Chapter 18: Endocrine System
I.
Endocrine System
A.
The endocrine system & nervous system work together to regulate/coordinate functions
of all body systems but by they do so by different means
i.
Nervous system
a.
Certain parts release hormones into the blood
b.
Other parts release neurotransmitters which excite or inhibit nerve, muscle
& gland cells (results can be seen within milliseconds and exhibits a brief
duration of effects)
II.
Endocrine vs. Exocrine
A.
Exocrine gland - secretes substances through ducts to the outside of the body
B.
Endocrine gland - secretes substances (hormones) into interstitial fluid (tissue found
between cells) surrounding secretory cells; the hormones then diffuse into blood
capillaries, enter the bloodstream, and travel through the body until they reach target cells
with the appropriate receptor sites / molecules (results may take hours but last longer)
III.
Hormones
A.
Hormones influence target cells by binding to specific receptors
B.
Autocrines - hormones that affect the cell that secreted them
C.
Pancrines - hormones that affect neighboring cells
D.
Circulatory hormones - pass into interstitial fluid & then into the bloodstream
E.
Local hormones - act on the neighboring cells or the same cell (autocrines & pancrines)
F.
Down regulation - decrease in the number of target cell receptors in response to an
excess of a hormone (likened to turning down the volume); lowers sensitivity of the
target cell
G.
Up regulation - increase in the number of target cell receptors in response to a deficiency
of a hormone (likened to turning up the volume); increases sensitivity of the target cell
H.
Secretion of hormones is controlled by:
i.
Other hormones
ii.
Signals from the nervous system
iii.
Chemical changes in the blood
I.
3 main mechanisms in which hormones interact with each other
i.
Permissive - exposure of a target to a second hormone is required for the first
hormone to have an effect on the target cell
ii.
Synergistic - the effect of two hormones together is greater than the sum of their
acting alone
iii.
Antagonistic - when one hormone opposes the action of another
IV.
Water Soluble Hormones vs. Lipid Soluble Hormones
A.
Water soluble hormones
i.
Bind to receptors on the surface / plasma membrane of a target cell
ii.
Can't get through the interior of a phospholipids plasma membrane
iii.
Can circulate in a "free" form through the blood stream
iv.
Initiate a cascading chain reaction involving enzymes where each step in the
domino effect multiples the effect of the next step
v.
Three main categories - amine hormones; peptide & protein hormones; eicosanoid
hormones
B.
Lipid soluble hormones
i.
ii.
iii.
iv.
C.
Activate receptors in the cytoplasm / cytosol or nucleus
Bound to a transport protein
Activate specific genes for making a given protein
Crosses the plasma membrane and activates receptors in the cytosol or nucleus;
genes are activated in response to lipid-soluble hormones combining with receptor
molecules; the proteins produced fulfill specific functions
v.
Three main categories - steroid hormones; thyroid hormones; nitric oxide
hormones
In contrast to water soluble hormones which activate specific pathways, lipid-soluble
hormones activate specific gene; each gene codes for a given protein
V.
Hormones of the Anterior Pituitary Gland
A.
Human growth hormone (hGH) or somatotropin - stimulates liver, muscle, bone, cartilage
& other tissues to synthesize or secrete IGF. IGF's promote growth of body cells, protein
synthesis, tissue repair, lypolysis & elevation of blood glucose
B.
Thyroid stimulating hormone (TSH) or thyrotropin - stimulates synthesis and secretion
of hormones by the thyroid gland
C.
Follicle stimulating hormone (FSH) - in females, it initiates development of oocytes and
induces ovarian secretion of estrogen; in males, it stimulates the testes to produce sperm
D.
Luteinizing hormone (LH) - in females, it stimulates secretion of estrogens &
progesterone, ovulation & formation of the corpus luteum; in males, it stimulates
interstitial cells of the testes to develop & produce testosterone
E.
Prolactin (PRL) - together with other hormones, it promotes milk secretion by the
mammary glands
F.
