B

basophilSee GRANULOCYTE.

blood The cell-filled fluid that carries vital
chemicals and NUTRIENTSvia the cardiovascular sys-
tem to tissues and cells throughout the body. The
HEARTpumps the blood, sending it under pressure
through a closed network of arteries and veins.
The blood provides volume within the cardiovas-
cular system, establishing both BLOOD PRESSUREand
osmotic pressure (the pressure that keeps fluid
within the blood vessels). The blood carries oxy-
gen and nourishment to and collects metabolic
wastes from the cells. The blood also serves as the
body’s primary IMMUNE RESPONSE mechanism,
transporting antibodies and specialized cells that
defend the body from INFECTIONas well as aid in
HEALINGwounds. The blood’s basic composition is
about 55 percent PLASMA(liquid) and 45 percent
cells. The adult human body contains about five
liters, or five and a half quarts, of blood account-
ing for 8 percent of total body weight.

Plasma
Plasma is 90 percent water. It contains a mix of
proteins, electrolytes, hormones, antibodies, min-
erals, GLUCOSE, and other dissolved substances,
forming a solution in which the blood’s cells float.
The constant churning and movement of the
blood as the heart pumps it through the blood
vessels keeps the cells and the plasma well mixed.
However, in a collected blood sample the cells
quickly settle to the bottom, leaving the plasma at
the top. The primary proteins in plasma are ALBU-
MIN, IMMUNOGLOBULIN, and CLOTTING FACTORS. Plasma
has a higher concentration of electrolytes (salts)
than fluid in the tissues, giving the blood a higher
osmotic pressure that draws fluid into the blood
rather than allows it to seep from the blood in the

CAPILLARY BEDS. Plasma is also essential for COAGU-

LATION(clotting) as it carries both clotting factors

and the enzymes that activate them.

Blood Cells

The blood contains three kinds of cells:

• erythrocytes, or red blood cells, which carry
  oxygen from the LUNGSto every other cell in
  the body


• leukocytes, or white blood cells, which fight
  infection and take one of three forms: MONO-
  CYTE, LYMPHOCYTE,orGRANULOCYTE


• platelets, also called thrombocytes, which cause
  blood to coagulate (clot)

Erythrocytes make up nearly the entire volume

of blood cells, while leukocytes and platelets com-

bined make up less than 1 percent. The red BONE

MARROWsynthesizes (produces) the vast majority

of blood cells, a process called HEMATOPOIESIS.

Other structures, such as the SPLEEN, can produce

limited numbers of blood cells when the body is in

crisis. The LIVERand the spleen cleanse damaged,

old, and deteriorating blood cells from the blood.

The liver breaks erythrocytes into their chemical

components, which the body then recycles to syn-

thesize new erythrocytes in the bone marrow.

For further discussion of the blood within the

context of blood and lymph structure and func-

tion please see the overview section “The Blood

and Lymph.”

See also BLOOD DONATION; BLOOD TRANSFUSION;

LY M P H.

blood donation The procedure of withdrawing

BLOODto prepare and use for BLOOD TRANSFUSION.

Volunteer donors provide all human blood and

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