Overview
Blood is a specialised connective tissue composed of a fluid component — plasma — and a cellular component comprising red cells (erythrocytes), white cells (leucocytes), and platelets. Each has a distinct morphology and a specific function that is critical to life.
All blood cells arise from a single progenitor called the Hematopoietic Stem Cell (HSC). The process of blood cell formation is called hematopoiesis, and the specific process of red cell formation is called erythropoiesis. Understanding these processes is the foundation of all hematology.
Learning Objectives
- Explain the composition of blood and the role of each component
- Describe the sites of hematopoiesis at different stages of life
- Explain what a Hematopoietic Stem Cell is and its properties
- Describe all stages in the development of red cells (erythropoiesis)
- State the normal lifespan and fate of mature red blood cells
- Explain universal precautions in handling blood and blood products
- Describe safe procedures for preventing injury from sharp items
Clinical Story
A 28-year-old woman presents with severe fatigue and breathlessness. Her doctor suspects anemia and orders a complete blood count. As the laboratory technician receiving this sample, you need to understand exactly what you are looking at — the cells in the tube, how they formed, and what their normal appearance should be. Only then can you recognise what is wrong. This lesson gives you that foundation.
Composition of Blood
Plasma is the straw-coloured fluid component of blood. It constitutes approximately 55% of total blood volume. Plasma consists mainly of water (~90%) and contains proteins, nutrients, hormones, electrolytes, and waste products in solution.
Plasma contains clotting factors. When blood is allowed to clot, the clotting factors are consumed and the remaining fluid is called serum. Serum is used for biochemical tests; plasma is used for coagulation studies.
Erythrocytes are biconcave discs — a unique shape that maximises surface area for gas exchange while allowing flexibility to squeeze through tiny capillaries. They have no nucleus and no mitochondria in mature form.
Their major constituent (~90%) is haemoglobin (Hb), which carries oxygen from the lungs to tissues and carbon dioxide from tissues back to the lungs. They also contain enzymes for energy production and maintaining haemoglobin in a functional, reduced state.
Leucocytes are nucleated cells that play a critical role in immune defence. They fight infection, destroy foreign substances, and mount immune responses. They are classified into granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monocytes).
Platelets are small, anucleate cell fragments derived from megakaryocytes. They are essential for haemostasis — the process of stopping bleeding. When a blood vessel is damaged, platelets aggregate at the site and form a platelet plug as the first step in clot formation.
Sites of Hematopoiesis
| Stage | Primary Site | Notes |
|---|---|---|
| Early Fetal (0–2 weeks) | Yolk sac (blood islands) | Primitive hematopoiesis; produces nucleated RBCs with embryonic Hb |
| Fetal (up to 3rd month) | Liver (main site) + Spleen | Definitive hematopoiesis begins in aorta-gonad-mesonephros region |
| Fetal (from 4th month) | Bone marrow (throughout skeleton) | Marrow gradually takes over; hepatic sites diminish |
| Birth | Bone marrow only | Only few foci of hepatic hematopoiesis remain |
| Childhood (until puberty) | All bones | Active hematopoiesis throughout the skeleton |
| Adult | Axial skeleton | Ribs, sternum, pelvis, ends of long bones |
The HSC is a pluripotent cell — the single common progenitor of all blood cells including red cells, granulocytes, monocytes, platelets and lymphocytes. It has three critical properties: self-renewal (makes more HSCs), proliferation (rapid multiplication), and differentiation (into any blood cell lineage). Morphologically it resembles a small lymphocyte. HSCs are used in bone marrow transplantation.
Stages of Erythropoiesis
From stem cell to RBCUnder the influence of Erythropoietin (EPO), erythroid progenitors develop from the HSC through BFU-E and CFU-E to produce recognisable precursors. The term erythroblast refers to all nucleated RBC precursors.
The earliest identifiable erythroid precursor. It is a large cell with a rim of deep blue (basophilic) cytoplasm due to abundant ribosomes. The large nucleus occupies most of the cell with coarse chromatin and prominent nucleoli.
Smaller than the proerythroblast. The nucleus is smaller but the cytoplasm is intensely basophilic (deep blue) due to increased ribosomes actively synthesising haemoglobin. Nucleoli are no longer visible.
