Blood cells and formed elements of the blood

Author: Dimitrios Mytilinaios MD, PhD

Last update:

Blood is a specialized type of connective tissue comprised of four main components:

  • Plasma
  • Red blood cells (erythrocytes)
  • White blood cells (leukocytes)
  • Platelets (thrombocytes)

These cellular components, also referred to as the formed elements of blood, are produced in the red bone marrow through a process called hematopoiesis. Blood cells are suspended in plasma, and together they form blood. Blood circulates through a network of blood vessels, propelled by the constant beating of the heart.

Blood serves numerous functions, including transporting oxygen and carbon dioxide, as well as carrying the products of tissue metabolism.

Blood plasma

Blood is a unique tissue in the body because it contains a liquid extracellular material called plasma. Plasma is a watery substance composed of three main components: electrolytes, plasma proteins, and formed elements of blood. Electrolytes include all dissolved nutrients, gases, hormones, ions, and waste products present in the plasma. Plasma proteins are larger molecules with various functions. The primary plasma proteins are:

  • Albumin, which maintains the osmotic pressure of the blood
  • Alpha- and beta-globulins, which transport certain substances
  • Gamma-globulins, which are antibodies produced by B lymphocytes
  • Fibrinogen, which participates in coagulation
  • Complement proteins, which are involved in the inflammation process

To obtain plasma in a laboratory, technicians first treat the sample with anticoagulants to prevent clotting. Then, they centrifuge the sample to separate the formed components of blood. This process differs from that of obtaining serum, which involves centrifuging a clotted sample without adding anticoagulants.

Blood cells

Red blood cells (RBC)

image descriptionRed blood cells, or erythrocytes, transport gasses such as oxygen and carbon dioxide, making up around 95% of the formed blood elements.

Erythrocytes are typically described as biconcave cells, a shape that maximizes their surface area for gas exchange. These cells lack nuclei and other organelles, but they are filled with hemoglobin, a protein capable of binding oxygen and carbon dioxide. Oxygen-bound hemoglobin is called oxyhemoglobin, while carbon-dioxide-bound hemoglobin is called carboxyhemoglobin. Hemoglobin can also bind other gases, such as carbon monoxide, which occurs during carbon monoxide poisoning—a topic explored within toxicology.

The primary function of erythrocytes is to transport oxygen from the lungs to tissues after inhalation, then pick up carbon dioxide from the tissues and carry it back to the lungs for exhalation. Erythrocytes have an average lifespan of around 120 days.

A section of capillary.
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A red blood cell in a section of capillary.

White blood cells (WBC)

White blood cells, or leukocytes, are the cells of the immune system. They are divided into two groups: granulocytes and agranulocytes, a classification based on the presence or absence of certain granules in the cells and the shape of the cellular nuclei. All leukocytes contain non-specific granules, which are actually lysosomes (cellular organelles).

Granulocytes have multilobular nuclei and specific granules in their cytoplasm. They are further divided into neutrophils, eosinophils, and basophils based on the type of specific granules they contain.

image descriptionNeutrophils account for 54-62% of leukocytes in the blood. Their granules are chemically neutral (neither acidic nor alkaline), giving them a light pink color. Neutrophils typically have 3-5 nuclear lobes, which is why they are also called polymorphonuclears or polys. With a lifespan of 1-4 days, they primarily fight bacterial infections.

A section of capillary.
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A neutrophil in a section of capillary.

image descriptionEosinophils make up 1-3% of leukocytes. Their granules stain with an acidic chemical called eosin, causing them to appear red on smears. Eosinophils usually have bilobed nuclei (two lobes) and a lifespan of 1-2 weeks. They mainly combat parasitic and helminthic infections.

A section of capillary.
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An eosinophils in a section of capillary.

image descriptionBasophils are the least abundant granulocytes, comprising less than 1% of total circulating leukocytes. Their granules stain with alkaline dyes, giving them a dark blue or purple appearance. Basophils have bilobed or S-shaped nuclei, and their lifespan typically lasts several months. Their numbers generally increase during hypersensitive immune reactions, allergies, and parasitic infections.

A section of capillary.
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A basophil in a section of capillary.

Agranulocytes lack specific granules, and their nuclei are usually spherical. There are two types of agranulocytes: lymphocytes and monocytes. image descriptionLymphocytes are the smallest leukocytes and usually constitute around 30% of circulating white blood cells. They can be divided into three types based on their functions and subtle morphological differences.

A section of capillary.
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A lymphocyte in a section of capillary.
  • B lymphocytes differentiate into plasma cells (plasmocytes) and memory cells. Plasma cells produce antibodies or immunoglobulins (Ig), which are classified into five types: D, E, M, A, and G (acronym: DEMAG). Memory cells form after encountering foreign particles, sensitizing the body to these pathogens during subsequent exposures.
  • T lymphocytes include cytotoxic or killer T cells and helper cells. Cytotoxic T lymphocytes physically contact and destroy diseased or foreign cells, while helper cells function similarly to B memory cells.
  • Natural killer cells (NK cells) provide non-specific immunity by targeting cells with abnormal features or particles, such as cancer cells or infected cells.

image descriptionMonocytes serve as precursor cells for phagocytes (macrophages, osteoclasts, microglia, etc.).

Learn to identify cells under the microscope with these histology quizzes and labelling exercises.

A section of capillary.
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A monocyte in a section of capillary.

Platelets

The image descriptionplatelets, or thrombocytes, are the smallest formed blood elements. They are actually fragments of large cells from the blood marrow called megakaryocytes. Their main function is to participate in and facilitate the process of blood clotting, which prevents blood loss when a blood vessel is injured.

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Platelets in a section of capillary.

References

  • Betts, J. G., Young, K. A., Wise, J. A., Johnson, E., Poe, B., & Kruse, D. H. (2022). Anatomy and Physiology (2nd ed.). OpenStax.
  • Mescher, A. L. (2013). Junqueira’s Basic Histology (13th ed.). McGraw Hill.
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