Bma Illustrated Medical Dictionary

be replaced by unrelated new ones.

(See also psychotherapy.)

bloodThe red fluid that circulates in the

body’s veins, arteries, and capillaries.

Blood is pumped by the heart via the

arteries to the lungs and all other tis-

sues and is then returned to the heart

in veins (see circulatory system). Blood

is the body’s transport system and plays

an important role in the defence against

infection. An average adult has about

5 litres of blood.

Almost half of the volume of blood

consists of blood cells; these include red

blood cells (erythrocytes), which carry

oxygen to tissues; white blood cells

(leukocytes), which fight infection; and

platelets (thrombocytes), which are in-

volved in blood clotting. The remainder

of the blood volume is a watery, straw-

coloured fluid called plasma, which

contains dissolved proteins, sugars,

fats, salts, and minerals. Nutrients are

transported in the blood to the tissues

after absorption from the intestinal

tract or after release from storage

depots such as the liver. Waste prod-

ucts, including ureaand bilirubinare

carried in the plasma to the kidneys and

liver respectively.

Plasma proteins include fibrinogen;

which is involved in blood clotting; im-

munoglobulins(also called antibodies)

and complement, which are part of the

immune system; and albumin. Hormones

are also transported in the blood to

their target organs.

blood cellsCells, also called blood cor-

puscles, present in blood for most or

part of their lifespan. They include red

blood cells, which make up about 45

per cent by volume of normal blood,

white blood cells, and platelets. Blood

cells are made in the bone marrow by a

series of divisions from stem cells.

Red blood cells (also known as RBCs,

red blood corpuscles, or erythrocytes)

transport oxygen from the lungs to

the tissues (see respiration). Each RBC

is packed with haemoglobin, enzymes,

minerals, and sugars. Abnormalities can

occur in the rate at which RBCs are

either produced or destroyed, in their

numbers, and in their shape, size, and

haemoglobin content, causing forms of

anaemiaand polycythaemia(see blood,

disorders of).

White blood cells (also called WBCs,

white blood corpuscles, or leukocytes)

protect the body against infection and

fight infection when it occurs. The 3

main types of WBC are granulocytes

(also called polymorphonuclear leuko-

cytes), monocytes, and lymphocytes.

Granulocytes are further classified as

neutrophils, eosinophils, or basophils,

and each type of granulocyte has a role

in either fighting infection or in inflam-

matory or allergic reactions. Monocytes

and lymphocytes also play an important

part in the immune system. Lympho-

cytes are usually formed in the lymph

nodes. One type, a T-lymphocyte, is re-

sponsible for the

delayed hypersen-

sitivity reactions

(see allergy) and

is also involved in

protection against

cancer. T-lympho-

cytes manufacture

chemicals, known

as lymphokines,

which affect the

function of other

cells. In addition,

the T-cells mod-

erate theactivity

of B-lymphocytes,

which form the

antibodiesthat can

prevent a second

attack of certain infectious diseases.

Platelets (also known as thrombocytes),

are the smallest blood cells and are

important in blood clotting.

The numbers, shapes, and appearance

of the various types of blood cell are of

great value in the diagnosis of disease

(see blood count; blood film).

blood clottingThe process of blood

solidification. Clotting is important in

stemming bleeding from damaged blood

vessels. However, unwanted blood clot-

ting can occur inside major blood vessels

and cause a myocardial infarction(heart

attack) or stroke (see thrombosis).

When a blood vessel is damaged, it

constricts immediately to reduce blood

flow to the area. The damage sets off a

BLOOD BLOOD CLOTTING

B

White

blood cell

(neutrophil)

White

blood cell

(lymphocyte)

Red blood

cell

Plasma

Platelet

BLOOD CELLS