What is blood made of?
Blood splits into two parts: a liquid — the plasma (≈ 55%) — and cells (≈ 45%): red blood cells, white blood cells and platelets. Here is a clear picture of its makeup, what each part does, where blood is made, and the tests that measure it.
Plasma
A pale-yellow liquid (~90% water) that carries cells, nutrients, hormones, wastes and proteins such as albumin.
Measured with: Total protein, albumin, metabolic panel…
Red blood cells
Erythrocytes carry oxygen to your organs using hemoglobin. They are by far the most numerous.
Measured with: CBC, hemoglobin, hematocrit, MCV
White blood cells
Leukocytes defend the body against infection and drive the immune response.
Measured with: CBC (WBC & differential), CRP
Platelets
Thrombocytes drive clotting and stop bleeding (they form the clot).
Measured with: CBC (platelets), coagulation panel
Two main parts: plasma and cells
If you let a tube of blood settle (or spin it in a centrifuge), it separates into two layers. On top is the plasma, a pale-yellow liquid that is about 90% water and carries proteins (including albumin), electrolytes, hormones, nutrients and wastes. At the bottom are the formed elements — the cells — whose share (the hematocrit) runs about 40–45% in a healthy adult.8
Between the two sits a thin whitish film, the buffy coat: these are the white blood cells and platelets, which together make up less than 1% of the volume but do outsized work in defense and clotting.4
The four components in detail
Plasma is the transport fluid of the blood. About 90% water, it carries hundreds of proteins — the most abundant being albumin. Albumin maintains the oncotic pressure that keeps fluid inside your vessels and ferries many substances (hormones, drugs, fatty acids); its level is also a marker of overall status.9 Plasma also carries the clotting factors and most of the chemistries measured in the lab.7
Red blood cells are the most numerous cells (about 4–6 million per µL). Their biconcave shape and flexible membrane let them squeeze through the smallest capillaries, and they are packed with hemoglobin, the iron-containing protein that binds oxygen. When their structure is disrupted, certain anemias follow.4
Far fewer in number, white blood cells are the body’s defenders. There are five main types — neutrophils, lymphocytes, monocytes, eosinophils and basophils — reported on the CBC differential. Some drive innate immunity (fast, non-specific), others adaptive immunity (targeted, with memory), often working with the antibodies carried in plasma.6
Platelets are the smallest elements in blood (about 150,000–400,000 per µL) — cell fragments, not whole cells. At the first break in a vessel wall, they stick, activate and clump to form an initial plug, then trigger the clotting cascade that firms up the clot. Beyond stopping bleeding, they also play a role in inflammation and immune defense.7
Reference figures (adult)
| Component | Typical US reference range |
|---|---|
| Red blood cells | 4.2 – 5.7 M/µL (men) · 3.8 – 5.1 (women) |
| Hemoglobin | 13 – 17 g/dL (men) · 12 – 15 (women) |
| Hematocrit | 40 – 52% (men) · 36 – 48% (women) |
| White blood cells | 4,500 – 11,000 /µL |
| Platelets | 150,000 – 400,000 /µL |
Approximate adult values; the limits depend on the laboratory, age and sex. Always read your result against the range printed on your own report.5
How are they measured?
The single most common test is the complete blood count (CBC), which counts the red cells, white cells and platelets and breaks down the white-cell differential.1 The plasma, in turn, is the medium for most chemistry tests (glucose, cholesterol, electrolytes…). To understand each result, browse our marker guides and the reference-range table.
What does blood do?
Blood is a true transport and defense network. Its main jobs:
- Carry oxygen from the lungs to your organs (and bring CO₂ back) using the hemoglobin in red cells.4
- Deliver nutrients (sugars, fats, vitamins) and hormones to every tissue, and carry wastes to the kidneys and liver.
- Defend the body against infection, through white blood cells and plasma antibodies.6
- Stop bleeding (clotting), thanks to platelets and clotting factors.7
- Regulate body temperature and acid–base (pH) balance.
Where is blood made?
Blood cells are born in the bone marrow (inside your bones), from hematopoietic stem cells — a process called hematopoiesis. A single stem cell can give rise to every lineage (red, white, platelet-forming) through a tightly regulated cascade of differentiation, guided by specialized bone-marrow “niches.”9 These cells have a limited lifespan and are constantly renewed: a red blood cell lives about 120 days, while some white blood cells last only days. When a lineage runs high or low, it is usually the CBC that reveals it.
Blood type: ABO and Rh
Not all blood is the same: the surface of your red blood cells carries antigens that define your blood type. The two most important systems are ABO (types A, B, AB, O) and Rh (Rh-positive or Rh-negative). They determine compatibility for a transfusion — with O-negative as the universal red-cell donor — and they matter in pregnancy (Rh incompatibility). For the full story, see our blood type guide.
How much blood is in the body?
A typical adult has about 5 liters of blood — roughly 10 US pints (about 1.3–1.5 gallons), or 7–8% of body weight (a little less in women, a little more in men).10 During a blood draw, the amount taken (a few milliliters per tube) is tiny next to that total: it does not “empty” you and is replaced quickly. That is also why blood donation is safe for a healthy donor.
Frequently asked questions
What is blood made of, in short?
Of plasma (≈ 55%, mostly water and proteins) and cells (≈ 45%): red blood cells (the most numerous), white blood cells and platelets.
What percentage of blood is plasma?
About 55% of the volume. The rest is cells — mostly red blood cells, whose share is called the hematocrit (≈ 40–45%).
Where are blood cells made?
In the bone marrow, from hematopoietic stem cells (a process called hematopoiesis).9
What is the difference between plasma and serum?
Plasma is the liquid part of unclotted blood; serum is what remains after clotting (without fibrinogen). See our blood plasma guide.
Which component is most numerous?
Red blood cells, by far: about 4–6 million per µL, versus a few thousand white blood cells.
Disclaimer. This overview is provided for general information and education. Values depend on the laboratory, age and sex; interpreting your own results is your clinician’s job.
Sources
- 1
NHLBI (National Heart, Lung, and Blood Institute) — “Blood Tests.” · nhlbi.nih.gov
- 2
American Red Cross — “What Is Plasma in Blood?” · redcrossblood.org
- 3
American Red Cross — “Whole Blood and Its Components.” · redcrossblood.org
- 4
MedlinePlus (NIH / U.S. National Library of Medicine) — “Blood” and the RBC-count reference range. · medlineplus.gov
- 5
MedlinePlus (NIH/NLM) — “RBC count,” “WBC count,” and “Platelet count” (reference ranges). · medlineplus.gov
- 6
MedlinePlus (NIH/NLM) — “WBC count” (white blood cells & immunity). · medlineplus.gov
- 7
Merck Manual (Consumer Version) — “Components of Blood.” · merckmanuals.com
- 8
Cleveland Clinic — “Blood: What It Is & Function” (plasma ~55%, hematocrit, ~5 L volume). · clevelandclinic.org
- 9
StatPearls (NCBI Bookshelf) — “Physiology, Blood Plasma.” Plus, on the marrow niche: Morrison SJ, Scadden DT. Nature, 2014. · ncbi.nlm.nih.gov · PubMed
- 10
StatPearls (NCBI Bookshelf) — “Physiology, Blood Volume.” On albumin: Fanali G, et al. “Human serum albumin.” Mol Aspects Med, 2012. · ncbi.nlm.nih.gov · PubMed