and blood pressure. Endocrine hormones also play vital roles
in reproduction, development, and growth.
Small groups of specialized cells produce some hor-
mones. However, the larger endocrine glands—the pituitary
gland, thyroid gland, parathyroid glands, adrenal glands,
and pancreas—are the subject of this chapter
Subsequent chapters discuss several other hormone-secret-
ing glands and tissues.
Hormones are released into the extracellular spaces sur-
rounding endocrine cells. From there, they diffuse into the
bloodstream and are carried to all parts of the body.
Chemistry of Hormones
Chemically, most hormones are either steroids or steroidlike
substances—or they are nonsteroids, including amines, pep-
tides, proteins, or glycoproteins. Thus, hormones are organic
compounds. Hormones can stimulate changes in target cells
even in extremely low concentrations.
Chemical communication. (
) Neurons release
neurotransmitters into synapses, af
ecting postsynaptic cells. (
release hormones into the bloodstream. Blood carries hormone
molecules throughout the body, but only target cells respond.
Types oF glands. (
) Endocrine glands release hormones
into the internal environment (body ±
) Exocrine glands secrete
to the outside environment through ducts that lead to body surFaces.
A Comparison Between the Nervous System and the Endocrine System
city oF action
Receptors on postsynaptic cell
Receptors on target cell
Speed oF onset
Seconds to hours
Duration oF action
Very brieF unless neuronal activity continues
May be brieF or may last For days even iF secretion ceases
Endocrine glands and their hormones help regulate
metabolic processes. They control the rates of certain chemi-
cal reactions; aid in transporting substances through mem-
branes; and help regulate water balance, electrolyte balance,
(cells with hormone
Hormones have no
effect on other cells