Human Physiology, 14th edition (2016)

410 Chapter 13

increased amount of oxygen. The World Anti-Doping Code bans

the use of recombinant erythropoietin for this reason, and urine

from athletes is tested for erythropoietin by World Anti-Doping

Agency (WADA) laboratories.

Scientists have identified a specific cytokine that stimulates

proliferation of megakaryocytes and their maturation into platelets.

By analogy with erythropoietin, they named this regulatory mol-

ecule thrombopoietin. The gene that codes for thrombopoietin

As the cells become differentiated during erythropoiesis
and leukopoiesis, they develop membrane receptors for chemi-
cal signals that cause further development along particular lines.
The earliest cells that can be distinguished under a microscope
are the erythroblasts (which become erythrocytes), myeloblasts
(which become granular leukocytes), lymphoblasts (which
form lymphocytes), and monoblasts (which form monocytes).
Erythropoiesis is an extremely active process. It is estimated
that about 2.5 million erythrocytes are produced every second
in order to replace those that are continuously destroyed by the
spleen and liver. The life span of an erythrocyte is approximately
120 days. Agranular leukocytes remain functional for 100 to
300 days under normal conditions. Granular leukocytes, by con-
trast, have an extremely short life span of 12 hours to 3 days.
The production of different subtypes of leukocytes is
stimulated by chemicals called cytokines. These are autocrine
regulators secreted by various cells of the immune system. The
production of red blood cells is stimulated by the hormone
erythropoietin, which is secreted by the kidneys. The gene for
erythropoietin has been commercially cloned so that this hor-
mone is now available for treatment of anemia, including the
anemia that results from kidney disease in patients undergo-
ing dialysis. Injections with recombinant erythropoietin sig-
nificantly improve aerobic physical performance, probably
because of increased hemoglobin allowing the blood to carry an

CLINICAL APPLICATION

Thrombocytosis is an abnormally elevated platelet count.

This occurs when conditions such as acute blood loss,

inflammation, cancer, and others stimulate the liver to pro-

duce an excess of thrombopoietin. However, the production

of thrombopoietin is normally adjusted to maintain homeo-

stasis of the platelet count. Because both megakaryocytes

in the bone marrow and circulating platelets have receptors

that bind to thrombopoietin, a decrease in platelets makes

more thrombopoietin available to stimulate the megakaryo-

cytes, raising the platelet count. Conversely, an increase in

the number of platelets results in less thrombopoietin that is

free to enter the bone marrow and stimulate the megakaryo-

cytes, reducing the platelet count to normal.

Table 13.2 | Formed Elements of the Blood

Component Description Number Present Function

Erythrocyte (red

blood cell)

Biconcave disc without nucleus; contains

hemoglobin; survives 100 to 120 days

4,000,000 to 6,000,000 / mm^3 Transports oxygen and carbon

dioxide

Leukocytes (white

blood cells)

5,000 to 10,000 / mm^3 Aid in defense against infections

by microorganisms

Granulocytes About twice the size of red blood cells;

cytoplasmic granules present; survive

12 hours to 3 days

1. Neutrophil Nucleus with 2 to 5 lobes; cytoplasmic
   granules stain slightly pink

54% to 62% of white cells

present

Phagocytic

2. Eosinophil Nucleus bilobed; cytoplasmic granules stain
   red in eosin stain

1% to 3% of white cells

present

Helps to detoxify foreign

substances; secretes enzymes

that dissolve clots; fights

parasitic infections

3. Basophil Nucleus lobed; cytoplasmic granules stain
   blue in hematoxylin stain

Less than 1% of white cells

present

Releases anticoagulant heparin

Agranulocytes Cytoplasmic granules not visible; survive

100 to 300 days (some much longer)

1. Monocyte 2 to 3 times larger than red blood cell;
   nuclear shape varies from round to lobed

3% to 9% of white cells

present

Phagocytic

2. Lymphocyte Only slightly larger than red blood cell;
   nucleus nearly fits cell

25% to 33% of white cells

present

Provides specific immune

response (including antibodies)

Platelet

(thrombocyte)

Cytoplasmic fragment; survives 5 to 9 days 130,000 to 400,000 / mm^3 Enables clotting; releases

serotonin, which causes

vasoconstriction