCl, D10W
water moves out of cells: they shrink