Blood Components and Hematopoiesis

Blood looks like a simple red liquid, but it is one of the busiest tissues in the body — a flowing organ that carries oxygen, defends against infection, plugs leaks, ferries hormones and nutrients, and hauls away waste, all at once. Understanding what blood is made of, and how the body manufactures billions of new blood cells every single day, is the foundation of all of hematology. Almost every disease you will meet on the wards — anemia, leukemia, bleeding disorders, infection — is ultimately a story about one of these components going wrong.

This page teaches you the four components of blood (red cells, white cells, platelets, plasma) and the process that keeps them replenished: hematopoiesis, the making of blood from stem cells. Get this map firmly in your head and the rest of hematology becomes far easier to reason about.

Learning Objectives

• Identify the four major components of blood and state the function of each
• Describe the structure and life cycle of red blood cells, white blood cells, and platelets
• Explain hematopoiesis from the hematopoietic stem cell down to mature circulating cells
• Recall the key growth factors (erythropoietin, thrombopoietin, G-CSF) and where they act
• Interpret a basic full blood count and recognise common abnormal patterns
• Understand the historical discovery of blood cells and why it mattered

Quick Answer

Blood is roughly 55% plasma (a straw-coloured fluid of water, proteins, and dissolved substances) and 45% formed elements (cells and cell fragments). The formed elements are red blood cells (erythrocytes) that carry oxygen via haemoglobin, white blood cells (leukocytes) that fight infection, and platelets (thrombocytes) that stop bleeding. All of these arise by hematopoiesis from a single self-renewing hematopoietic stem cell in the red bone marrow, which divides and differentiates down myeloid and lymphoid lineages under the control of growth factors. Because mature blood cells are short-lived, the marrow must produce hundreds of billions of new cells daily to maintain steady numbers.

Where It Came From

For most of history blood was a mystery understood only through what could be seen with the naked eye — it clotted, it separated into layers, it was central to ancient theories of the four "humours." The real story begins with the microscope. In 1658 the Dutch naturalist Jan Swammerdam was the first person to see red blood cells, describing tiny globules in the blood of a frog. A few years later, in the 1670s, Antonie van Leeuwenhoek — a Delft cloth merchant who ground his own extraordinary lenses — described human red cells in detail, estimating their size with startling accuracy and noting their flattened, disc-like shape.

Why did this matter? Because it transformed medicine from theory into observation. Once you could see the cells, you could count them, measure them, and ask what they did. In the 19th century, as staining techniques and better microscopes arrived, Paul Ehrlich (1870s–1880s) developed aniline dyes that let scientists distinguish the different white blood cells for the first time — neutrophils, eosinophils, basophils — laying the foundation for the differential white cell count still used today. The problem being solved was diagnostic: physicians needed a way to tell why a patient was pale, feverish, or bleeding, and the microscope gave them a window into the causes. The discovery that blood cells are continuously made — traced to the bone marrow by pathologists such as Ernst Neumann and Giulio Bizzozero around 1868–1870 — answered the deeper question of where blood comes from and set hematopoiesis on its modern footing.

Plasma: The Fluid Matrix

If you spin a tube of anticoagulated blood in a centrifuge, it separates into layers. The top ~55% is plasma, a pale-yellow fluid that is about 92% water. Dissolved in it are the plasma proteins, the most abundant being albumin (maintains oncotic pressure and transports many substances), globulins (including antibodies), and fibrinogen (the soluble precursor of the clot). Plasma also carries electrolytes, glucose, hormones, lipids, and metabolic waste such as urea.

A useful distinction: plasma still contains clotting factors; serum is what remains after blood has been allowed to clot, so it lacks fibrinogen and the consumed clotting factors. This matters in the lab — some assays require one and not the other.

Between the plasma and red cells sits a thin greyish-white layer called the buffy coat, which contains the white cells and platelets — a detail worth remembering because it is where these cells are harvested for study or transfusion.

Red Blood Cells: Oxygen Carriers

Red cells are the most numerous cells in the body — roughly 25 trillion in an adult, about 5 million per microlitre of blood. Their whole design serves one job: carrying oxygen. A mature human erythrocyte is a biconcave disc about 7–8 micrometres across, and it has no nucleus and no mitochondria (both extruded during development). This is elegant engineering: losing the nucleus makes room for more haemoglobin and lets the cell flex through capillaries narrower than itself; lacking mitochondria means the cell does not consume the oxygen it carries.

Each red cell is packed with haemoglobin, a protein of four globin chains each cradling an iron-containing haem group. Adult haemoglobin (HbA) is two alpha and two beta chains. Each haem binds one oxygen molecule, so one haemoglobin carries up to four. Red cells live about 120 days, after which aged cells are removed by macrophages in the spleen and liver; the iron is recycled and the haem is broken down to bilirubin.

Key numbers (approximate normal adult ranges): haemoglobin 13.5–17.5 g/dL (men), 12.0–15.5 g/dL (women); MCV (mean cell volume) 80–100 fL. A low haemoglobin is anaemia; the MCV then tells you a lot — small cells (microcytic) point toward iron deficiency, large cells (macrocytic) toward B12 or folate deficiency.

