Endocrine System
The body has two communication systems to coordinate the activities of different tissues and
organs. One communication system is the nervous system
and the other is the endocrine system. The endocrine system is the network
of glands and tissues that secrete hormones, chemical messengers produced in one tissue and carried
by the blood to act on other parts of the body. Compared to the nervous system, the signals
conveyed by the endocrine system take much more time. A nervous impulse is produced in a millisecond
and travels anywhere in the body in less than a second. Hormones require time to be synthesized,
can travel no more quickly than the blood can carry them, and often cause actions through inducing
protein synthesis or transcription that require time. However, hormone signals will tend to be more
long-lasting than nerve impulses. When the nerve impulse ends, a target such as skeletal muscle
returns quickly to its starting state. When a hormone induces protein synthesis, the proteins
remain long after the hormone is gone. Often the two systems work together. The endocrine glands,
such as the pancreas or the adrenal cortex, can be the direct targets (effectors) of the autonomic
nervous system. The hormone adrenaline acts in concert with the sympathetic nervous system to
produce a set of results similar to those produced directly by sympathetic neurons.
Many in this class plan to major in psychology. Because of this, we will go into the endocrine
system in more detail than the other systems. The action of hormones can have profound effects on
psychology.
Endocrine glands secrete hormones directly into the bloodstream. This is in contrast to exocrine
secretions that do not contain hormones and are released through ducts into a body compartment. An
example of exocrine secretion is the secretion by the pancreas of digestive enzymes into the small
intestine through the pancreatic duct. Both endocrine and exocrine functions can be found in the
same organ. The pancreas simultaneously produces exocrine secretions like digestive enzymes and
endocrine secretions like insulin and glucagon that are released into the blood to exert their
effects throughout the body.
This animation (Audio - Important) describes the glands of
the endocrine system.
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