HUMAN BIOLOGY

BLooD 143

What is blood?

• Blood consists of plasma, red blood cells, white blood cells,
  and cell fragments called platelets.

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Other leukocytes don’t have visible granules in their

cytoplasm (and so are agranulocytes). Those known as

monocytes develop into macrophages, “big eaters” that

enter tissues and engulf and destroy invading microbes and

debris. Another type, lymphocytes, operates in immune

responses discussed in Chapter 9.

Some types of white blood cells may

live for years, but most types live for

only a few days or, during a major

infection, perhaps a few hours.

platelets help clot blood

Some stem cells in bone marrow

develop into “giant” cells called

megakaryocytes (mega = “large”).

These cells shed bits of cytoplasm

enclosed in a plasma mem brane.

The fragments, known as platelets,

last only about a week, but millions

are always circulating in our blood.

Platelets release substances that

begin the process of blood clotting

described in Section 8.7.

red blood cells carry oxygen and Co 2

About 45 percent of whole blood—the bottom portion in
the test tube in Figure 8.1—consists of erythrocytes, or
red blood cells (RBCs). Each red blood cell is a biconcave
disk, like a thick pancake with a dimple on each side. The
cell’s red color comes from the iron-containing protein
hemoglobin. Hemoglobin transports oxygen that body
cells need for aerobic respiration. Red blood cells also carry
away some carbon dioxide wastes.

White blood cells defend and clean up

Leukocytes, or white blood cells, make up a tiny fraction
of whole blood. Even so, they play crucial roles in body
defense. Some remove dead or worn-out cells, or material
identified as foreign to the body. Others target or destroy
disease agents such as bacteria or viruses. Most white
blood cells go to work after they squeeze out of blood ves-
sels and enter tissues. How many are in the body at any one
time varies, depending on your activity level and whether
you are healthy or fighting an infection.
In different types of white blood cells, the nucleus var-
ies in its size and shape, and there are other differences
as well. For example, some contain granules that become
visible when the cells are stained for viewing under a
microscope. This group (called granulocytes) includes
neutrophils, eosinophils, and basophils. All have roles
in body defenses that you will read more about in
Chapter 9.

Figure 8.2 Animated! Various types of blood cells form, each with its particular function. Chemicals

called growth factors stimulate the growth and specialization of the different subgroups of blood cells.

Neutrophil B lymphocyte

Monocyte/macrophage

T lymphocyte

In bone

marrow

In circulation

Pluripotent

stem cell

Myeloid stem cell

Megakaryocytes Granulocyteprecursors Erythrocyteprecursors Monocyte

precursors

Lymphocytes in

lymphoid tissues

Platelets Granulocytes Erythrocytes Monocytes Lymphocytes

Lymphoid stem cell

Basophil

Eosinophil

© Cengage Learning

blood Fluid connective tis-

sue consisting of plasma,

blood cells, and platelets.

erythrocyte Red blood cell

(RBC).

leukocyte White blood cell.

Two subgroups of white

blood cells, the granulo-

cytes and agranulocytes,

give rise to types of white

blood cells involved in body

defenses.

plasma The fluid portion of

whole blood.

platelet Cell fragment that

produces some of the sub-

stances required for blood

clotting.

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