9.4: Cells and Platelets of the Blood
- Page ID
- 100099
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)The formed elements of blood include RBCs, WBCs, and platelets — each with distinct structures, essential for their function in transporting gases, defending against disease, and preventing blood loss.
- Identify all the formed elements of blood and describe their origins.
- Compare the structure and function of these formed elements.
- Explain what a CBC is and relate it to health or disease.
Formed Elements
Blood cells and cell fragments are collectively known as formed elements. This group includes erythrocytes (red blood cells, or RBCs), leukocytes (white blood cells, or WBCs), and thrombocytes (platelets). All of them are produced primarily in the red bone marrow rather than within the bloodstream itself. Because platelets are actually fragments of much larger cells called megakaryocytes, it is technically incorrect to refer to all formed elements as “cells.” For this reason, the term cellular components is not used to describe them.
Erythrocytes function to carry oxygen through the bloodstream so it may be delivered throughout the body.
There are several different types of leukocytes that have unique functions contributing to the body's housekeeping and sickness fighting efforts.
Platelets, finally, function in helping to maintain blood volume (a process called hemostasis) by forming blood clots to prevent blood loss.
The formed elements are summarized in the table below. Each category of formed element is covered in more detail in a dedicated section below.
|
Formed Element |
Major Subtypes |
Numbers Present per Microliter (μL); Mean (Range) |
Appearance in a Standard Blood Smear |
Summary of Functions |
Comments |
|---|---|---|---|---|---|
|
Erythrocytes |
5.2 million |
Flattened biconcave disc; no nucleus; pale red color |
Transport oxygen and some carbon dioxide between tissues and lungs |
Lifespan of approximately 120 days |
|
|
Leukocytes |
Granulocytes (neutrophils, eosinophils, and basophils) |
7000 (5000-10,000) |
Obvious dark-staining nucleus |
All function in body defenses |
Exit capillaries and move into tissues; lifespan of usually a few hours or days |
|
Neutrophils
|
4150 (1800-7300) |
Nuclear lobes increase with age; pale lilac granules |
Phagocytic; particularly effective against bacteria; Release cytotoxic chemicals from granules |
Most common leukocyte; lifespan of minutes to days |
|
|
Eosinophils
|
165 (0-700) |
Nucleus generally two-lobed; bright red-orange granules |
Phagocytic cells; effective with antigen-antibody complexes; release antihistamines; increase in allergies and parasitic infections. |
Lifespan of minutes to days. |
|
|
Basophils
|
44 (0-150) |
Nucleus generally two-lobed but difficult to see due to presence of heavy, dense, dark purple granules |
Promotes inflammation |
Least common leukocyte; lifespan unknown |
|
|
Agranulocytes (lymphocytes and monocytes) |
2640 (1700-4950) |
Lack abundant granules in cytoplasm; have a simple-shaped nucleus that may be indented |
Body defenses |
Group consists of two major cell types from different lineages |
|
|
Lymphocytes
|
2185 (1500-4000) |
Spherical cells with a single often large nucleus occupying much of the cell’s volume; stains purple; seen in large (natural killer cells) and small (B and T cells) variants |
Primarily specific (adaptive) immunity: T cells directly attack other cells; B cells release antibodies; natural killer cells are like T cells but nonspecific |
Initial cells originate in bone marrow, but secondary production occurs in lymphatic tissue; several distinct subtypes; memory cells form after exposure to a pathogen and rapidly increase responses to subsequent exposure; lifespan of many years |
|
|
Monocytes
|
455 (200-950) |
Largest leukocyte with an indented or horseshoe-shaped nucleus |
Very effective phagocytic cells engulfing pathogens or worn-out cells; also serve as antigen- presenting cells (APCs) for other components of the immune system |
Produced in red bone marrow; referred to as macrophages after leaving circulation |
|
|
Platelets |
 |
350,000 (150,000-500,000) |
Cellular fragments surrounded by a plasma membrane and containing granules; purple stain |
Hemostasis plus release growth factors for repair and healing of tissue |
Formed from megakaryocytes that remain in the red bone marrow and shed platelets into circulation
|
In addition to measuring hematocrit, another very common blood test is the complete blood count (CBC) with differential. In this test, a small sample of blood is placed on a microscope slide, spread into a thin film (see image on the right), and stained so that the different formed elements — RBCs, WBCs, and platelets —can be clearly distinguished. A trained laboratory technician then examines the sample under a microscope to count and identify each type of cell or cell fragment.
The differential portion of the CBC focuses on determining the numbers and percentages of each type of leukocyte, while the rest of the test provides detailed information about the sizes, shapes, and overall appearance of all the formed elements. In some cases, a slide imprinted with a fine grid is used to make it easier to calculate cell concentrations per unit volume of blood. These values are then compared with standard reference ranges (such as those shown in the table above) to help identify possible abnormalities.
Elevated or decreased numbers of specific cell types can give important clues about a person’s health. For example, unusually high neutrophil counts — called neutrophilia— might suggest a bacterial infection. If, on the other hand, the neutrophil count is low, a condition known as neutropenia, it may indicate that the body’s ability to fight bacterial infections is reduced. Neutropenia can occur with certain viral infections, bone marrow disorders, autoimmune diseases, or as a side effect of chemotherapy.
Figure \(\PageIndex{1}\): RBCs, Neutrophil and Thrombocytes in a Peripheral Blood Smear. In a typical sample, there are about 600 - 700 times more RBCs than WBCs. Most of the pink, non-nucleated cells seen here are erythrocytes, which transport oxygen and carbon dioxide. The large cell with a multilobed purple nucleus is a neutrophil, the most common type of WBC and an important defender against bacterial infection through phagocytosis. The tiny, irregular purple-stained fragments are thrombocytes (platelets), which aid in hemostasis by forming temporary plugs and releasing clot-promoting factors. Image credit: OpenStax, CC BY 4.0.