Introduction to Hematology: Blood, Hematopoiesis and CBC Interpretation

Introduction to Hematology: Blood, Hematopoiesis, Cells, Laboratory Tests and Emergency Interpretation

Study level: Clinical medicine, emergency medicine and pathology | Topic: Hematology

Learning objectives

• Describe the composition and functions of whole blood and plasma.
• Explain adult and fetal hematopoiesis and the roles of marrow, liver, spleen, thymus and lymph nodes.
• Identify the morphology and functions of erythrocytes, leukocytes and platelets.
• Interpret the main components of a CBC, differential count, red-cell indices, reticulocytes and blood film.
• Recognise patterns that suggest anaemia, infection, marrow failure, haemolysis, leukaemia or thrombocytopenia.
• Collect blood correctly and identify urgent hematologic emergencies.

1. What is blood?

Blood is a specialised connective tissue that circulates through arteries, veins and capillaries. It transports oxygen, carbon dioxide, nutrients, hormones, heat, drugs and metabolic waste; provides immune surveillance; maintains acid–base, fluid and electrolyte balance; and prevents blood loss through haemostasis.

Component	Approximate proportion	Major functions
Plasma	About 55% of blood volume.	Water, electrolytes, albumin, clotting proteins, immunoglobulins, nutrients, hormones and waste transport.
Red blood cells (erythrocytes)	Most of the cellular volume; haematocrit varies with sex, age, pregnancy and altitude.	Oxygen delivery through haemoglobin; carbon-dioxide transport and buffering.
White blood cells (leukocytes)	Small fraction of blood volume.	Innate and adaptive immunity, inflammation and tissue repair.
Platelets	Cell fragments derived from megakaryocytes.	Primary haemostasis, clot formation, vascular repair and inflammatory signalling.

2. Plasma and its proteins

• Water: solvent and major determinant of circulating volume and viscosity.
• Albumin: maintains oncotic pressure and transports bilirubin, fatty acids, calcium and many drugs.
• Globulins: immunoglobulins, complement, transport proteins and acute-phase proteins.
• Fibrinogen and clotting factors: form fibrin and support coagulation; most are synthesised in the liver.
• Electrolytes and buffers: sodium, chloride, bicarbonate, calcium and phosphate influence cell function and coagulation.
• Serum: plasma after clotting, so it lacks fibrinogen and some clotting factors. This distinction matters when choosing a specimen.

3. Hematopoiesis

Hematopoiesis is the continuous production of mature blood cells from self-renewing multipotent haematopoietic stem cells (HSCs). In adults it occurs mainly in the red marrow of the pelvis, vertebrae, ribs, sternum and proximal long bones.

3.1 Developmental sites

Life stage	Major site
Embryo	Yolk sac initially, then fetal liver and spleen.
Late fetus/newborn	Most bones contain active red marrow.
Childhood	Marrow gradually converts from red to fatty yellow marrow in long-bone shafts.
Adult	Axial skeleton and proximal humerus/femur; liver and spleen can resume extramedullary hematopoiesis in severe demand or marrow disease.

3.2 Lineage commitment and growth factors

• HSCs give rise to a common myeloid progenitor and common lymphoid progenitor.
• Erythropoietin (EPO), produced mainly by renal peritubular cells in response to hypoxia, increases erythroid production.
• Thrombopoietin (TPO), produced largely by the liver, regulates megakaryocyte and platelet production.
• G-CSF promotes neutrophil production and release; GM-CSF supports granulocyte and monocyte lineages.
• Interleukins coordinate proliferation, differentiation and survival; the marrow microenvironment provides stromal signals and niches.

3.3 Marrow response principles

Increased demand can increase production if stem cells, nutrients and marrow architecture are intact. A high reticulocyte response suggests preserved erythropoiesis; an inappropriately low response suggests inadequate production. Marrow infiltration, fibrosis, cytotoxic drugs, nutritional deficiency and kidney disease can blunt compensation.

4. Red blood cells and haemoglobin

4.1 Structure and function

Mature erythrocytes are flexible, anucleate biconcave discs with a large surface area and no mitochondria. Their membrane and cytoskeleton allow passage through capillaries; haemoglobin binds oxygen reversibly and carries some carbon dioxide. The average red-cell lifespan is about 120 days, after which senescent cells are removed by splenic and hepatic macrophages.

