Human Physiology, 14th edition (2016)

Blood, Heart, and Circulation 409

term formed elements is used instead of blood cells to describe

erythrocytes, leukocytes, and platelets.) The fragments that

enter the circulation as platelets lack nuclei but, like leuko-

cytes, are capable of amoeboid movement. The platelet count

per cubic millimeter of blood ranges from 130,000 to 400,000,

but this count can vary greatly under different physiological

conditions. Platelets survive for about five to nine days before

being destroyed by the spleen and liver.

Platelets play an important role in blood clotting. They

constitute most of the mass of the clot, and phospholipids in

their cell membranes activate the clotting factors in plasma that

result in threads of fibrin, which reinforce the platelet plug.

Platelets that attach together in a blood clot release serotonin,

a chemical that stimulates constriction of blood vessels, thus

reducing the flow of blood to the injured area. Platelets also

secrete growth factors (autocrine regulators—chapter 11, sec-

tion 11.7), which are important in maintaining the integrity of

blood vessels. These regulators also may be involved in the

development of atherosclerosis, as described in section 13.7.

The formed elements of the blood are illustrated in fig-

ure 13.3 , and their characteristics are summarized in table 13.2.

Hematopoiesis

Blood cells are constantly formed through a process called

hematopoiesis (also called hemopoiesis ). The hematopoietic

stem cells —those that give rise to blood cells—originate in the

yolk sac of the human embryo and then migrate in sequence to

regions around the aorta, to the placenta, and then to the liver of

a fetus. The liver is the major hematopoietic organ of the fetus,

but then the stem cells migrate to the bone marrow and the liver

ceases to be a source of blood cell production shortly after birth.

Scientists estimate that the hematopoietic tissue of the bone

marrow produces about 500 billion cells each day. The hema-

topoietic stem cells form a population of relatively undifferen-

tiated, multipotent adult stem cells (chapter 20, section 20.6)

that give rise to all of the specialized blood cells. The hemato-

poietic stem cells are self-renewing, duplicating themselves by

mitosis so that the parent stem cell population will not become

depleted as individual stem cells differentiate into the mature

blood cells. Hematopoietic stem cells are rare, but they prolifer-

ate in response to the proinflammatory cytokines released dur-

ing infection (chapter 15, section 15.3) and in response to the

depletion of leukocytes during infection. Hematopoietic stem

cells are the only cells capable of restoring complete hemato-

poietic ability (producing all blood cell lines) upon transplanta-

tion into the depleted bone marrow of a recipient.

The term erythropoiesis refers to the formation of eryth-

rocytes, and leukopoiesis to the formation of leukocytes. These

processes occur in two classes of tissues after birth, myeloid and

lymphoid. Myeloid tissue is the red bone marrow of the long

bones, ribs, sternum, pelvis, bodies of the vertebrae, and portions

of the skull. Lymphoid tissue includes the lymph nodes, tonsils,

spleen, and thymus. The bone marrow produces all of the differ-

ent types of blood cells; the lymphoid tissue produces lympho-

cytes derived from cells that originated in the bone marrow.

nuclei become lobulated, with two to five lobes connected by
thin strands. At this stage, the neutrophils are also known as
polymorphonuclear leukocytes (PMNs).
There are two types of agranular leukocytes: lymphocytes
and monocytes. Lymphocytes are usually the second most
numerous type of leukocyte; they are small cells with round
nuclei and little cytoplasm. Monocytes, by contrast, are the
largest of the leukocytes and generally have kidney- or horse-
shoe-shaped nuclei. In addition to these two cell types, there
are smaller numbers of plasma cells, which are derived from
lymphocytes. Plasma cells produce and secrete large amounts
of antibodies. The immune functions of the different white
blood cells are described in more detail in chapter 15.

CLINICAL APPLICATION

Whereas anemia refers to an abnormally low red blood

cell count (as previously discussed), polycythemia is an

abnormally high red blood cell count. This can have many

causes, including the low oxygen of life at high altitudes

(discussed in chapter 16). Leukopenia is an abnormally low

white blood cell count, which may be produced by radia-

tion for cancer, among other causes. Leukocytosis is the

opposite—an abnormally high white blood cell count, which

may be caused by cytokines released from an inflammation

during an infection. Leukemia is cancer of the bone marrow

that causes a high number of abnormal and immature white

blood cells to appear in the blood.

Figure 13.3 The blood cells and platelets. The
white blood cells depicted above are granular leukocytes; the
lymphocytes and monocytes are nongranular leukocytes.

Neutrophils Eosinophils Basophils

Lymphocytes Monocytes Platelets Erythrocytes

Platelets

Platelets, or thrombocytes, are the smallest of the formed ele-
ments and are actually fragments of large cells called mega-
karyocytes, which are found in bone marrow. (This is why the