17.1 Endocrine

-The hypothalamus is an important control center for hormone

action, so it is important to know its anatomy and functions.

-Many hormones are proteins and other peptides. In discussing

the synthesis of this class of hormones, it will be assumed

that you are familiar with protein structure and synthesis.

-Many hormones exert their effects by binding to proteins of

the plasma membranes of their target cells. It is important to

be familiar with membrane proteins as receptors and with G

proteins, cAMP, and second messengers.

-A review of the transport maximum (Tm) of membrane proteins

will enhance your understanding of diabetes mellitus in this chapter.

If the body is to function as an integrated whole, its organs

must communicate with each other and coordinate their activities.

Even simple organisms composed of only a few cells have

mechanisms for intercellular communication, suggesting that

such mechanisms evolved very early in the history of life. In humans,

two such systems are especially prominent—the nervous

and endocrine systems, which communicate with neurotransmitters

and hormones, respectively.

Nearly everyone has heard of at least some hormones—

growth hormone, thyroid hormone, estrogen, and insulin, for

example. At least passingly familiar, too, are some of the glands

that secrete them (such as the pituitary and thyroid glands) and

some disorders that result from hormone deficiency, excess, or

dysfunction (such as diabetes, goiter, and dwarfism).

This chapter is primarily about the endocrine (hormonal) system

of communication. We will start with the relatively familiar

and large-scale aspects of this system—a survey of the endocrine

glands, their hormones, and the principal effects of those

hormones. We will then work our way down to the finer and less

familiar details—the chemical identity of hormones, how they

are made and transported, and how they produce their effects

on their target cells. Shorter sections at the end of the chapter

discuss the role of the endocrine system in adapting to stress,

some hormonelike paracrine secretions, and the pathologies

that result from endocrine dysfunction.

The body has four principal avenues of communication from cell

to cell:

-Gap junctions join single-unit smooth muscle, cardiac

muscle, epithelial, and other cells to each other. They enable

cells to pass nutrients, electrolytes, and signaling molecules

directly from the cytoplasm of one cell to the cytoplasm

of the next through pores in their plasma membranes.

-Neurotransmitters are released by neurons, diffuse across a

narrow synaptic cleft, and bind to receptors on the surface of

the next cell.

-Paracrines are secreted by one cell, diffuse to nearby cells

in the same tissue, and stimulate their physiology. Some call

them local hormones.

-Hormones, in the strict sense, are chemical messengers

that are transported by the bloodstream and stimulate physiological

responses in cells of another tissue or organ, often a

considerable distance away. Certain hormones produced by

the pituitary gland in the head, for example, act on organs as

far away as the pelvic cavity.

This chapter is concerned mainly with hormones and, to

some extent, paracrine secretions. The glands, tissues, and cells

that secrete hormones constitute the endocrine system; the study

of this system and the diagnosis and treatment of its disorders is

called endocrinology. The most familiar hormone sources are the

organs traditionally recognized as endocrine glands, such as the

pituitary, thyroid, and adrenal glands, among others.

Growing knowledge of endocrinology has revealed, however, that

hormones are also secreted by numerous organs and tissues not

usually thought of as glands, such as the brain, heart, small intestine,

bones, and adipose tissue.

Comparison of Endocrine and Exocrine Glands

The classical distinction between exocrine and

endocrine glands has been the presence or absence of ducts. Most

exocrine glands secrete their products by way of a duct onto an

epithelial surface such as the skin or the mucosa of the digestive

tract. Endocrine glands, by contrast, are ductless and release their

secretions into the bloodstream. For this reason, hormones were originally called the body’s “internal secretions”; the word endocrine still alludes to this fact. Exocrine secretions have extracellular effects such as the digestion of food, whereas endocrine secretions have intracellular effects—they alter cell metabolism.

As we see in the photo that opens this chapter, endocrine

glands have an unusually high density of blood capillaries; these

serve to pick up and carry away their hormones. These vessels are

an especially permeable type called fenestrated capillaries, which

have patches of large pores in their walls allowing for the easy uptake

of matter from the gland tissue.

Some glands and secretory cells defy simple classification as

endocrine or exocrine. Liver cells, for example, behave as exocrine

cells in the traditional sense by secreting bile into ducts that lead

ultimately to the small intestine. However, they also secrete hormones

into the blood, and in this respect they act as endocrine

cells. They secrete albumin and blood-clotting factors directly into

the blood as well. These do not fit the traditional concept of exocrine

secretions, because they are not released by way of ducts or

onto epithelial surfaces; nor do they fit the concept of endocrine secretions, because they are not hormones. Liver cells are just one of nature’s myriad ways of confounding our impulse to rigidly classify things.

Comparison of the Nervous and Endocrine Systems

The nervous and endocrine systems both serve for internal communication,

but they are not redundant; they complement rather

than duplicate each other’s function. One important

difference is the speed with which they start and stop responding

to a stimulus. The nervous system typically responds within a few

milliseconds, whereas it takes from several seconds to days for a

hormone to act. When a stimulus ceases, the nervous system stops

responding almost immediately, whereas hormonal effects can last

for days or even longer. On the other hand, under long-term stimulation,

most neurons quickly adapt and their response declines. The

endocrine system shows more persistent responses.

Another difference is that an efferent nerve fiber innervates

only one organ and a limited number of cells within that organ,

so its effects are usually precisely targeted and relatively specific.

Hormones, by contrast, circulate throughout the body and some of

them, such as growth hormone and epinephrine, have more widespread

effects than any one nerve fiber does.

But these differences should not blind us to the similarities between

the two systems. Both communicate chemically, and several

chemicals function as both neurotransmitters and hormones—for

example, norepinephrine, dopamine, and antidiuretic hormone (arginine

vasopressin). Thus, a particular chemical such as dopamine

can be considered a hormone when secreted by an endocrine cell

but a neurotransmitter when secreted by a nerve cell. Another similarity

is that some hormones and neurotransmitters produce identical

effects on the same organ. For example, both norepinephrine

and glucagon stimulate the liver to break down glycogen and

release glucose. The nervous and endocrine systems continually

regulate each other as they coordinate the activities of other organ

systems. Some neurons trigger hormone secretion, and some hormones

stimulate or inhibit neurons.

Some cells defy any attempt to rigidly classify them as neurons

or gland cells. They act like neurons in many respects,

but like endocrine cells, they release their secretions (such as

oxytocin) into the bloodstream. Thus, we give them a hybrid

name—neuroendocrine cells.

We have seen that neurotransmitters depend on receptors in the receiving cell; they cannot exert any effect unless the receiving cell is equipped to bind and respond to them. This is true of hormones as well. When a hormone enters the bloodstream, it goes wherever the blood goes; there is no way to send it selectively to a particular organ. However, only certain target organs or target cells respond to it. Thyroid-stimulating hormone, for example, circulates everywhere the blood goes, but stimulates only the thyroid gland. In most cases, such selective responses are because only the target cells have receptors for a given hormone. They can also occur, however, because the circulating hormone is inactive and only the target cells have the enzyme needed to convert it to active form. Circulating testosterone, for example, is relatively inactive, but its target cells have an enzyme that converts it to dihydrotestosterone, which is much more potent.