HUMAN BIOLOGY

142 Chapter 8

blood: plasma, blood Cells, and platelets

fragments called platelets, and over a hundred other sub-

stances. Most of these “substances” are different plasma

proteins, which have a variety of functions.

Plasma proteins determine blood’s fluid volume—how

much of it is water. Two-thirds of plasma proteins are

albumin molecules made in the liver. Because there is so

much of it—that is, because its concentration is so high—

albumin has a major influence on the osmotic movement

of water into and out of blood. Albumin also carries many

substances in blood, from wastes to therapeutic drugs.

Other plasma proteins include certain hormones and

proteins involved in immunity and blood clotting. Lipo-

proteins carry lipids, and still other plasma proteins

transport fat-soluble vitamins. Plasma proteins are in

high demand for various therapeutic uses, and in many

countries people who wish to donate plasma can do so at

government-regulated collection centers.

Plasma also contains ions, glucose and other simple

sugars, amino acids, various communication molecules,

and dissolved gases—mostly oxygen, carbon dioxide, and

nitrogen. The ions (such as Na^1 , Cl^2 , H^1 , and K^1 ) help main-

tain the volume and pH of extracellular fluid.

The blood cells in your test tube arose from stem cells in

red bone marrow. Remember from Chapter 4 that a stem cell

is like a “blank slate”—it stays unspecialized and retains

the ability to divide. Some of the daughter cells, however,

differentiate—they become specialized to carry out particu-

lar functions. As Figure 8.2 shows, the formation of special-

ized blood cells begins with pluripotent (“many powers”)

stem cells from which two lines of precursor cells arise. The

descendants of lymphoid stem cells circulate mainly in the

lymphatic system. Myeloid (“from bone marrow”) stem cells

give rise to the other types of circulating blood cells.

n blood is a sticky fluid that consists of water, blood cells, and

other substances.

n Links to Properties of water 2.5, Proteins 2.11, Osmosis

3.10, skeleton 5.2

It is literally true that “blood is thicker than water.” Blood

consists of plasma, blood cells, and cell fragments called

platelets. If you are a woman of average size, your body has

about 4 to 5 liters of this unusual fluid. Men have slightly

more. In all, blood amounts to about 6 to 8 percent of your

body weight.

plasma is the fluid part of blood

If you whirl a prepared blood sample in a centrifuge,

the test tube’s contents should look like what you see in

Figure 8.1. About 55 percent of whole blood is plasma.

Plasma is mostly water. It transports blood cells and

(

(

):

):

Components Relative Amounts Functions

Figure 8.1 Blood consists
of cells, platelets, and
plasma. In the micrograph
(A) the red disks are red
blood cells. Platelets are
pink. The wrinkled gold balls
are white blood cells. The
plasma portion (B) makes
up a little over half of the
volume of blood.

platelets

white

blood

cell

red

blood

cell

National Cancer Institute/Science Source

8.1

A

B © Cengage Learning

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