Basic Hematology

Hematology Overview

Hematology is the study of blood and the disorders related to it. Human blood is composed of ∼45% cellular components and ∼55% plasma. Blood has many functions, including transporting oxygen, regulating body temperature, and supporting the immune system. The cellular component consists of three major cell types, all of which arise from hematopoietic stem cells located primarily in the bone marrow: red blood cells (RBCs), white blood cells (WBCs), and platelets. Red blood cells (RBC, erythrocytes) are hemoglobin-carrying cells that primarily transport oxygen. White blood cells (WBC, leukocytes) take the form of either granulocytes or lymphocytes. Granulocytes are part of the innate immune system and play a key role in the immune response to bacteria, fungi, and parasites. Lymphocytes are highly antigen-specific, meaning they are able to target specific infectious agents, primarily viruses. Platelets are the other major cellular component of blood and are small anucleate cells produced from megakaryocytes in the bone marrow. This learning card provides an overview of the production, morphology, and most important functions of blood cells.

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Introduction

Function of blood

Transports oxygen, nutrients, metabolic waste products

Regulation of body temperature, signal transmission, acid-base homeostasis, and immune response

Composition of blood

60–80 mL blood/kg body weight ≅ 5 L blood for an adult weighing 70 kg/154 lbs

45% cellular components (→ hematocrit), 55% plasma

Production of blood

Location: bone marrow; the spleen and liver are the primary sites before birth

Hematopoietic stem cells can differentiate into any type of blood cell and give rise to all blood cells through the following two precursor cells:

Lymphoid precursor cells: origin of lymphocytes and natural killer cells

Myeloid precursor cells: origin of erythrocytes, granulocytes, and megakaryocytes

Left shift (or blood shift): an increase in immature precursor cells (e.g., increase in band neutrophils with rod-shaped nuclei in response to acute inflammation) in the blood. In most cases, infection is the cause of a left shift.

A reactive increase of a cell line ends in "-cytosis" (e.g., leukocytosis); neoplastic increase is referred to as "-emia" (e.g., leukemia).

Overview

Bone marrow is the spongy, soft tissue that fills the hollow spaces within cancellous (spongy) bone. It is composed of stromal cells and stem cells, which are the progenitors of all five blood cell lineages. Hematopoiesis is the main function of the bone marrow. It also filters aging RBCs and houses immune cells (e.g., plasma cells).