IV. Endocrine System
A. Overall functions
1. Regulatory System
2. Maintains homeostasis internally
3. Responds to help cope with environmental changes
4. Growth & development
5. Reproduction
B. Exocrine vs Endocrine glands
Exocrine releases contents into ducts or skin surface, endocrine glands are ductless and release contents into blood
C. Hormone
1. Definition – Chemical messenger carried by bloodstream, made by endocrine glands
2. Structure
Organic, amino acids, cholesterol derivatives
3. Transport
Many are bound to specific carrier proteins
4. Action
a. Target cell
b. Receptors
1. Plasma membrane
2. Within target cell
c. Second messenger cAMP
1. Stimulatory
2. Inhibitory
d. Activation of genes
5. Hormonal interrelationships
6. N.S. & Endocrine relationships
D. Controls
1. Negative feedback
2. Releasing or inhibiting hormones- Hypothalamus
E. Pituitary Gland {Hypophysis}
1. Location
Sella turcica of sphenoid bone
2. Adenohypophysis
Anterior lobe of pituitary gland
a. Gonadotropins
– Stimulates gonads
–FSH – Follicle stimulating hormone, stimulates follicle development in ovaries, stimulates sperm production and maturation in male
–LH / ICSH Luteinizing hormone results in ovulation in female, Interstitial cell stimulating hormone results in testosterone production in male
b. TSH -Thyroid stimulating hormone
c. ACTH -Adrenocorticotropic hormone
d. GH -Growth hormone
e. Prolactin– Initiates and maintains milk production in female
f. MSH – Melanocyte stimulating hormonestimulates skin melanin (pigment)
g. Endorphins – Peptides made by Adenohypophysis in addition to the hormones that has opiate-like effects
3. Controls
a. Inhibiting/releasing hormones (GnRH Gondadotropin releasing hormone, TRH Thyrotropin releasing hormone, PIF Prolactin inhibiting factor, CRF Corticotropin releasing factor, Somatostatin Growth hormone inhibiting hormone)
b. Brain (Emotions)
c. Feedback
Negative feedback mechanisms prevail
4. Neurohypophysis
Posterior lobe of pituitary gland
a. Infundibulum (Hypothalamic-hypophyseal tract)
b. ADH (vasopressin) – Antidiuretic hormone causes water reabsorption from kidney to bloodstream
c. Oxytocin
Uterine contractions for childbirth and milk letdown
F. Thyroid Gland
1. Location
Inferior larynx
2. Histology (follicle cells, colloid)
3. Thyroid hormones
a. Iodine
b. T4 (thyroxine) 90%
c. T3(triiodothyronine) 10%
d. TBG – Most thyroid hormones are carried in plasma bound to thyroid binding globulin
e. Regulates metabolic rate
f. Controls
TSH, TRH, negative feedback
4. Calcitonin
a. Parafollicular = C cells
b. Decreases Ca & P
c. Controls
If blood calcium levels are high, Calcitonin is secreted
G. Parathyroid glands
1. Location
4 – embedded in posterior surface of thyroid gland
2. PTH (=Parathormone)function (increases Ca; decreases P)
3. Controls
If blood calcium levels are low, PTH is secreted
H. Adrenal gland
1. Location
Superior to kidney
2. Adrenal medulla – Inner portion
a. Chromaffin cells
b. Epinephrine(80%), Norepinephrine (20%)
c. Action (glucose, ht, bl. vessels, bl. pressure, airways)
d. Release (Pregang. neurons, excitement, injury)
3. Adrenal cortex – Outer portion
a. Mineralocorticoids
1. Zona glomerulosa – Outermost layer
2. Aldosterone(90%)
3. Na reabsorption, K excretion – In kidneys
4. Release/controls
a. Renin/Angiotensinogen
Renin is an enzyme made by the kidneys in response to low blood pressure. It causes inactive angiotensinogen to activate into Angiotensin, which stimulates Aldosterone release.
b. High K, Low Na
c. ACTH, CRH
b. Glucocorticoids
1. Zona fasciculata – Very thick middle layer
2. Conserves & supplements energy
3. Anti-inflammatory
4. Release/controls
a. ACTH, CRH
b. Stress
c. Sex hormones
1. Zona reticularis – Innermost layer
I. Pancreas
1. Location
Behind stomach and along duodenum (first part of small intestine)
2. Exocrine & endocrine
3. Pancreatic islets (of Langerhans) – ENDOCRINE portion
4. Insulin(beta cells, lowers blood glucose)
5. Glucagon(alpha cells, raises blood glucose)
6. Somatostatin(delta cells, inhibits growth hormone release)
7. Release/Controls
a. Blood glucose level
b. GI hormones – meals/between meals
c. ANS
J. Gonads
Testes make androgens to be discussed later, Ovaries make estrogens and progesterones to be discussed later.
K. Pineal gland
1. Location
Roof of third ventricle in brain
2. Melatonin Helps regulate circadian rhythms including sleep. Darkness stimulates melatonin release naturally. ( Melatonin inhibits gonadotropin-releasing hormone from hypothalamus in humans, thus inhibiting some reproductive functions. In animals that breed in springtime, as daylight increases, melatonin decreases. Therefore the inhibitory actions of reproduction stops and animals can breed.)
L. Thymus
1. Location
Within mediastinum above heart
2. Puberty
Decreases in size as we mature
3. Immunity
Thymic hormone stimulates T cells after they leave the thymus
M. Prostaglandins
1. Fatty acids
2. Localized
3. Functions vary
Useful Videos:
Crash Course A&P Endocrine System Part 1