4.2 Haemoglobin

• Adult haemoglobin HbA is predominantly alpha-2 beta-2; HbA2 and fetal HbF are minor fractions after infancy.
• Oxygen affinity is shifted by pH, carbon dioxide, temperature and 2,3-BPG (the Bohr effect), allowing unloading in metabolically active tissues.
• Iron cycles between ferrous and ferric states; haem synthesis requires iron, pyridoxine and functioning mitochondria, while globin production depends on intact genes and protein synthesis.

4.3 Erythropoiesis requirements

Healthy erythropoiesis requires iron, vitamin B12, folate, protein, EPO, thyroid and marrow function. Iron is absorbed mainly in the duodenum and transported by transferrin; B12 absorption requires intrinsic factor and an intact terminal ileum. Folate is absorbed in the small intestine and is rapidly depleted when demand is high.

5. White blood cells

Cell	Morphology/function	Clinical interpretation
Neutrophils	Multilobed nucleus; rapid phagocytosis and killing of bacteria/fungi.	Neutrophilia in infection, stress, steroids and inflammation; neutropenia raises bacterial/fungal sepsis risk.
Lymphocytes	T cells, B cells and NK cells; adaptive memory and cytotoxicity.	Lymphocytosis in many viral infections; lymphopenia in HIV, steroids, malnutrition and severe illness.
Monocytes/macrophages	Large cells that enter tissue and become macrophages or dendritic cells.	Chronic inflammation, tissue repair and phagocytosis; monocytosis may indicate infection or marrow disease.
Eosinophils	Bilobed nucleus and red-orange granules; parasite defence and allergic inflammation.	Eosinophilia in helminths, drug reactions, asthma and some neoplasms.
Basophils	Granules containing histamine and heparin; IgE-associated responses.	Basophilia can occur in myeloproliferative disease and allergic states.

6. Platelets and primary haemostasis

Platelets are anucleate fragments shed from megakaryocytes. Vascular injury exposes collagen and von Willebrand factor (vWF). Platelets adhere through glycoprotein receptors, activate, release ADP/thromboxane and aggregate through glycoprotein IIb/IIIa-fibrinogen bridges. Coagulation then generates thrombin and fibrin to stabilise the platelet plug.

• Platelet count reflects quantity, not necessarily function.
• Thrombocytopenia causes petechiae and mucosal bleeding; severe spontaneous bleeding risk rises as the count falls, but clinical context matters more than a single threshold.
• Thrombocytosis may be reactive (infection, inflammation, iron deficiency, splenectomy) or clonal.
• Platelet dysfunction can occur with aspirin, uraemia, inherited disorders and antiplatelet medicines despite a normal count.

7. The complete blood count (CBC/FBC)

A CBC measures the number and selected physical properties of red cells, white cells and platelets. It is a screening and monitoring test, not a final diagnosis.

Parameter	Meaning	Common clinical uses
Haemoglobin (Hb)	Concentration of haemoglobin in whole blood.	Detects anaemia/polycythaemia and estimates oxygen-carrying capacity.
Haematocrit/packed cell volume (Hct/PCV)	Fraction of blood occupied by red cells.	Supports anaemia/polycythaemia assessment; affected by plasma volume.
RBC count	Number of erythrocytes per volume.	Helps classify anaemia or erythrocytosis with indices.
MCV	Mean red-cell volume.	Microcytic, normocytic or macrocytic classification.
MCH/MCHC	Mean haemoglobin per cell / concentration in packed cells.	Hypochromia and red-cell disorders.
RDW	Variation in red-cell size (anisocytosis).	Iron deficiency, mixed deficiency and evolving marrow disease.
WBC count	Total leukocyte number.	Infection, inflammation, marrow suppression or leukaemia.
WBC differential	Absolute and percentage neutrophils, lymphocytes, monocytes, eosinophils and basophils.	Identify neutropenia, lymphocytosis, eosinophilia and blasts.
Platelet count	Number of circulating platelets.	Bleeding/thrombosis risk, marrow production and treatment monitoring.
MPV	Mean platelet size.	May support peripheral destruction versus production failure; interpret cautiously.

Reference ranges vary by age, sex, pregnancy, altitude, analyser and laboratory. Use the local report interval rather than memorising one universal range.

8. Absolute counts and useful calculations

8.1 Absolute neutrophil count (ANC)

ANC = WBC × (neutrophil fraction + band fraction). Use fractions, not percentages, in the calculation. Infection risk rises as ANC falls; profound neutropenia with fever is an emergency.

