Download The Endocrine System 1 Prof. Dr. Sa`ad Merza Alaraji

The Endocrine System 1
Prof. Dr. Sa'ad Merza Alaraji
University Of Babylon ,College Of Medicine, Dep. Of Medical Physiology
The Endocrine System (1)
The Endocrine System And Homeostasis:
Circulating or local hormones of the endocrine system contribute to
homeostasis by regulating the activity and growth of target cells in your body.
Hormones also regulate your metabolism.
As girls and boys enter puberty, they start to develop striking differences in
physical appearance and behavior. Perhaps no other period in life so
dramatically shows the impact of the endocrine system in directing development
and regulating body functions.
In girls, estrogens promote accumulation of adipose tissue in the breasts and
hips, sculpting a feminine shape.
At the same time or a little later, increasing levels of testosterone in boys begin
to help build muscle mass and enlarge the vocal cords, producing a lower
pitched voice.
These changes are just a few examples of the powerful influence of endocrine
secretions.
Less dramatically, perhaps, multitudes of hormones help maintain homeostasis
on a daily basis. They regulate the activity of smooth muscle, cardiac muscle,
and some glands; alter metabolism; spur growth and development; influence
reproductive processes; and participate in circadian (daily) rhythms established
by the suprachiasmatic nucleus of the hypothalamus.
Comparison Of Control By The Nervous And Endocrine Systems
The nervous and endocrine systems act together to coordinate functions of all
body systems. Recall that the nervous system acts through nerve impulses
(action potentials) conducted along axons of neurons. At synapses, nerve
impulses trigger the release of mediator (messenger) molecules called
Neurotransmitters (shown in Figure 1).
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The Endocrine System 1
Prof. Dr. Sa'ad Merza Alaraji
University Of Babylon ,College Of Medicine, Dep. Of Medical Physiology
Figure 1: Signal transmission at a chemical synapse. Through exocytosis of synaptic vesicles, a
presynaptic neuron releases neurotransmitter molecules. After diffusing across the synaptic cleft, the
neurotransmitter binds to receptors in the plasma membrane of the postsynaptic neuron and
produces a postsynaptic potential.
At a chemical synapse, a presynaptic neuron converts an electrical signal (nerve impulse) into a
chemical signal (neurotransmitter release). The postsynaptic neuron then converts the chemical signal
back into an electrical signal (postsynaptic potential).
The endocrine system also controls body activities by releasing mediators, called
hormones, but the means of control of the two systems are very different.
A hormone (hormon = to excite or get moving) is a mediator molecule that is
released in one part of the body but regulates the activity of cells in other parts of
the body. Most hormones enter interstitial fluid and then the bloodstream. The
circulating blood delivers hormones to cells throughout the body. Both
neurotransmitters and hormones exert their effects by binding to receptors on or
in their “target” cells. Several mediators act as both neurotransmitters and
hormones. One familiar example is norepinephrine, which is released as a
neurotransmitter by sympathetic postganglionic neurons and as a hormone by
chromaffin cells of the adrenal medullae.
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The Endocrine System 1
Prof. Dr. Sa'ad Merza Alaraji
University Of Babylon ,College Of Medicine, Dep. Of Medical Physiology
Responses of the endocrine system often are slower than responses of the
nervous system; although some hormones act within seconds, most take several
minutes or more to cause a response. The effects of nervous system activation
are generally briefer than those of the endocrine system. The nervous system acts
on specific muscles and glands. The influence of the endocrine system is much
broader; it helps regulate virtually all types of body cells.
We will also have several opportunities to see how the nervous and endocrine
systems function together as an interlocking “supersystem.” For example, certain
parts of the nervous system stimulate or inhibit the release of hormones by the
endocrine system.
Table 1 compares the characteristics of the nervous and endocrine systems. In
this chapter, we focus on the major endocrine glands and hormone-producing
tissues and examine how their hormones govern body activities.
Table 1: Comparison of Control by the Nervous and Endocrine Systems
CHARACTERISTIC
NERVOUS SYSTEM
ENDOCRINE SYSTEM
Mediator molecules
Neurotransmitters released locally Hormones
delivered
to
in response to nerve impulses.
tissues throughout the body
by the blood.
Site
of
mediator Close to site of release, at a Far from site of release
action
synapse; binds to receptors in (usually); binds to receptors
postsynaptic membrane.
on or in target cells
Types of target cells
Muscle (smooth, cardiac, and Cells throughout the body.
skeletal) cells, gland cells, other
neurons.
Time to onset of Typically
within
milliseconds Seconds to hours or days.
action
(thousandths of a second).
Duration of action
Generally briefer (milliseconds).
Generally longer (seconds to
days).
Endocrine Glands
The body contains two kinds of glands: exocrine glands and endocrine glands.
Exocrine glands (exo- = outside) secrete their products into ducts that carry the
secretions into body cavities, into the lumen of an organ, or to the outer surface
of the body. Exocrine glands include sudoriferous (sweat), sebaceous (oil),
mucous, and digestive glands.
Endocrine glands (endo- = within) secrete their products (hormones) into the
interstitial fluid surrounding the secretory cells rather than into ducts. From the
interstitial fluid, hormones diffuse into blood capillaries and blood carries them
to target cells throughout the body. Because most hormones are required in very
small amounts, circulating levels typically are low.
The endocrine glands include the pituitary, thyroid, parathyroid, adrenal, and
pineal glands (Figure 2). In addition, several organs and tissues are not
exclusively classified as endocrine glands but contain cells that secrete
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The Endocrine System 1
Prof. Dr. Sa'ad Merza Alaraji
University Of Babylon ,College Of Medicine, Dep. Of Medical Physiology
hormones. These include the hypothalamus, thymus, pancreas, ovaries, testes,
kidneys, stomach, liver, small intestine, skin, heart, adipose tissue, and placenta.
Taken together, all endocrine glands and
hormone-secreting cells constitute the endocrine system. The science of the
structure and function of the endocrine glands and the diagnosis and treatment of
disorders of the endocrine system is endocrinology.
Figure 2: Location of many endocrine glands. Also shown are other organs that contain endocrine
cells and associated structures.
Endocrine glands secrete hormones, which circulating blood delivers to target tissues.
Thank You
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