Red blood cell formation is controlled through a negative feed-
back system involving the kidney and red bone marrow that is
mediated by the hormone erythropoietin.
Regulation of red blood cell formation
• Figure 10.5
Some stimulus disrupts
homeostasis by
Decreasing
Oxygen delivery to kid
neys (and other tissues)
If a patient has lost a lot of blood following a car acci-
dent, which of the following would you expect to see?
a. Decreased secretion of erythropoietin
b. No change in blood cell production
c. Increased production of erythropoietin
d. Bone marrow reducing production of reticulocytes
During red blood cell formation, also called erythro-
poiesis, the pre-red blood cell, or reticulocyte, ejects its
nucleus to form the biconcave erythrocyte. Therefore, the
mature RBC can no longer reproduce and will die within
120 days. Platelets, which are fragments of the bone mar-
row’s megakaryocytes, are also unable to reproduce.
Besides routine maintenance, the major stimulus for
producing RBCs is reduced oxy-
gen delivery to tissues
(tissue hypox-
ia ).
The kidneys sense hypoxia and
release a hormone called
erythro-
poietin
, which stimulates the red
bone marrow to produce addition-
al RBCs (
Figure 10.5
).
Like RBCs, granular leukocytes develop in red bone
marrow from myeloid stem cells, while lymphocytes de-
velop in the red bone marrow from lymphoid stem cells.
Some
lymphocytes
later
develop
in
lymphoid
tissues
through mitosis. An increase in WBCs called leukocyto-
sis (loo'-ko-si-TO-sis) is a normal response to infections,
strenuous exercise, anesthesia, and surgery.
erythropoietin
(e-rith'-ro-POY-e-tin)
A hormone released
by the kidneys that
stimulates red blood
cell production.
When Blood Cells Are Destroyed,
Many of Their Components
are Recycled
What happens to red blood cells after 120 days? Old, worn-
out RBCs are dismantled in the spleen, liver, and red bone
marrow (see Figure 10.4). Hemoglobin breaks apart. The
iron is recycled to red bone marrow, where it ends up back
in new RBCs. The non-iron portions of heme get broken
down, processed in the liver, and ultimately excreted by
the kidneys and intestines. The globin protein degrades
into amino acids, which are released into the bloodstream
and reused.
Usually, the rate of RBC production equals the rate of
RBC destruction. To maintain normal numbers of RBCs,
new mature cells must enter the bloodstream at an as-
tonishing rate of at least 2 million per second. However,
if tissue hypoxia occurs, then RBC production is stimu-
lated at a rate faster than its destruction (
Figure 10.6
).
CONCEPT CHECK
1.
How
long does a red blood cell survive?
2.
What
causes most white blood cells to die?
3.
What
is bilirubin?
4.
Which
hormone stimulates RBC production?
292 CHAPTER 10
The Cardiovascular System: Blood
previous page 327 Craig Freudenrich, Gerard J  Tortora   Visualizing Anatomy and Physiology   2011 read online next page 329 Craig Freudenrich, Gerard J  Tortora   Visualizing Anatomy and Physiology   2011 read online Home Toggle text on/off