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370 Hormones

Basics subject to hormonal regulation are the up-
take and degradation of storage substances
Hormones are chemical signaling substances. (glycogen, fat), metabolic pathways for bio-
They are synthesized in specialized cells that synthesis and degradation of central me-
are often associated to form endocrine glands. tabolites (glucose, fatty acids, etc.), and
Hormones are released into the blood and the supply of metabolic energy.
transportedwith the bloodto their effector • Digestive processes
organs. In the organs, the hormones carry Digestive processes are usually regulated
out physiological and biochemical regulatory by locally acting peptides (paracrine; see
functions. In contrast to endocrine hormones, p. 372), but mediators, biogenic amines,
tissue hormones areonlyactivein the imme- and neuropeptides are also involved (see
diate vicinity of the cells that secrete them. p. 270).
The distinctions between hormones and • Maintenance of ion concentrations (ho-
other signaling substances (mediators, neuro- meostasis)
–
+
+
transmitters, and growth factors) are fluid. Concentrations of Na ,K ,and Cl in body
Mediators is the term used for signaling sub- fluids, and the physiological variables de-
stances that do not derive from special hor- pendent on these (e. g. blood pressure), are
mone-forming cells, but are form by many cell subject to strict regulation. The principal
types. They have hormone-like effects in their site of action of the hormones involved is
immediate surroundings. Histamine (see the kidneys, where hormones increase or
p. 352) and prostaglandins (see p. 390) are reduce the resorption of ions and recovery
important examples of these substances. of water (see pp. 326–331). The concentra-
Neurohormones and neurotransmitters are tions of Ca 2+ and phosphate, which form
signaling substances that are produced and the mineral substance of bone and teeth,
released by nerve cells (see p. 348). Growth are also precisely regulated.
factors and cytokines mainly promote cell Many hormones influence the above pro-
proliferation and cell differentiation (see cesses only indirectly by regulating the syn-
p. 392). thesis and release of other hormones (hormo-
nal hierarchy; see p. 372).

A. Hormones: overview
B. Hormonal regulation system
The animal organism contains more than 100
hormones and hormone-like substances, Each hormone is the center of a hormonal
which can be classified either according to regulation system. Specialized glandular cells
their structure or according to their function. synthesize the hormone from precursors,
In chemical terms, most hormones are amino store it in many cases, and release it into the
acid derivatives, peptides or proteins, or ste- bloodstream when needed (biosynthesis). For
roids. Hormones regulate the following pro- transport, the poorly water-soluble lipophilic
cesses: hormones are bound to plasma proteins
• Growth and differentiation of cells, tissues, known as hormone carriers. To stop the ef-
and organs fects of the hormone again, it is inactivated by
Theseprocesses includecellproliferation, enzymatic reactions, most of which take place
embryonic development, and sexual dif- in the liver (metabolism). Finally, the hor-
ferentiation—i. e., processes that require a mone and its metabolites are expelled via
prolonged time period and involve proteins the excretory system, usually in the kidney
de novo synthesis. For this reason, mainly (excretion). All of these processes affect the
steroid hormones which function via tran- concentration of the hormone and thus con-
scription regulation are active in this field tribute to regulation of the hormonal signal.
(see p. 244). Intheeffectororgans,targetcellsreceivethe
• Metabolic pathways hormone’smessage. Thesecellshavehormone
Metabolic regulation requires rapidly act- receptors for thepurpose,which bind thehor-
ing mechanisms. Many of the hormones mone. Binding of a hormone passes informa-
involved therefore regulate interconversion tion to the cell and triggers a response (effect).
of enzymes (see p. 120). The main processes

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