White Blood Cells: The Immune Defenders

White cells are far less numerous (4,000–11,000 per microlitre) but critical. They fall into two families:

• Granulocytes (myeloid): Neutrophils (40–75%, the first responders that phagocytose bacteria and form pus), eosinophils (parasites and allergy), and basophils (release histamine in allergic responses).
• Agranulocytes: Lymphocytes (20–45%: B cells make antibodies, T cells coordinate and kill, NK cells target virus-infected and tumour cells) and monocytes (which mature into tissue macrophages and dendritic cells).

The classic teaching mnemonic for relative abundance is "Never Let Monkeys Eat Bananas" (Neutrophils > Lymphocytes > Monocytes > Eosinophils > Basophils).

Unlike red cells, most white cells spend little time in the blood — the bloodstream is a highway to the tissues where they actually do their work. A high neutrophil count (neutrophilia) often signals bacterial infection; a high lymphocyte count can signal viral illness; a very high, abnormal white count can signal leukemia.

Platelets: The Clotting Fragments

Platelets are not whole cells at all but fragments of megakaryocytes — giant marrow cells that shed thousands of platelets from their cytoplasm. They are tiny (2–3 micrometres), have no nucleus, and number about 150,000–400,000 per microlitre. When a vessel is injured, platelets adhere to exposed collagen (via von Willebrand factor), activate, change shape, release granules, and aggregate to form the initial platelet plug — the first phase of haemostasis, later reinforced by the fibrin mesh of the coagulation cascade. They circulate for about 7–10 days.

A low platelet count (thrombocytopenia) causes bleeding and bruising; a very high count (thrombocytosis) can predispose to clotting.

Hematopoiesis: How Blood Cells Are Made

Because mature blood cells are short-lived, the body must replace them constantly — an astonishing ~200–300 billion cells per day. This is hematopoiesis, and in adults it happens in the red bone marrow, mainly of the pelvis, sternum, vertebrae, ribs, and skull. (In the fetus it occurs first in the yolk sac, then the liver and spleen, migrating to the marrow before birth — a fact that explains why the liver and spleen can resume blood production in some diseases, called extramedullary hematopoiesis.)

Everything starts with the hematopoietic stem cell (HSC), a rare cell with two defining powers: self-renewal (making more stem cells so the supply never runs out) and differentiation (giving rise to every blood cell type). The HSC produces two main progenitor lines:

• The common myeloid progenitor, giving rise to red cells, platelets (via megakaryocytes), neutrophils, eosinophils, basophils, and monocytes.
• The common lymphoid progenitor, giving rise to B lymphocytes, T lymphocytes, and NK cells.

Differentiation is steered by growth factors — signalling proteins that tell progenitors which path to take and how fast to divide. Three are essential to know:

• Erythropoietin (EPO) — made by the kidney in response to low oxygen; drives red cell production. This is why kidney failure causes anaemia and why EPO is given therapeutically.
• Thrombopoietin (TPO) — made by the liver; drives megakaryocyte and platelet production.
• Granulocyte colony-stimulating factor (G-CSF) — drives neutrophil production; given clinically to patients whose counts are suppressed by chemotherapy.

Worked example: tracing a red cell

Imagine a patient who moves to high altitude, where oxygen is scarce. Their kidneys sense the low oxygen and release more EPO. In the marrow, EPO acts on erythroid progenitors, accelerating their maturation: proerythroblast → erythroblast (haemoglobin accumulates) → the nucleus is extruded to form a reticulocyte, which enters the blood and matures into an erythrocyte within a day or two. Over a few weeks the haemoglobin rises to carry more oxygen. A blood test would show a raised reticulocyte count — the marrow's "response signal" — which is exactly what clinicians look for to confirm the marrow is working. The same reticulocyte rise appears after treating iron-deficiency anaemia, confirming the treatment is working.

Real-World Applications

• The full blood count (FBC/CBC) is the single most ordered blood test in medicine. Every value on it maps directly to the components above, so understanding this page lets you read one.
• Blood transfusion relies on separating components: packed red cells for anaemia, platelet concentrates for thrombocytopenia, and fresh frozen plasma for clotting factor replacement — one donation helps several patients.
• Bone marrow transplantation is, at heart, transplanting hematopoietic stem cells to rebuild a patient's entire blood and immune system after leukemia or marrow failure.
• Recombinant growth factors (EPO for renal anaemia, G-CSF after chemotherapy, TPO agonists for low platelets) are direct therapeutic applications of understanding hematopoiesis.

