Essentials of Anatomy and Physiology

the lymph nodes, and the thymus gland. The thymus
also contains stem cells, but they produce only a
subset of lymphocytes. Stem cells are present in the
spleen and lymph nodes as well, though the number of
lymphocytes they produce is a small fraction of the
total.
The blood cells make up 38% to 48% of the total
blood, and each type of cell has its specific function.
Red blood cells(RBCs) carry oxygen bonded to the
iron in their hemoglobin. White blood cells(WBCs)
destroy pathogens by phagocytosis, the production of
antibodies, or other chemical methods, and provide us
with immunity to some diseases. Platelets prevent
blood loss; the process of blood clotting involves
platelets.

AREOLAR CONNECTIVE TISSUE

The cells of areolar(or loose) connective tissueare

called fibroblasts. A blastcell is a “producing” cell,

and fibroblasts produce protein fibers. Collagenfibers

are very strong; elastinfibers are elastic, that is, able

to return to their original length, or recoil, after being

stretched. These protein fibers and tissue fluid make

up the matrix, or non-living portion, of areolar con-

nective tissue (see Fig. 4–4). Also within the matrix are

mast cells that release inflammatory chemicals when

tissue is damaged, and many white blood cells, which

are capable of self-locomotion. Their importance here

is related to the locations of areolar connective tissue.

Areolar tissue is found beneath the dermis of the

Tissues and Membranes 75

Blood

Areolar

Adipose

White blood cell

Fibroblast

Collagen fibers

Elastin fiber

Adipocytes

(Approximately 430X)

Red blood cells

White blood cell

(Approximately 300X)

(Approximately 150X)

A

B

C

Platelets

Figure 4–4. Connective tissues. (A) Blood. (B) Areolar. (C) Adipose.

QUESTION:What is the matrix of blood, and what is found in adipocytes?