Adrenocorticotrophic hormone (ACTH) or corticotropin - stimulates secretion of
glucocorticoids (mainly cortosol) by the adrenal cortex
G.
Melanocyte-stimulating hormone (MSH) - exact role in humans is not known; it may
influence brain activity; when it is present in excess, it can cause darkening of the skin
VI.
Hormones of the Posterior Pituitary Gland
A.
Oxytocin (OT) - stimulates contractions of smooth muscle cells of the uterus during
childbirth and also stimulates contraction of myoepithelial cells in mammary glands to
cause milk ejection
B.
Antidiuretic hormone (ADH) or vasopressin - conserve body water by decreasing urine
volume & decreasing perspiration; raises blood pressure by vasoconstriction of arterioles
VII.
Thyroid Gland
A.
Positioned on the anterior side of the trachea and above the sternum; weighs about one
ounce
B.
Considered the "gas pedal" or "throttle" for the body
C.
Thyroid hormone increases BMR (basal metabolic rate)
D.
The thyroid gland requires iodine to function
E.
The thyroid produces the following molecules
i.
T1 - one iodine atom
ii.
T2 - two iodine atoms
iii.
T3 - three iodine atoms (triodothyronine)
iv.
T4 - four iodine atoms (thyroxin)
v.
Calcitonin - lowers blood calcium levels
vi.
T1 & T2 are considered building blocks for hormones; T1 + T2 = T3; T2 + T2 = T4
vii.
T3 & T4 are considered hormones
VIII.
Parathyroid Gland
A.
Consists of four small glands on the posterior side of the thyroid
B.
Produces parathormone (parathyroid hormone) which raises blood calcium levels
i.
Calcitonin & parathormone are an antagonistic pair
IX.
Adrenal Glands
A.
Cortex - outer portion of adrenal gland; responsible for everyday housekeeping similar to
the parasympathetic division of the autonomic nervous system; controlled by ACTH;
effects on the adrenal cortex take minutes or hours and last a long time
i.
Zona glomerulosa - produce mineralcorticoids which regulate homeostasis of
sodium & potassium
ii.
Zona fasciculate - produce glucocorticoids which increase protein breakdown,
formation of glucose & lipid breakdown
iii.
Zona reticularis - produce androgens which regulate libido & secondary sex
characteristics; is a source of estrogen after menopause
B.
Medulla - inner portion of adrenal gland; responsible for fight or flight similar to the
sympathetic division of the autonomic nervous system; controlled by hypothalamus & the
sympathetic division of the autonomic nervous system; effects of the nervous system on
the adrenal medulla is milliseconds and last a short time
i.
Produces adrenaline (eponephrin) & noradrenaline (norepinephrine) which is
involved in the bodys fight or flight response to emergencies
X.
Pancreas
A.
Both endocrine & exocrine; the exocrine portion makes up the majority of digestive
enzymes; the endocrine portion is referred to as the islets of langerhans
B.
Pancreatic hormones
i.
Insulin - lowers blood glucose levels
ii.
Glucagon - increases blood glucose levels
iii.
Somatostatin - inhibits insulin & glucagons secretion and slows absorption of
nutrients by the GI tract
iv.
Pancreatic polypeptide - inhibits somatostatin secretion; inhibits gall bladder
contraction; inhibits secretion of pancreatic digestive enzymes
XI.
Ovaries & Testes
A.
Estrogens - made by the ovaries
B.
Progesterone - made by the ovaries
C.
Relaxin - made by the ovaries
D.
Inhibin -made by both testes and the ovaries
E.
Testosterone - made by the testes
XII.
Pineal Gland
A.
Produces melatonin which is thought to play a role in daily rhythms of sleeping and
waking as well as seasonal affective disorder
XIII.
Thymus
A.
Part of the immune & endocrine systems
B.
T cells mature in the thymus
i.
Maturation is influenced by the thymus hormone secretion
C.
The thymus, through its hormonal production, helps to slow the aging process
XIV. Factoids
A.
A gland can be both exocrine & endocrine (ex. pancreas)
B.
One hormone molecule can cause the release of millions of glucose molecules