What Is Whole Blood Made Of?
When you draw blood from a patient, you are collecting whole blood — a mixture of cells, proteins, and fluid. Understanding what that mixture contains is not just anatomy trivia. It directly affects how you collect specimens, which tubes you choose, and why certain tests require one sample type over another.
Whole blood separates into two broad fractions:
- Plasma (approximately 55%): The liquid portion carrying cells, proteins, nutrients, waste, and clotting factors.
- Formed elements (approximately 45%): The cellular components — red blood cells, white blood cells, and platelets.
You will see these proportions referenced on the NHA CPT exam in the context of hematocrit, which measures the percentage of whole blood occupied by red blood cells.
Exam domain alert: Blood composition appears in the Anatomy & Physiology domain of the NHA CPT, ASCP PBT, AMT RPT, and NCCT NCPT exams. Expect 2–4 questions directly tied to this material.
Plasma: The Liquid Fraction
Plasma makes up about 55% of whole blood volume. It is a pale yellow fluid that is roughly 90% water. The remaining 10% is packed with substances the body needs to function:
| Component | Examples | Function |
|---|---|---|
| Plasma proteins | Albumin, globulins, fibrinogen | Transport, immunity, clotting |
| Electrolytes | Sodium, potassium, chloride, calcium | Fluid balance, nerve and muscle function |
| Nutrients | Glucose, amino acids, lipids | Energy and building materials for cells |
| Waste products | Urea, creatinine, bilirubin | Transported to kidneys and liver for elimination |
| Hormones & enzymes | Insulin, cortisol, ALT, AST | Regulation and metabolic signaling |
| Gases | Carbon dioxide (dissolved) | Respiratory transport |
Key Plasma Proteins to Know
- Albumin: The most abundant plasma protein. Maintains osmotic pressure and transports drugs, fatty acids, and hormones.
- Globulins: Include antibodies (immunoglobulins) that defend against infection, plus transport proteins like transferrin.
- Fibrinogen: A clotting factor. This is what separates plasma from serum — fibrinogen stays in plasma but is consumed when blood clots.
Plasma vs. Serum: A Critical Distinction
This is one of the highest-yield topics on the phlebotomy exam, and it comes up directly in tube selection. Students mix these terms up constantly. Here is the cleanest way to keep them straight:
- Plasma = whole blood minus cells. It still contains fibrinogen and the other clotting factors because you prevented clotting by adding an anticoagulant.
- Serum = plasma minus clotting factors. Blood was allowed to clot. During clotting, fibrinogen converts to fibrin and is consumed. Once you spin the clotted sample, the liquid left above the clot is serum.
| Property | Plasma | Serum |
|---|---|---|
| Contains clotting factors? | Yes (including fibrinogen) | No (consumed during clotting) |
| How obtained | Anticoagulated tube, centrifuge | Clot-activator or plain tube, allow to clot, centrifuge |
| Tube color (common) | Green (heparin), Lavender (EDTA) | Red (no additive), Gold/Tiger-top (SST) |
| Processing time | Centrifuge immediately after collection | Allow 30 minutes to clot before centrifuge |
| Used for | Coagulation studies, stat chemistry, blood bank | Most routine chemistry and serology tests |
How to remember it: Serum is what is left after the clot takes the clotting factors away. If you add an anticoagulant and stop the clot from forming, nothing is consumed — that is plasma.
Formed Elements: The Cellular 45%
The solid 45% of whole blood is divided into three cell types. On the exam, you need to know the scientific name, primary function, and how each one connects to phlebotomy.
Red Blood Cells (Erythrocytes)
RBCs are by far the most numerous cells in blood — about 4.5 to 5.5 million per microliter. They are biconcave discs with no nucleus, packed with hemoglobin, which carries oxygen from the lungs to tissues and returns carbon dioxide to the lungs.
- Hematocrit: The percentage of whole blood volume made up by RBCs. Normal range is approximately 37–47% in adults (slightly higher in males). Hematocrit is measured from EDTA tubes (lavender top) on a complete blood count (CBC).
- Hemolysis risk: RBCs are fragile. Rough handling, drawing through a narrow gauge needle under high vacuum, or prolonged tourniquet time can rupture them. Hemolysis releases intracellular contents into the sample and will invalidate tests like potassium, LDH, and AST.
