Essentials of Anatomy and Physiology

may recall are folds of the cell membrane on their free
surfaces (see Fig. 3–2). These microscopic folds
greatly increase the surface area for absorption.
Yet another type of columnar cell is the goblet cell,
which is a unicellular gland. Goblet cells secrete
mucusand are found in the lining of the intestines
and the lining of parts of the respiratory tract such as
the trachea. Mucous membranes will be described in a
later section.

CILIATED EPITHELIUM

Ciliated epitheliumconsists of columnar cells that
have ciliaon their free surfaces (see Fig. 4–3). Recall
from Chapter 3 that the function of cilia is to sweep
materials across the cell surface. Ciliated epithelium
lines the nasal cavities, larynx, trachea, and large
bronchial tubes. The cilia sweep mucus, with trapped
dust and bacteria from the inhaled air, toward the
pharynx to be swallowed. Bacteria are then destroyed
by the hydrochloric acid in the stomach. The air that
reaches the lungs is almost entirely free of pathogens
and particulate pollution.
Another location of ciliated epithelium in women is
the lining of the fallopian tubes. The cilia here sweep
the ovum, which has no means of self-locomotion,
toward the uterus.
The epithelial tissues are summarized in Table 4–1.

GLANDS

Glandsare cells or organs that secrete something;

that is, they produce a substance that has a function

either at that site or at a more distant site.

Unicellular Glands

Unicellularmeans “one cell.” Goblet cells are an

example of unicellular glands. As mentioned earlier,

goblet cells are found in the lining of the respiratory

and digestive tracts. Their secretion is mucus (see also

Box 4–1: Cystic Fibrosis).

Multicellular Glands

Most glands are made of many similar cells, or of a

variety of cells with their secretions mingled into a

collective secretion. Multicellular glands may be

divided into two major groups: exocrine glands and

endocrine glands.

Exocrineglands have ducts(tubes) to take the

secretion away from the gland to the site of its func-

tion. Salivary glands, for example, secrete saliva that is

carried by ducts to the oral cavity. Sweat glands secrete

sweat that is transported by ducts to the skin surface,

where it can be evaporated by excess body heat. The

gastric glands of the stomach lining contain different

kinds of cells (see Fig. 16–5), which produce

Tissues and Membranes 73

Table 4–1 TYPES OF EPITHELIAL TISSUE

Type Structure Location and Function

Simple squamous

Stratified squamous

Transitional

Cuboidal

Columnar

Ciliated

One layer of flat cells

Many layers of cells; surface cells

flat; lower cells rounded;

lower layer undergoes mitosis

Many layers of cells; surface cells

change from rounded to flat

One layer of cube-shaped cells

One layer of column-shaped cells

One layer of columnar cells with

cilia on their free surfaces

• Alveoli of the lungs—thin to permit diffusion of gases


• Capillaries—thin to permit exchanges of materials;
  smooth to prevent abnormal blood clotting


• Epidermis—surface cells are dead; a barrier to pathogens


• Lining of esophagus, vagina—surface cells are living; a
  barrier to pathogens


• Lining of urinary bladder—permits expansion without
  tearing the lining


• Thyroid gland—secretes thyroxine


• Salivary glands—secrete saliva


• Kidney tubules—permit reabsorption of useful materials
  back to the blood


• Lining of stomach—secretes gastric juice


• Lining of small intestine—secretes enzymes and absorbs
  end products of digestion (microvilli present)


• Lining of trachea—sweeps mucus and dust to the pharynx


• Lining of fallopian tube—sweeps ovum toward uterus