Common Mistakes

• "Serum and plasma are the same thing." They are not. Plasma contains fibrinogen and clotting factors; serum is the fluid left after clotting, so it lacks them. Ordering the wrong tube can invalidate a test.
• "Platelets are cells." Platelets are anucleate fragments of megakaryocytes, not complete cells. This explains their short life and why they cannot divide.
• "Red cells have mitochondria like other cells." Mature erythrocytes have expelled both nucleus and mitochondria. They generate energy by anaerobic glycolysis, which conveniently means they do not consume the oxygen they transport.
• "All blood cells are made in the same place throughout life." Site changes with age — yolk sac and liver in the fetus, then red marrow after birth, with fatty (yellow) marrow gradually replacing red marrow in the long bones of adults.

Comparison and Connections

Feature	Red cells	White cells	Platelets
Also called	Erythrocytes	Leukocytes	Thrombocytes
Main job	Carry oxygen	Fight infection	Stop bleeding
Nucleus	No (mature)	Yes	No (fragment)
Approx. count per microlitre	~5 million	4,000–11,000	150,000–400,000
Lifespan	~120 days	Hours to years	7–10 days
Precursor	Erythroblast	Myeloblast / lymphoblast	Megakaryocyte

A common point of confusion is myeloid versus lymphoid lineage. Myeloid gives rise to red cells, platelets, and most white cells except lymphocytes; lymphoid gives rise only to lymphocytes (B, T, NK). This split explains the classification of leukemias into myeloid and lymphoid types.

Practice Questions

Recall

Q: What are the four components of blood, and roughly what fraction of blood is plasma? A: Red cells, white cells, platelets, and plasma. Plasma makes up about 55% of blood volume.

Understanding

Q: Why do mature red blood cells lack a nucleus and mitochondria? A: Removing the nucleus frees space for more haemoglobin and lets the cell deform to squeeze through capillaries. Lacking mitochondria means the cell relies on anaerobic glycolysis and does not consume the oxygen it is carrying — both adaptations maximise oxygen delivery.

Application

Q: A patient with chronic kidney disease is anaemic despite normal iron stores. Why, and what treatment addresses the cause? A: Damaged kidneys produce insufficient erythropoietin, so the marrow lacks the signal to make red cells. Recombinant EPO restores the signal and corrects the anaemia.

Analysis

Q: A blood film shows large numbers of abnormal, immature white cells and the platelet and red cell counts are low. Which process is likely disrupted, and why are the other lineages affected? A: This pattern suggests acute leukemia — malignant clonal proliferation of an immature white cell precursor. The leukemic cells crowd out the marrow, suppressing normal hematopoiesis, so red cell and platelet production fall too (causing anaemia and thrombocytopenia). It illustrates that all lineages share one crowded marrow space.

FAQ

How many blood cells does the body make each day? On the order of 200–300 billion, dominated by red cells and platelets, simply to replace those that die of old age.

If red cells have no DNA, how do they survive 120 days? They are pre-loaded with all the enzymes and haemoglobin they need before losing the nucleus. Without DNA they cannot repair themselves indefinitely, which is precisely why they wear out and are removed at around 120 days.

Where exactly is the bone marrow that makes blood in an adult? Active (red) marrow in adults is concentrated in the pelvis, sternum, vertebrae, ribs, skull, and the proximal ends of the femur and humerus. This is why marrow biopsies are usually taken from the posterior iliac crest of the pelvis.

What is the difference between a stem cell and a progenitor cell? A hematopoietic stem cell can both self-renew indefinitely and become any blood cell. A progenitor is more committed — it can still divide but is already destined for a limited set of cell types and cannot self-renew forever.

Why does chemotherapy cause low blood counts? Chemotherapy targets rapidly dividing cells, and the marrow's hematopoietic cells divide constantly. Damaging them drops white cells (infection risk), platelets (bleeding risk), and red cells (anaemia) — the counts fall a week or two later and recover as the marrow regenerates.

Quick Revision

• Blood = ~55% plasma + ~45% formed elements (red cells, white cells, platelets).
• Red cells: biconcave, no nucleus/mitochondria, carry O2 via haemoglobin, live ~120 days.
• White cells: neutrophils, eosinophils, basophils, lymphocytes, monocytes; fight infection.
• Platelets: megakaryocyte fragments, form the platelet plug, live 7–10 days.
• Plasma has clotting factors; serum does not.
• Hematopoiesis: all cells arise from the hematopoietic stem cell (self-renew + differentiate) via myeloid and lymphoid lines.
• Key growth factors: EPO (kidney → red cells), TPO (liver → platelets), G-CSF (→ neutrophils).
• Adult site = red bone marrow (pelvis, sternum, vertebrae); fetal = yolk sac → liver/spleen.
• History: Swammerdam and van Leeuwenhoek saw red cells (1600s); Ehrlich's dyes distinguished white cells (1800s).

Related Topics

Prerequisites

• Hematology overview
• Cell biology and the structure of proteins (see ../../3._Biochemistry/index.md)

Related Topics

• Physiology of oxygen transport (see ../../2._Physiology/index.md)
• Immune cell function (see ../../34._Immunology/index.md)

Next Topics

• Anemia: classification and approach
• Hemostasis and the coagulation cascade
• Leukemias and lymphomas