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Amy Young
Three Definitions – Erythrocyte
In this assignment we are to choose a somewhat complex term used in our discipline or profession
and define it in three ways, including: a parenthetical definition, a sentence definition, and an
expanded definition. We do this in order to study and practice the writing techniques associated with
defining terms and with peer review. This will help us understand how important definitions are to
technical writing and what role they play, as well as how the situation and audience determine
whether a definition is needed and in what level of detail it should be given.
An introductory biology laboratory or class at a college or university teaching students about the
human circulatory system. The audience is made up of mainly first year students, with little to no
background in biology. They have likely been educated, earlier in this course, about the general
structure of a cell and the basic organic molecules: carbohydrates, proteins and fats.
Parenthetical Definition
An erythrocyte (red blood cell) carries oxygen in the blood.
Sentence Definition
An erythrocyte is a common type of blood cell that contains the protein hemoglobin, carries oxygen
from the lungs to other tissues in the body, and gives blood its red colour.
Expanded Definition
Erythrocytes (e-RITH-rō-sīts) are a type of blood cell, commonly called Red Blood Cells (RBCs).
Their name originates from the Greek words erythro, meaning red, and cyte, which means cell
(Chabner 504).
Why are erythrocytes red?
Erythrocytes contain multiple copies
of an oxygen-carrying protein called
hemoglobin. When oxygen and
hemoglobin combine they produce
the bright red colour of blood.
Figure 1. normal red blood cells flowing freely in a blood vessel.
The inset image shows a cross-section of a red blood cell with
hemoglobin. (Red Blood Cells – PubMed Health)
What do erythrocytes look like?
Erythrocytes are generally 8 µm in
diameter and have a biconcave shape
(Figure 1). Biconcave means concave
on both sides. Their shape gives them
a larger surface area, which is ideal
for gas exchange.
Figure 2. Components of blood, shows the breakdown of blood into liquid plasma
and solid formed elements, and is then further broken down into their component
parts. (Components of Blood - Mosby)
How are
erythrocytes different
from other cells?
Erythrocytes are made
up of a selectively
permeable membrane,
cytosol, and
hemoglobin. Mature
red blood cells lack a
nucleus and other
organelles, features
other cells in the body
commonly have. The
lack of a nucleus,
which houses a cell’s
DNA and ribosomes,
means erythrocytes are
unable to reproduce.
Instead, all internal
space is available to
store and transport
oxygen and carbon
dioxide (Tortora et al.
What is the lifespan of an erythrocyte?
Erythrocytes live around 120 days in the bloodstream because of the wear and tear their membranes
faces as they squeeze through capillaries (tiny blood vessels in the lungs). Worn out red blood cells
are destroyed by macrophages (scavenger cells) in the spleen, liver, and bone marrow. When red
blood cells are broken down, the iron from the hemoglobin protein is recycled to be used in making
hemoglobin for new erythrocytes (Chabner 506).
How many erythrocytes are there in a drop of blood?
A healthy adult has 5–5.5 million red blood cells per microliter (µL) of blood. To put this in
perspective, one drop of blood is about 50 µL. Every second, 2-10 million worn out erythrocytes are
destroyed (Tortora et al. 359). The production of red blood cells must keep up with this rate in order
to maintain an adequate amount of erythrocytes in the blood.
What are the components of blood?
Blood is made up of cells suspended in plasma, a clear, light yellow liquid. Cells make up 45% of the
volume of blood and include erythrocytes, leukocytes (White Blood Cells), and platelets (Clotting
factors). The other 55% of the volume of blood is plasma, a solution of water, proteins, sugar, salts,
hormones, and vitamins (Figure 2).
Where do erythrocytes come
All blood cells come from the
same source (type of stem cell),
but they develop in different ways
(Figure 3). Erythrocytes are made
in the bone marrow. When the
Blood stem cell is exposed to the
hormone erythropoietin (EPO) it
is stimulated to produce new red
blood cells (Chabner 506).
How do erythrocytes reach
their mature form?
Near the end of erythrocyte
formation, the red blood cell
Figure 3. Blood cell development. A blood stem cell goes through
precursor ejects its nucleus. The
several steps to become a red blood cell, platelet, or white blood cell.
loss of the nucleus causes the
(White Blood Cells - PubMed Health)
center of the cell to indent,
producing the erythrocyte’s
characteristic shape. At the time red blood cells are released into the bloodstream they still contain a
few organelles, but after 1-2 days they reach their mature adult state (Tortora et al. 361-62).
Chabner, Davi-Ellen. The Language of Medicine, 9th ed. Saint Louis: Saunders, 2011: 504-506. Print.
“Components of Blood.” PetalSchools. 2001. Web. 27 May 2016.
“Red Blood Cells (Erythrocytes).” PubMed Health. September 2015. Web. 27 May 2016.
Tortora, Gerard J., and Bryan Derrickson. Introduction to the Human Body: the essentials of anatomy and
physiology, 8th ed. New York: John Wiley & Sons, 2010: 358-362. Print.
“White Blood Cells (Leukocytes).” PubMed Health. September 2015. Web. 27 May 2016.