Physiology
Fluids & blood
The numbers you’ll be asked about — and why clinical context always wins.
Estimated blood volume
≈ 70 mL/kg adult male · ≈ 65 mL/kg adult female (higher in infants).
Maximum allowable blood loss
MABL = EBV × (Hctinitial − Hcttarget) / Hctinitial — a teaching estimate. Calculator →
Transfusion
- Restrictive threshold: hemoglobin 7 g/dL for most hospitalized adults; 7.5–8 g/dL in cardiac surgery or with pre-existing cardiovascular disease (AABB 2023). Active bleeding and physiology override numbers.
- 1 unit PRBC raises Hgb ≈ 1 g/dL · 1 apheresis platelet raises platelets ≈ 30 × 10⁹/L · FFP replaces factors · cryoprecipitate replaces fibrinogen.
- Massive transfusion follows LA General’s MTP — a team protocol, not a student task. Calcium, temperature and acidosis matter.
Crystalloids
| Fluid | Notes |
|---|---|
| 0.9% NaCl | Chloride 154 mEq/L — large volumes cause hyperchloremic metabolic acidosis |
| Lactated Ringer’s | Balanced; contains potassium (4) and calcium (3) — don’t run with blood through the same line (calcium + citrate) |
Pimp question 1
Why not run LR with blood in the same line?
Answer
LR contains calcium, which can bind citrate anticoagulant and promote clotting in the line.
Pimp question 2
MABL for a 70 kg man, Hct 42% → 27%?
Answer
EBV 70 × 70 = 4,900 mL; 4,900 × (42 − 27)/42 ≈ 1,750 mL.