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Knowledge

Understanding Blood and Its Components

Learn about plasma, red blood cells, white blood cells, and platelets and their basic roles.

4 min read Hematology #rbc-indices #hematology #differential
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Learning Objectives

By the end of this resource you should be able to:

- Distinguish plasma from formed blood elements.
- Describe the basic roles of red blood cells, white blood cells, and platelets.
- Identify major circulating white-cell populations.
- Explain why blood components are measured in laboratory medicine.

Understanding Blood and Its Components

Overview

Blood is a specialized connective tissue that transports gases, nutrients, hormones, metabolic products, cells, and other substances throughout the body. It also contributes to immune defense, hemostasis, temperature regulation, and maintenance of physiological balance.

Blood is commonly divided into plasma and formed elements.

Plasma

Plasma is the liquid component of blood. It is predominantly water and contains dissolved proteins, electrolytes, nutrients, hormones, gases, and metabolic products.

Important plasma proteins include:

  • Albumin
  • Globulins
  • Fibrinogen and other coagulation-related proteins

The exact composition of plasma changes with physiological state and disease.

Red blood cells

Red blood cells, or erythrocytes, contain hemoglobin and are specialized for transporting oxygen and participating in carbon-dioxide transport.

Laboratory assessment

Common red-cell measurements include:

  • RBC count
  • Hemoglobin
  • Hematocrit
  • MCV
  • MCH
  • MCHC
  • RDW

These measurements are related. For example, cell size and hemoglobin content provide complementary information about the characteristics of the circulating red-cell population.

Red-cell morphology

A blood film can provide additional observations about red-cell size, shape, color, inclusions, and distribution. Morphological findings should be described and interpreted using the laboratory's validated terminology and reporting practice.

White blood cells

Leukocytes participate in immune and inflammatory processes.

Neutrophils

Neutrophils are important in innate immune defense, particularly in responses involving bacterial and fungal pathogens.

Lymphocytes

Lymphocytes include major populations involved in adaptive and innate immune functions, including B cells, T cells, and natural killer cells.

Monocytes

Monocytes circulate in blood and can differentiate into macrophage and related cell populations in tissues.

Eosinophils

Eosinophils participate in immune responses and are associated with several inflammatory and hypersensitivity processes.

Basophils

Basophils are granulocytic leukocytes involved in immune and inflammatory signaling.

Laboratories may assess total leukocyte count and differential counts or percentages.

Platelets

Platelets are small cellular fragments produced from megakaryocytes. They contribute to primary hemostasis by participating in adhesion, activation, and aggregation at sites of vascular injury.

Laboratory platelet assessment commonly includes platelet count and may include additional indices depending on the analyzer and laboratory scope.

Blood-cell production

Blood-cell production is called hematopoiesis. In healthy adults, bone marrow is a major site of hematopoiesis.

The process involves stem and progenitor cells that develop through progressively differentiated lineages. Regulation involves growth factors and signals from the marrow microenvironment and the wider body.

Laboratory relevance

Changes in blood-cell number or characteristics can provide information that contributes to the investigation of many conditions. However, laboratory findings should be considered with the clinical context and the laboratory's validated reference and interpretive framework.

Specimen quality

Reliable blood-cell measurements depend on appropriate specimen collection and handling. Common problems include:

  • Misidentification
  • Incorrect anticoagulant
  • Inadequate volume
  • Clotting
  • Excessive delay
  • Improper storage
  • Platelet aggregation
  • Other specimen abnormalities

Key points

  • Blood consists broadly of plasma and formed elements.
  • RBCs, WBCs, and platelets have distinct functions.
  • Hematopoiesis produces the cellular components of blood.
  • Laboratory testing evaluates both quantitative and qualitative characteristics.
  • Specimen quality can substantially influence hematology results.

References

  • MedlinePlus. Blood Count Tests.
  • NCBI Bookshelf. Physiology, Blood.
  • NCBI. MeSH: Hematology.