Table 2.

Body Fluid Compartments and Serum Na⁺ with Hypothetical 1 L Fluid Losses.

Body Fluid Compartment	Decrease in Volume of Respective Compartment (mL)
	Acute Blood Loss	Slow Blood Loss	Isotonic Fluid	Pure Water	Half-Isotonic Fluid
Intracellular	400	400	0	550	275
Extracellular	600	600	1000	450	725
Interstitial	0	1250	750	375	562.5
Plasma	600	−650*	250	75	162.5
Blood	1000	250	250	125	187.5
Hct (%)	40	33.7	42.1	40	41
Δ Serum Na+	0	0	0	⇑ 3.6 mEq/L	⇑ 1.8 mEq/L

Assuming TBH₂O = 40L, blood volume = 5L, baseline % Hct = 40 & serum Na⁺ = 140 mEq/L

^*Transcapillary refill expands plasma volume at the expense of interstitial fluid in the setting of slow blood loss.