HUMAN BIOLOGY

BLooD 145

complete blood count

(CBC) The number of red

blood cells, white blood

cells, and platelets in a

microliter of blood.

hoW does the body make neW red blood

cells?

• As needed, the kidneys release erythropoietin, a hormone that
  stimulates the production of new red blood cells by stem cells
  in bone marrow.

taKe-Home message

This hormone stimulates the pro-

duction of new red blood cells when

they are needed.

The process relies on a negative

feedback loop (Figure 8.4). In this

loop, the kidneys monitor the level

of oxygen in your blood. When it falls below a set point,

kidney cells detect the change and soon release EPO. It

stimulates stem cells in bone marrow to produce more red

blood cells. As new red blood cells enter your bloodstream,

the blood can carry more oxygen and the oxygen level rises

in your blood and tissues. This information feeds back to

the kidneys. They make less erythropoietin, and produc-

tion of red blood cells in bone marrow drops.

In “blood doping,” some of an athlete’s blood is

withdrawn and stored. Erythropoietin then stimulates

the production of replacement red blood cells. The stored

blood is reinjected several days prior to an athletic event,

so that the athlete has more than the normal number of

red blood cells to carry oxygen to body muscles—and an

unethical competitive advantage. Some cyclists, runners,

and other “distance” athletes have used lab-made EPO,

even though it is a banned performance-enhancing drug.

Better drug testing is helping to curb this practice.

n Red blood cells do not live long. in response to hormones,

stem cells in bone marrow constantly produce new ones.

Every minute, about 180 million new red blood cells enter
your bloodstream. Each one gradually loses its nucleus and
other organelles. Those structures are unnecessary because
red blood cells don’t divide or make new proteins.
Red blood cells have enough enzymes and other proteins
to function for about 120 days. As they near the end of their
life, die, or become damaged or abnormal, phagocytes
called macrophages remove them from the blood. Much
of this cleanup occurs in the spleen, which is located in
the upper left abdomen. As a macrophage dismantles a
hemoglobin molecule, amino acids from its proteins return
to the bloodstream and the iron in its heme groups returns
to red bone marrow. There the iron may be recycled in new
red blood cells. The rest of the heme group is converted
to the orangish pigment bilirubin. Liver cells take up this
pigment, which is mixed with bile that is released into the
small intestine during digestion.
Steady replacements from stem cells in bone marrow
keep a person’s red blood cell count fairly constant over
time. A complete blood count (CBC) is a tally of the
number of red blood cells, white blood cells, and platelets
in a microliter of blood. On average, an adult male’s red
blood cell count is around 5.4 million. In an adult female
the count averages about 4.8 million red blood cells.
Having a stable red blood cell count is important for
homeostasis, because body cells need a reliable sup-
ply of oxygen. Your kidneys make erythropoietin (EPO).

making new red blood Cells

Figure 8.4 A negative feedback loop helps maintain a normal red blood cell count. (© Cengage Learning)

Kidney

Red blood cells

develop in red

bone marrow

Relieves

RBCs

Reduced oxygen

in blood

Increased oxygen

in blood

Erythropoietin

C

D

E

A

B

A The kidneys detect reduced

O 2 in the blood.

B When less O 2 is delivered to the

kidneys, they secrete the hormone

erythropoietin into the blood.

C Erythropoietin stimulates produc-

tion of red blood cells in bone marrow.

D The additional circulating RBCs

increase O 2 carried in blood.

E The increased O 2 relieves the initial

stimulus that triggered erythropoietin

secretion.

8.3

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