8.2 Corrected reticulocyte response

The reticulocyte percentage may appear normal or high simply because the red-cell population is reduced. Correct it for the patient’s haematocrit and, in significant anaemia, use a reticulocyte production index (RPI) to estimate whether marrow response is appropriate.

8.3 Mentzer index

For microcytosis, Mentzer index = MCV/RBC count (with RBC count expressed in millions/µL). A lower value may suggest thalassaemia trait and a higher value iron deficiency, but it is a screening clue—not a substitute for ferritin, haemoglobin electrophoresis and clinical assessment.

9. Peripheral blood film

A blood film is often the fastest way to identify a serious abnormality missed by numbers alone. Assess red-cell size, colour, shape and inclusions; white-cell maturity and abnormal populations; and platelet number, size and clumping.

Film finding	Possible associations
Microcytosis/hypochromia	Iron deficiency, thalassaemia, anaemia of inflammation or sideroblastic disease.
Macro-ovalocytes and hypersegmented neutrophils	Vitamin B12/folate deficiency.
Spherocytes	Hereditary spherocytosis or warm autoimmune haemolysis.
Schistocytes	Microangiopathic haemolysis, DIC, prosthetic valve or severe mechanical injury; urgent context.
Sickle cells/target cells	Haemoglobinopathy or liver disease; correlate with history and electrophoresis.
Howell–Jolly bodies	Asplenia or reduced splenic function.
Blasts	Possible acute leukaemia; urgent haematology review.
Rouleaux	Raised plasma proteins, chronic inflammation or paraproteinaemia.
Platelet clumps	Pseudothrombocytopenia from in-vitro EDTA aggregation; repeat in citrate if suspected.

10. Interpreting a CBC as a clinical pattern

10.1 Anaemia

1. Confirm low Hb against the correct reference interval and assess symptoms/hemodynamics.
2. Classify by MCV and examine RDW and film.
3. Use reticulocytes to separate underproduction from blood loss/haemolysis.
4. Check iron studies, B12/folate, renal function, inflammation, thyroid status, haemolysis markers and bleeding source as indicated.

10.2 Leukocytosis

Identify which lineage is increased. Neutrophilia may reflect infection, stress, steroids or inflammation; lymphocytosis may be reactive or clonal; eosinophilia suggests parasites, allergy, drugs or malignancy. Persistent extreme counts, blasts, constitutional symptoms or cytopenias require urgent specialist evaluation.

10.3 Leukopenia/neutropenia

Review drugs, viral infection, sepsis, nutritional deficiencies, autoimmune disease, marrow disorders and hypersplenism. A febrile patient with severe neutropenia needs immediate sepsis management even when the examination is deceptively normal.

10.4 Pancytopenia

Low red cells, white cells and platelets suggest marrow failure/infiltration, aplasia, nutritional deficiency, hypersplenism, severe infection or chemotherapy. Examine the film, reticulocytes, B12/folate, liver/renal tests, infection screen and marrow when indicated.

11. Specimen collection and quality

• Confirm patient identity with two identifiers, label at the bedside and record date/time, posture and relevant therapy.
• Use the correct tube: EDTA for CBC, citrate for coagulation, serum or heparin plasma for chemistry according to local laboratory instructions.
• Fill citrate tubes to the correct ratio; underfilling alters PT/aPTT.
• Avoid prolonged tourniquet time, vigorous shaking, haemolysis, clotting and sample contamination from an IV line.
• Transport promptly at the recommended temperature; delay can change cell morphology and potassium.
• Repeat an unexpected critical result with a fresh specimen while treating the patient’s clinical emergency.

