Atissueis a group of cells with similar structure

and function. The tissue contributes to the function-
ing of the organs in which it is found. You may recall
that in Chapter 1 the four major groups of tissues were
named and very briefly described. These four groups
are epithelial, connective, muscle, and nerve tissue.
This chapter presents more detailed descriptions of
the tissues in these four categories. For each tissue, its
functions are related to the organs of which it is a part.
Also in this chapter is a discussion of membranes,
which are sheets of tissues. As you might expect, each
type of membrane has its specific locations and func-
tions.

EPITHELIAL TISSUE

Epithelial tissuesare found on surfaces as either cov-
erings (outer surfaces) or linings (inner surfaces).
Because they have no capillaries of their own, epithe-
lial tissues receive oxygen and nutrients from the
blood supply of the connective tissue beneath them.
Many epithelial tissues are capable of secretion and
may be called glandular epithelium, or more simply,
glands.

Classification of the epithelial tissues is based on

the type of cell of which the tissue is made, its charac-

teristic shape, and the number of layers of cells. There

are three distinctive shapes: squamouscells are flat,

cuboidalcells are cube shaped, and columnarcells

are tall and narrow. “Simple” is the term for a single

layer of cells, and “stratified” means that many layers

of cells are present (Fig. 4–1).

SIMPLE SQUAMOUS EPITHELIUM

Simple squamous epitheliumis a single layer of flat

cells (Fig. 4–2). These cells are very thin and very

smooth—these are important physical characteristics.

The alveoli (air sacs) of the lungs are simple squamous

epithelium. The thinness of the cells permits the dif-

fusion of gases between the air and blood.

Another location of this tissue is capillaries, the

smallest blood vessels. Capillary walls are only one cell

thick, which permits the exchange of gases, nutrients,

and waste products between the blood and tissue fluid.

The interior surface of capillaries is also very smooth

(and these cells continue as the lining of the arteries,

veins, and heart); this is important because it prevents

abnormal blood clotting within blood vessels.

70 Tissues and Membranes

Columnar

Cuboidal

Squamous

Simple columnar

Simple cuboidal

Simple squamous

Stratified squamous

Shapes Simple Stratified

Figure 4–1. Classification of epithelial tissues

based on the shape of the cells and the number of

layers of cells.

QUESTION:Which of these might be best for effi-

cient diffusion, and why?