White Blood Cells (Leukocytes)
WBCs are the immune system's workforce. There are far fewer of them than RBCs — about 4,500 to 11,000 per microliter in a healthy adult. There are five types, which you need to know by name and function:
| WBC Type | Approximate % of WBCs | Primary Role |
|---|---|---|
| Neutrophils | 50–70% | First responders to bacterial infection; phagocytize bacteria |
| Lymphocytes | 20–30% | Adaptive immunity; B cells make antibodies, T cells destroy infected cells |
| Monocytes | 3–8% | Phagocytosis of debris and pathogens; mature into macrophages in tissues |
| Eosinophils | 1–4% | Respond to parasitic infections and allergic reactions |
| Basophils | <1% | Release histamine; involved in allergic and inflammatory responses |
A memory trick many students use: Never Let Monkeys Eat Bananas — Neutrophils, Lymphocytes, Monocytes, Eosinophils, Basophils — in order from most to least common.
Platelets (Thrombocytes)
Platelets are tiny cell fragments, not full cells. They have no nucleus. When a blood vessel is damaged, platelets rush to the site, clump together, and form a temporary platelet plug. They also release chemicals that activate the clotting cascade. Normal platelet count is roughly 150,000 to 400,000 per microliter.
For phlebotomy: a patient with a very low platelet count (thrombocytopenia) will bruise easily and bleed longer after a stick. Apply pressure to the site for extra time and watch for hematoma formation.
The Buffy Coat
When you centrifuge an anticoagulated tube of whole blood, the components separate into distinct layers based on density:
- Top layer — Plasma: Pale yellow liquid, lightest fraction, sits on top (approximately 55% of volume).
- Middle layer — Buffy coat: A thin, whitish-gray band. This layer contains all the WBCs and platelets. It is called the buffy coat because of its off-white color.
- Bottom layer — Packed RBCs: Dark red, densest fraction, settles at the bottom (approximately 45% of volume).
Buffy coat on the exam: Questions will describe centrifuged blood and ask you to identify which layer is which, or they will ask what the buffy coat specifically contains. The answer is always WBCs and platelets — not RBCs, not plasma.
Clinical Significance for Phlebotomy Practice
Understanding blood composition is not just memorization. It guides real decisions you make during collection:
Tube Selection: Plasma vs. Serum
The ordering physician specifies which analyte to test, and that analyte determines whether the lab needs serum or plasma. As a phlebotomist, you need to match the tube to the requirement:
- Serum required: Collect in a red-top (no additive) or gold/tiger-top SST tube. Allow 30 minutes for the clot to fully retract before centrifuging. If you spin too early, you get an incomplete clot that may re-aggregate and clot the sample.
- Plasma required: Collect in the appropriate anticoagulant tube (green for heparin, lavender for EDTA, blue for citrate). Mix immediately by gentle inversion 8–10 times. Centrifuge promptly.
- Coagulation tests (PT, aPTT): These require plasma from a blue-top sodium citrate tube. The tube must be filled to exactly the correct volume — the citrate-to-blood ratio is critical. An underfilled tube will give falsely prolonged results.
Why the Distinction Matters for Test Results
Sending the wrong sample type can invalidate a result entirely. For example, some analytes have different reference ranges in serum vs. plasma. Fibrinogen is measurable in plasma but not in serum (it's gone). Drug levels and certain hormone assays specify one type. If you collect a serum sample for a test that requires plasma (or vice versa), the lab will reject the specimen and you will need to re-draw the patient.
Being solid on this distinction prevents delays in patient care and repeat venipunctures — which nobody wants.
Practice Questions
-
After centrifuging a green-top heparin tube, the phlebotomist collects the liquid above the cell layer. This liquid is:
A) Serum
B) Plasma
C) The buffy coat
D) Interstitial fluid
Answer: B — Green-top tubes contain heparin, an anticoagulant. Because clotting was prevented, the liquid fraction retains fibrinogen and clotting factors. That makes it plasma, not serum. -
Which layer of centrifuged blood contains white blood cells and platelets?
A) The top plasma layer
B) The packed RBC layer at the bottom
C) The buffy coat
D) The gel separator layer
Answer: C — The buffy coat is the thin whitish band between plasma and packed RBCs. It contains all the WBCs and platelets. -
A lab requisition calls for a serum potassium level. Which tube is most appropriate?
A) Lavender-top EDTA tube
B) Blue-top sodium citrate tube
C) Red-top or gold SST tube
D) Gray-top fluoride/oxalate tube
Answer: C — Serum is produced by allowing blood to clot and then centrifuging. Red-top (no additive) and gold SST tubes both yield serum. Lavender and blue tubes are anticoagulated and yield plasma. Gray tubes yield plasma and are used specifically for glucose and alcohol. -
A patient's blood is centrifuged and you observe that the bottom red layer takes up about 42% of the total tube volume. This measurement is called the:
A) Mean corpuscular volume
B) Hematocrit
C) Hemoglobin concentration
D) Erythrocyte sedimentation rate
Answer: B — Hematocrit is the percentage of whole blood volume occupied by packed red blood cells. A value of 42% is within the normal range for adult females (37–47%).