The last precursor capable of mitosis. Smaller than the basophilic erythroblast. The cytoplasm appears grey due to increasing haemoglobin (acidophilic) mixing with remaining ribosomes (basophilic). Nuclear chromatin undergoes condensation.
The smallest precursor, only slightly larger than a mature RBC. Haemoglobinisation is complete, so the cytoplasm is eosinophilic (pink). The nucleus shrinks and becomes pyknotic (dark, dense, homogeneous). Cannot undergo mitosis.
Larger than a mature RBC. The nucleus has been extruded. Retains remnants of cytoplasmic ribosomal RNA which appear as a fine reticulum when stained with supravital stains (new methylene blue, brilliant cresyl blue). On Romanowsky stains it appears uniformly blue (polychromatophil). Released into circulation; spends 1–2 days in spleen maturing before becoming a mature RBC.
Anucleate biconcave disc. No nucleus, no mitochondria, no ribosomes. Contains ~90% haemoglobin. Highly deformable — can squeeze through splenic sinusoids. Average lifespan: 100–120 days, after which it is removed by macrophages in the liver and spleen (extravascular haemolysis). A small fraction undergoes intravascular destruction.
Erythropoiesis — Maturation Flow
As the cell matures: Size ↓ · Cytoplasm colour: Blue → Grey → Pink · Nucleus: Large → Shrinks → Pyknotic → Absent. Think of it as the nucleus "dying" while the cell fills with haemoglobin (pink).
RBC Lifespan & Destruction
Universal Precautions in Handling Blood
Human blood components and products are handled as if known to be infectious, with risk of transmitting HIV, HBV and other pathogens. This is the Universal Precaution (UP) approach.
- •Gloves — always when handling blood, blood products or applicable body fluids
- •Masks, goggles, face shields — for procedures involving splashing or aerosol generation
- •Gowns, lab coats, aprons — to prevent body exposure to blood/body fluids
- •Remove contaminated PPE immediately after task completion; remove all PPE before leaving the work area
- ✓Wash hands immediately after any contact with blood
- ✓Wash hands each time gloves are removed
- ✓Minimise splashing and splattering during hand washing
BSCs are required for all procedures that may generate an aerosol — such as vortexing, grinding, blending, or centrifugation of blood samples. Employees with exudative lesions or weeping dermatitis must not handle blood until the condition resolves.
Wash exposed skin with warm water and soap immediately. For mouth: rinse with water or mouthwash. For eyes: flush with warm water or saline.
Report how, when and where the incident occurred and with whose blood/secretions.
Your blood may be tested for HIV with your consent. The source individual's blood will also be tested if available.
Interactive Quiz
5 QuestionsFlashcards
Tap to flipClinical Case Study
Apply Your KnowledgeMaria presents with progressive fatigue for 3 months, pallor, and mild breathlessness on exertion. She has been a vegetarian for 2 years. Her doctor suspects a nutritional deficiency and requests a full blood count and peripheral blood film examination.
The low haemoglobin and reduced reticulocyte count suggest hypoproliferative anaemia — the bone marrow is not producing enough red cells. In a vegetarian with low haemoglobin, iron or B12/folate deficiency should be suspected. The peripheral film will show abnormal red cell morphology that can guide the diagnosis.
- →A low reticulocyte count with anaemia indicates the marrow is failing to respond — a hypoproliferative cause
- →Understanding normal erythropoiesis helps you recognise when and where the process has failed
- →Haemoglobin is synthesised during erythropoiesis — any deficiency in its building blocks impairs red cell production
Quick Revision
10-Minute ReviewKey Takeaways
- Erythroid cells develop from a hematopoietic stem cell in the bone marrow
- The earliest identifiable erythroid precursor is the Proerythroblast
- As cells mature, the cytoplasm changes from deep blue to pink and the nucleus shrinks and disappears
- The Reticulocyte is larger than a mature RBC, lacks a nucleus, and has RNA remnants in the cytoplasm
- The mature RBC is an anucleate biconcave disc whose major constituent is haemoglobin
- Average RBC lifespan is 100–120 days; removed by macrophages extravascularly
- Universal Precautions must be applied to all blood and body fluids without exception
- Needles must never be recapped, bent or broken — dispose immediately into sharps containers