12. Haematology emergencies

Emergency	Red flags	First priorities
Severe anaemia/acute blood loss	Shock, syncope, chest pain, dyspnoea, confusion or ongoing bleeding.	ABCDE, control bleeding, large-bore access, cross-match, resuscitate and involve transfusion/haematology teams.
Febrile neutropenia	Fever or hypothermia with ANC below local high-risk threshold.	Sepsis assessment, cultures and immediate empiric antimicrobials per protocol; do not wait for imaging.
Thrombotic thrombocytopenic purpura (TTP)	Thrombocytopenia, microangiopathic haemolytic anaemia, neurologic/renal/fever features.	Urgent blood film/haemolysis tests and specialist plasma-exchange pathway; do not delay for ADAMTS13 result.
DIC	Bleeding and thrombosis with severe illness, prolonged clotting times, low fibrinogen/high D-dimer.	Treat the cause, support bleeding and involve critical care/haematology.
Acute leukaemia/hyperleukocytosis	Blasts, leukostasis symptoms, headache, confusion, dyspnoea or visual change.	Urgent haematology/ICU, manage tumour lysis and leukostasis; avoid delay from routine outpatient work-up.
Sickle-cell crisis	Severe pain, acute chest syndrome, stroke, priapism or splenic sequestration.	Analgesia, oxygen if hypoxic, hydration tailored to status, infection assessment and urgent specialist care.

13. Clinical cases

Case 1: microcytic anaemia

A patient has fatigue, low Hb, MCV 68 fL and high RDW. Do not stop at “microcytic anaemia.” Review reticulocytes, ferritin/transferrin saturation, blood loss (including GI or menstrual), diet, pregnancy, infection and chronic inflammation. A thalassaemia trait often has marked microcytosis with a relatively preserved RBC count.

Case 2: fever with neutropenia

A patient on chemotherapy has temperature 38.3°C and ANC 0.2 × 10⁹/L. This is febrile neutropenia until proven otherwise. Obtain cultures promptly, start empiric broad-spectrum antibiotics under the local protocol and assess for line, lung, gut and skin sources.

Case 3: schistocytes

A patient has confusion, thrombocytopenia, anaemia, raised LDH, low haptoglobin and schistocytes. Suspect a thrombotic microangiopathy such as TTP. This is not “simple anaemia”; urgent haematology treatment is required while distinguishing DIC, malignant hypertension, HELLP, renal disease and mechanical haemolysis.

14. Exam-focused summary

Question	Answer
Where is adult hematopoiesis concentrated?	Red marrow of the axial skeleton and proximal long bones.
What does EPO regulate?	Erythroid progenitor survival and red-cell production in response to tissue hypoxia.
Why is the reticulocyte count useful?	It shows whether marrow is responding appropriately to anaemia.
What does MCV classify?	Average red-cell size: microcytic, normocytic or macrocytic.
What do schistocytes suggest?	Fragmentation haemolysis/microangiopathic injury, requiring urgent clinical correlation.
Which CBC finding is immediately dangerous with fever?	Severe neutropenia because infection may progress rapidly with few localising signs.

15. Quick self-test

1. List four functions of blood.
2. Differentiate plasma and serum.
3. Match EPO, TPO and G-CSF with their primary lineages.
4. How do MCV, RDW and reticulocytes help classify anaemia?
5. What are the five leukocyte types?
6. Why can a platelet count be normal in a patient with serious bleeding?
7. What is the immediate approach to fever with severe neutropenia?

Answers

1. Transport, immunity, haemostasis, acid–base/fluid regulation and thermoregulation.
2. Plasma is the liquid portion of anticoagulated blood and contains clotting factors; serum is the fluid after clotting and lacks fibrinogen.
3. EPO: erythrocytes; TPO: megakaryocytes/platelets; G-CSF: neutrophils.
4. MCV gives cell size, RDW gives size variation and reticulocytes show marrow response versus underproduction.
5. Neutrophils, lymphocytes, monocytes, eosinophils and basophils.
6. Platelet function may be impaired by aspirin, uraemia or inherited/acquired disorders even when the number is adequate.
7. Treat as febrile neutropenia: ABCDE, cultures and immediate empiric antimicrobials under local protocol.

Key take-home points

• Blood is a transport, defence and haemostatic tissue; marrow continuously replaces its cellular components.
• Interpret a CBC by pattern, absolute counts, symptoms, trend and specimen quality.
• MCV classifies red-cell size, reticulocytes assess production, and the film often reveals the mechanism.
• Normal ranges are context-dependent; pregnancy, age, altitude, hydration and laboratory method matter.
• Blasts, schistocytes, pancytopenia, severe thrombocytopenia and febrile neutropenia are urgent findings.

Selected references

• NCBI Bookshelf: Blood and the cells it contains
• NCBI Bookshelf: Histology, Hematopoiesis
• MedlinePlus: Complete Blood Count
• MedlinePlus: Blood Differential
• NCBI Bookshelf: Peripheral Blood Smear
• American Society of Hematology: Blood Basics