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Summary Human Endocrine System
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HUMAN ENDOCRINE SYSTEMGENERAL
- Human body must adapt to external changes, this is accomplished by maintaining
constant internal equilibrium in body [homeostasis].
- Nervous system and hormones enable animals to respond to external changes and to
control conditions inside body.
- The endocrine system consists of endocrine glands that secrete organic compounds,
chemical messengers called hormones.
- Different endocrine glands don’t function in isolation but interact continuously by
means of their hormone secretions.
- This contributes to maintenance of homeostasis in that all metabolic activities in body
are controlled and coordinated by endocrine system.
- Metabolic activities either anabolic [build things up] or catabolic [break things
down].
- The type of control and coordination brought by hormones in blood is known as
chemical coordination.
Endocrine glands
Ductless glands that release their secretions [hormones] directly into bloodstream.
Blood transports the hormones to target organs where they perform their function.
Exocrine glands
Glands that release their secretions via ducts to a body cavity / to the outside [e.g.
salivary glands that secrete saliva into ducts that open in the mouth cavity].
HORMONES
- Organic chemical messengers.
- Most are proteins, some are steroids [lipid like organic compounds].
- Secreted in small quantities directly into bloodstream by endocrine glands.
- Transported to target organs [an organ affected by specific hormone. Has receptors
for this specific hormone, enabling it to respond to hormone] by bloodstream.
- Have regulatory function that can be stimulating or inhibiting.
- May be regulated by negative / positive feedback mechanism.
NEGATIVE FEEDBACK
- In negative feedback mechanism, any deviation from normal levels of a
substance or temp elicits a response that restores normal levels.
Example: Hormone A and B
- Hormone A stimulates secretion of hormone B. Hormone B levels rise.
- Hormone B inhibits secretion of hormone A. Hormone A levels decrease.
- Lower levels of hormone A reduces stimulation of secretion of hormone B.
Hormone B levels also drop.
- Lower levels of hormone B don’t inhibit secretion of hormone A anymore.
Hormone A levels rise and return to normal.
- In this way both levels of hormones are kept in balance through ongoing cycles.
, PITUITARY GLAND [HYPOPHYSIS]
- Located at base of brain, attached to hypothalamus by a short stalk. It fits into a
small bony cavity in cranium.
- Hypothalamus is part of brain located just below corpus callousness and controls
functions of pituitary gland. It is the link between nervous and endocrine systems.
- Pituitary gland consists of two lobes;
o Anterior Kobe [adenohypophysis].
o Posterior lobe [neurohypophysis].
- Pituitary gland originally known as “master gland” of body, because it secretes many
hormones that affect many of the other endocrine glands.
- Relationship between hypothalamus and pituitary gland;
o Hypothalamus is connected to pituitary gland in two ways:
▪ By blood vessels to anterior lobe.
▪ By neurons to posterior lobe.
o Hypothalamus produces hormones which are transported via neurons to
posterior lobe of pituitary gland for storage.
o Hypothalamus controls pituitary gland by secreting releasing factors via
neurons [to posterior lobe] or via blood [to anterior lobe]. The releasing factors
stimulate the pituitary gland to produce and/or release hormones into blood.
HORMONES OF ANTERIOR LOBE
TSH [THYROID STIMULATING HORMONE]
- Stimulates thyroid to secrete hormone thyroxin.
GROWTH HORMONE
- Promotes growth of skeleton and muscles by stimulating synthesis of proteins.
- Growth disorders;
o Caused by abnormal secretion of growth hormone.
o Under-secretion of growth hormone in children causes dwarfism. Body is very
short, w. body parts in proportion. Mental ability is normal, but person doesn’t
reach sexual maturity. Some cases of dwarfism are genetic conditions: in
these cases, body parts are out of proportion.
o Over-secretion of growth hormone in children causes gigantism. Usually a
result of a tumor in pituitary gland.
o Over-secretion of growth hormone in adults causes acromegaly. The long
bones in body have completed growth and cannot lengthen anymore.
Growth plates in epiphyses of long bones close during puberty. The bone of
face, especially jawbone, and hands and feet increase in size.
Under-secretion: hyposecretion.
Over-secretion: hypersecretion.
FSH [FOLLICLE-STIMULATING HORMONE]
- Stimulates development of primary follicles in female ovary into mature Graafian
follicles.
- In males it, activates germinal epithelium that produces sperm cells in testes.
LH [LUTEINISING HORMONE]
- In females, it stimulates ovulation [release of secondary oocyte] and the
development of corpus luteum in ovary.
- In males, it stimulates the cells of Leydig, in testes, to produce male hormone
testosterone.
PROLACTIN
- Stimulates mammary glands to produce milk after childbirth.
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