HUMAN BIOLOGY

What are hormones?

• Hormones are signaling molecules secreted by endocrine
  glands, endocrine cells, and some neurons.


• The bloodstream carries hormones to distant target cells.


• Together, the glands and cells that secrete hormones make
  up the endocrine system. Their activity usually is regulated
  by negative feedback.


• Hormones are secreted in small amounts. Different ones may
  have opposing effects or may exert their effects in concert
  with other hormones. Still other hormone effects require a
  target cell to be primed by exposure to one hormone in order
  to respond to a second one.

taKe-home message

endocrine system The
glands, organs, and cells
that produce hormones.

hormones Signaling
molecules carried in the
bloodstream.

target cell A cell that has
receptors for a signal-
ing molecule and so may
respond to the molecule in
some way.

the endocrine system: hormones

n Hormones help coordinate and manage the activities of the

billions of body cells.

n    Links to Receptor proteins in cell membranes 3.4,

Neurotransmitters 13.3, Pheromones 14.3

hormones are long-distance signaling

molecules carried in the bloodstream

Cells signal other cells by various chemical means. Previ-

ous chapters have discussed several types of signaling

molecules, including neurotransmitters that carry ner-

vous system messages. Pheromones,

the “social signals” mentioned in

Chapter 14’s discussion of chemical

senses, are another example. These

and the other chemical messengers

listed in Table 15.1 are all alike in

one key way: They act on target

cells. A target cell is any cell that

has receptors for the signaling mol-

ecule and that may change its activi-

ties in response. A target cell may or

may not be next to the cell that sends

the signal.

Hormones, our main topic here, are secreted by the

body’s endo crine glands, endo crine cells, and some

neurons. They travel the bloodstream to target cells some

distance away. Many types of cells also release “local”

signaling molecules that change conditions in nearby tis-

sues. Prostaglandins are an example. Their targets include

smooth muscle cells in the walls of bronchioles, which then

close up or dilate and so change air flow in the lungs (Sec-

tion 10.6). Prostaglandins that affect smooth muscle in the

uterus cause menstrual cramps.

The word hormone—from the Greek horme, “to excite”—

was coined in 1900 by scientists studying food digestion in

dogs. They discovered that a substance released by gland

cells in a dog’s GI tract could stimulate the pancreas. Later

on, other researchers identified a variety of hormones and

their sources (Figure 15.1).

Hormone-producing glands, organs, and cells form

the endocrine system. For several reasons the name

is misleading, however. First, it implies that there is an

independent hormone-based control system for the body,

when in fact almost all organ systems produce hormones.

Section 15.11 will describe some major examples, including

the digestive tract hormones introduced in Chapter 11. In

addition, as you will soon see, the functioning of endocrine

glands, cells, and organs is closely allied with operations of

the central nervous system.

hormones are produced in small

amounts and often interact

In general, endocrine glands usually release small amounts

of hormones in short bursts. Controls usually prevent hor-

mones from being either overproduced or underproduced.

Negative feedback is the most common control mechanism.

It’s not unusual for two or more hormones to affect the

same process. There are three common kinds of these hor-

mone “partnerships”:

1. Opposing interaction. The effect of one hormone
   may oppose the effect of another. Insulin, for example,
   reduces the level of glucose in the blood, and glucagon
   increases it.


2. Synergistic interaction. The combined action of two
   or more “cooperating” hormones may be required to
   trigger a certain effect on target cells. For instance, a
   woman’s mammary glands can’t produce and secrete
   milk without the synergistic interaction of three other
   hormones: prolactin, oxytocin, and estrogen.


3. Permissive interaction. One hormone can exert its
   effect on a target cell only when a different hormone
   first “primes” the target cell. For example, even if one
   of a woman’s eggs is fertilized, she can’t become
   pregnant unless the lining of her uterus has been
   exposed to reproductive hormones.

15.1

Type Route to Target cells

Hormones Carried by blood to distant targets

Neurotransmitters Released at synapses between neurons

and target cells

Prostaglandins Released in tissues and diffuse to target

cells

Pheromones Possibly reach target cells in other

individuals

Table 15.1 Examples of Chemical Signals in the Body

286 Chapter 15

Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).