Blood (formation, RBC indices) explanation in simple way

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Blood: Formation & RBC Indices - Simple Explanation

Part 1 - What Is Blood?

Blood is a fluid connective tissue (yes, it counts as connective tissue!) that flows through the cardiovascular system. An average adult has about 6 litres of it - roughly 7-8% of body weight.

Blood has two main parts:

Plasma (the liquid - ~55%) - mostly water (91-92%), proteins (albumin, globulins, fibrinogen), electrolytes, glucose, hormones
Formed elements (the cells - ~45%) - red blood cells (RBCs), white blood cells (WBCs), and platelets

Part 2 - Blood Formation (Hemopoiesis/Hematopoiesis)

Hemopoiesis = the continuous process of making ALL blood cells. Think of it as a factory that never stops, because blood cells have a limited lifespan and must be replaced constantly.

RBCs live ~120 days | Platelets live ~10 days | WBCs live variable, short periods

Where does it happen?

Blood formation starts in the embryo and shifts location over time:

Phase	Location	When
1. Yolk-sac phase	Yolk sac "blood islands"	Week 3 of gestation
2. Hepatic phase	Liver (+ spleen)	2nd month to birth
3. Bone marrow phase	Red bone marrow	2nd trimester onward - continues for LIFE

After birth, the red bone marrow is the sole factory. In adults it's found in flat bones (skull, sternum, ribs, pelvis) and the ends of long bones.

The Family Tree of Blood Cells

All blood cells come from one common ancestor: the Hemopoietic Stem Cell (HSC).

Hemopoiesis chart showing all blood cell lineages from the hemopoietic stem cell

The HSC splits into two major "families":

1. Common Myeloid Progenitor (CMP) → gives rise to:

RBCs (erythrocytes) via the erythroid line
Platelets (thrombocytes) via megakaryocytes
Neutrophils, eosinophils, basophils (granulocytes)
Monocytes → become macrophages in tissues

2. Common Lymphoid Progenitor (CLP) → gives rise to:

T cells, B cells (→ plasma cells), NK cells

Part 3 - RBC Formation Specifically (Erythropoiesis)

This is the most clinically important lineage. Here is the maturation sequence:

Hemopoietic Stem Cell
       ↓
Erythrocyte Progenitor (ErP)
       ↓
Proerythroblast  ← large cell, basophilic (blue) cytoplasm, big nucleus
       ↓
Basophilic erythroblast  ← starts making hemoglobin
       ↓
Polychromatophilic erythroblast  ← mixed pink/blue; actively dividing
       ↓
Orthochromatophilic erythroblast (Normoblast)
    ← cytoplasm now pink (full of Hgb), nucleus becomes dense & small
    ← NUCLEUS IS EJECTED here (Howell-Jolly bodies = leftover fragments)
       ↓
Reticulocyte (Polychromatophilic erythrocyte)
    ← no nucleus, still has ribosomes making Hgb
    ← stays in marrow ~24 hrs, then blood ~24 hrs
    ← normally 1-2% of circulating RBCs
       ↓
Mature Erythrocyte (RBC)
    ← biconcave disc, no nucleus, no organelles, full of hemoglobin

Key driver: The hormone erythropoietin (EPO) - made by the kidneys when they sense low oxygen. EPO signals the marrow to produce more RBCs.

Part 4 - RBC Indices (Wintrobe Indices)

These were developed by Maxwell Wintrobe in 1932 and are still used in every CBC (Complete Blood Count) today. They tell you the size and hemoglobin content of your RBCs, which is key to classifying anaemia.

You only need three raw values to calculate them: RBC count, Haemoglobin (Hb), and Haematocrit (HCT).

The Four Main Indices

1. MCV - Mean Corpuscular Volume

"How big is an average RBC?"

Formula: MCV (fL) = 10 × HCT (%) ÷ RBC (millions/µL)

Normal range: ~80-96 fL

Result	Name	What it means
Low MCV (<80 fL)	Microcytic	Small RBCs - think iron deficiency, thalassaemia
Normal MCV	Normocytic	Normal-sized - e.g. acute blood loss, renal anaemia
High MCV (>96 fL)	Macrocytic	Large RBCs - think B12 or folate deficiency

2. MCH - Mean Corpuscular Haemoglobin

"How much haemoglobin is in an average RBC?"

Formula: MCH (pg) = Hb (g/dL) × 10 ÷ RBC (millions/µL)

Normal range: ~27-33 pg

Low = Hypochromic (pale RBCs, not enough Hgb)
Normal = Normochromic
High = occurs with macrocytic states

3. MCHC - Mean Corpuscular Haemoglobin Concentration

"What is the CONCENTRATION of haemoglobin inside the RBC?"

Formula: MCHC (g/dL) = Hb (g/dL) × 100 ÷ HCT (%)

Normal range: ~32-36 g/dL

Low = Hypochromic (diluted, pale RBCs)
Normal = Normochromic
High = Seen in hereditary spherocytosis (densely packed cells)

4. RDW - Red Cell Distribution Width

"Are all my RBCs a uniform size, or are they different sizes?"

Normal RDW = uniform sizes (isocytosis)
High RDW = mix of sizes (anisocytosis) - can help distinguish, e.g., iron deficiency anaemia (high RDW) vs. thalassaemia trait (normal-low RDW), even if both are microcytic

Quick Summary Table

Index	Measures	Normal	Low means	High means
MCV	RBC size (fL)	80-96	Microcytic	Macrocytic
MCH	Hgb per RBC (pg)	27-33	Hypochromic	Macrocytic
MCHC	Hgb concentration (g/dL)	32-36	Hypochromic	Spherocytosis
RDW	Size uniformity	11.5-14.5%	Uniform	Anisocytosis

Putting It Together: Anaemia Classification

Type	MCV	MCH	Example Cause
Microcytic hypochromic	Low	Low	Iron deficiency, thalassaemia
Normocytic normochromic	Normal	Normal	Acute blood loss, renal failure
Macrocytic	High	High	B12/folate deficiency, liver disease

Sources: Histology: A Text and Atlas with Correlated Cell and Molecular Biology (Pawlina) | Tietz Textbook of Laboratory Medicine, 7th ed.

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