Basics of hematology

Last updated: November 7, 2023

Summarytoggle arrow icon

Hematology is the study of blood and the disorders related to it. Human blood consists of blood cells and plasma. Blood has many functions, including transporting oxygen and nutrients to tissues, removing waste materials (e.g., carbon dioxide, urea), regulating body temperature, and carrying cells responsible for coagulation and immune response. There are three main types of blood cells, all of which originate from hematopoietic stem cells, which are located primarily in the bone marrow: red blood cells (RBCs; erythrocytes), white blood cells (WBCs; leukocytes), and platelets (thrombocytes). RBCs are hemoglobin-carrying cells that primarily transport oxygen. There are two types of WBCs: granulocytes and lymphocytes. Granulocytes are part of the innate immune system and play a key role in the immune response to bacteria, fungi, and parasites. Lymphocytes include cells that are responsible for both the innate (natural killers) and adaptive (T and B cells) immune system. T and B cells, in contrast to the cells involved in innate immunity, can target specific antigens presented to them by antigen-presenting cells. Platelets, small anucleate cells produced by megakaryocytes, are required for hemostasis.

Overviewtoggle arrow icon

Functions of blood [1][2]

  • Transport of:
  • Protection from pathogens
  • Hemostasis
  • Regulation of body temperature, signal transmission, and acid-base homeostasis

Blood volume

  • On average, blood constitutes ∼ 7% of the total body weight of adults (∼ 70 mL/kg). [1][3]
  • The actual circulating blood volume varies and depends on age, weight, and height.
  • For children ≥ 35 kg, adolescents, and adults, the total blood volume can be estimated using the Nadler equation: [4]
    • Men: BV = (0.3669 × H3) + (0.03219 × W) + 0.6041
    • Women: BV = (0.3561 × H3) + (0.03308 × W) + 0.1833
      • BV: blood volume in liters
      • H: height in meters
      • W: weight in kilograms
  • For neonates, infants, and children up to 14 years, the following estimates can be used: [5]
Blood volume in individuals of different age
Age Estimated blood volume (mL/kg)
0–30 days 80–85
1–12 months 75–80
13–24 months 85
2–3 years 80
4–6 years 76–77
7–14 years 70–75

Blood constituents [1][2][3]

Overview of blood cell types
Cell Characteristics Function
RBC (erythrocyte)

WBC (leukocyte)

Blood cells in peripheral tissue
Mast cell
Dendritic cell
Plasma cell

Never Let Monkeys Eat Bananas: Neutrophils > Lymphocytes > Monocytes > Eosinophils > Basophils.

Bone marrow and hematopoiesistoggle arrow icon

Bone marrow [2][7]

Types of bone marrow
Characteristic Red marrow Yellow marrow

Stromal cells of the bone marrow [2][7]

Cells of the stromal compartment of bone marrow are not hematopoietic themselves, but they play a critical role in maintaining and regulating hematopoiesis (e.g., by removing potentially harmful cells and substances).

Hematopoiesis (blood cell production) [2]

Growth factors for hematopoiesis [1][2]

Overview of hematopoietic growth factors
Growth factor Produced by Function
Stem cell factor (SCF)

Granulocyte-monocyte colony-stimulating factor (GM-CSF)

Granulocyte colony-stimulating factor (G-CSF)

Macrophage colony-stimulating factor (M-CSF)

Erythropoietin (EPO)
Thrombopoietin (TPO)

RomiPLoSTIM STIMulates PLatelets. SarGRaMoSTIM STIMulates GRanulocytes and Monocytes.

Red cell line: erythrocytestoggle arrow icon

Erythropoiesis (RBC production) [2]

To recall the sequence of prenatal erythropoiesis sites, think Young Livers Synthesize Blood: Yolk sac, Liver, Spleen, Bone marrow.

RBC morphology and physiology

White cell line: leukocytestoggle arrow icon

Myeloid cell linetoggle arrow icon

Myelopoiesis [1][2]

Granulocytes [1][2][17][18][19]

Types of granulocytes
Cell Characteristics Function Causes of increase Causes of decrease
Neutrophil granulocyte
  • Migrate to inflammation sites (chemotaxis)
  • Identification, destruction, and phagocytosis of extracellular pathogens (e.g., bacteria, fungi)

Eosinophil granulocyte

Basophil granulocyte

  • Basopenia: difficult to assess because the normal basophil count is already very low


Causes Pretty Major Collapse To Defense Cells”: some drugs that can cause leukopenia Clozapine, Propylthiouracil, Methimazole, Carbamazepine, Ticlopidine, Dapsone, Chloramphenicol.

Monocytes, macrophages, and mast cells [1][2][27]][28]

Characteristics of tissue-residing immune cells
Cell type Characteristics Function Causes of increase Causes of decrease
  • 3–7% of all leukocytes
  • Diameter: ∼ 5–20 μm
  • Remain in the bloodstream for only ∼ 8 hours before migrating to tissue
Mast cell [30]
  • N/A

Lymphoid cell linetoggle arrow icon

Lymphopoiesis (lymphocyte production) [2][31]

Lymphocyte physiology

T lymphocytes (T cells)

CD4/CD8 ratio

Natural killer T cells (NKT cells) [32]

B lymphocytes (B cells)

Natural killer cells (NK cells) [2][33][34]

Plateletstoggle arrow icon

Thrombopoiesis (platelet production) [2]

Platelet physiology

Referencestoggle arrow icon

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