Standard Unit of Blood Volume

A standard unit of blood contains approximately 450 mL of blood. 1

Blood Components and Volumes

Licensed blood packs for collection typically contain approximately 450 mL of blood (±10%) mixed with anticoagulant 1
Smaller packs containing approximately 250 mL (±10%) are also available for specific purposes 1
The volume is carefully measured using a calibrated balance during collection to ensure proper anticoagulant-to-blood ratios 1

A single unit of packed red blood cells (approximately 300 mL) typically raises hemoglobin by about 1.9% ± 1.2% 2
The hemoglobin response to transfusion is greater at lower starting hemoglobin levels 3
In pediatric patients, 10 mL/kg of red blood cells should increase hemoglobin by approximately 20 g/L 1

In hemodialysis patients, annual blood loss from various sources can range from 2.68 L (with native fistula) to 5.32 L (with double-lumen catheter) 1
Blood losses in hemodialysis include:
- Dialytic technique: 165 mL/year 1
- Occult gut bleeding: 2,257 mL/year 1
- Regular blood sampling: 428 mL/year 1
- Catheter care: 2,680 mL/year 1

Blood in children should be prescribed by volume rather than units 1
Pediatric blood volume is estimated at 70 mL/kg but may be as high as 100 mL/kg in newborns 1
Recommended transfusion volumes for children:
- Red blood cells: 10 mL/kg 1
- Platelets: 10-20 mL/kg 1
- Fresh frozen plasma: 10-15 mL/kg 1
- Cryoprecipitate: 5-10 mL/kg 1

During normal pregnancy, plasma volume increases by approximately 1,250 mL (nearly 50% above non-pregnant volume) 4
Red cell mass increases by about 250-450 mL (18-25% above non-pregnant volume), depending on iron supplementation 4
These physiological changes result in a dilutional drop in hematocrit from about 40% to 33% during the last trimester 4

Blood is collected into packs containing CPD-A1 anticoagulant 1
During collection, the pack should be gently agitated to mix blood with anticoagulant 1
Proper labeling must include patient identifiers, date/time of collection, and responsible medical officer 1
The donor tubing should be properly clamped before needle removal to prevent air entry 1

When determining transfusion needs, consider that one "unit" of packed red blood cells is typically 300 mL, not 500 mL as sometimes assumed 2
The variability in hemoglobin response to transfusion is substantial, as indicated by the standard deviation of ±1.2% per unit 2
Whole blood contains all elements necessary for oxygen delivery and hemostasis in nearly physiologic ratios 5
Modern blood component therapy allows separation of one whole blood unit into multiple components (PRBCs, platelets, plasma, cryoprecipitate) to maximize utility 6