1.
Overview of the Endocrine System
Function: Regulates metabolism, growth, reproduction, and homeostasis using
hormones.
Hormones: Chemical messengers transported through the bloodstream.
Types of Hormones
Amino acid-based: Peptides, proteins, or derivatives.
Steroids: Derived from cholesterol (lipid-soluble).
2. Mechanisms of Hormone Action
Hormones bind to specific target cells and alter their activity.
Effects include:
Changing membrane permeability.
Enzyme activation/deactivation.
Stimulating protein synthesis.
Inducing secretion or mitosis.
Hormonal Signaling Pathways
1. Second Messenger System (Water-Soluble Hormones)
Hormone binds to membrane receptor → Activates G-protein → Stimulates
cAMP production → Activates protein kinases → Triggers cell response.
2. Direct Gene Activation (Lipid-Soluble Hormones)
Hormone enters cell nucleus, binds DNA, and stimulates mRNA transcription →
Protein synthesis.
3. Hormonal Regulation & Control of Release
1. Humoral Stimuli:
Blood ion/nutrient levels regulate hormone release.
Example: Low calcium → PTH secretion from the parathyroid gland.
2. Neural Stimuli:
Nerve signals trigger hormone release.
Example: Sympathetic stimulation of adrenal medulla → Epinephrine release
(fight-or-flight response).
3. Hormonal Stimuli:
One hormone regulates another hormone’s release.
Example: Hypothalamus releases TRH → Stimulates pituitary to release TSH →
Stimulates thyroid hormone production.
4. Endocrine Organs & Their Hormones
A. Hypothalamus & Pituitary Gland ("Master Gland")
Hypothalamus
Controls hormone release from the pituitary gland.
Secretes releasing & inhibiting hormones via the hypophyseal portal system.
Pituitary Gland (Hypophysis)
Anterior Pituitary (Adenohypophysis) (Produces hormones):
Growth Hormone (GH): Stimulates growth, metabolism.
Thyroid-Stimulating Hormone (TSH): Stimulates thyroid hormone production.
Adrenocorticotropic Hormone (ACTH): Stimulates adrenal cortex → Releases
cortisol.
Follicle-Stimulating Hormone (FSH): Stimulates ovarian follicle/sperm production.
Luteinizing Hormone (LH): Triggers ovulation/testosterone production.
Prolactin (PRL): Stimulates milk production.
Posterior Pituitary (Neurohypophysis) (Stores hormones from hypothalamus):
Oxytocin: Stimulates uterine contractions, milk ejection, bonding.
Antidiuretic Hormone (ADH/Vasopressin): Regulates water balance, reduces
urine output.
B. Pineal Gland
Secretes melatonin → Regulates sleep-wake cycles.
Light exposure suppresses melatonin production.
C. Thyroid Gland
Located in the neck (C5-T1), highly vascularized.
Produces thyroid hormones (T3, T4) → Increase metabolism, body temperature, and
growth.
Calcitonin (from C-cells) → Lowers blood calcium by inhibiting bone resorption.
D. Parathyroid Glands
Secrete Parathyroid Hormone (PTH)
Increases blood calcium
by:
Stimulating osteoclasts to break down bone.
Enhancing calcium reabsorption in kidneys.
Promoting Vitamin D activation → Increases calcium absorption in intestines.
E. Adrenal Glands
Located above kidneys; consist of cortex & medulla.
Adrenal Cortex (Steroid Hormones)
1. Aldosterone (Mineralocorticoid):
Regulates sodium & potassium balance.
Controlled by renin-angiotensin system (RAAS).
2. Cortisol (Glucocorticoid):
Increases blood sugar & suppresses immune response.
Released in response to ACTH from pituitary.
3. Androgens (Sex Hormones):
Precursors to testosterone & estrogen.
Adrenal Medulla (Catecholamines)
Epinephrine & Norepinephrine (Sympathetic activation → "Fight-or-flight" response).
F. Pancreas (Endocrine & Exocrine Functions)
Pancreatic Islets (Islets of Langerhans)
Alpha Cells → Glucagon: Raises blood glucose.
Beta Cells → Insulin: Lowers blood glucose.
Glucagon Functions:
Glycogenolysis (Breakdown of glycogen to glucose).
Gluconeogenesis (Synthesis of glucose from non-carbohydrates).
Insulin Functions:
Promotes glucose uptake into cells.
Stimulates glycogen synthesis.
Inhibits fat & protein breakdown.
G. Gonads (Ovaries & Testes)
Female Reproductive Hormones (Ovaries)
Estrogen
Regulates menstrual cycle, secondary sex characteristics.
Stimulates endometrial growth.
Progesterone
Maintains pregnancy.
Prepares uterus for implantation.
Male Reproductive Hormones (Testes)
Testosterone
Regulates sperm production, libido, muscle mass.
Promotes secondary sexual characteristics (hair, deep voice, etc.).
5. Hormonal Regulation of the Menstrual Cycle
1. Follicular Phase (Days 1-14):
FSH stimulates follicle growth → Estrogen release.
Estrogen inhibits FSH (negative feedback).
Prepares endometrium for implantation.
2. Ovulation (~Day 14):
LH surge → Triggers ovulation (egg release).
3. Luteal Phase (Days 15-28):
Corpus luteum releases progesterone & estrogen → Prepares for pregnancy.
If no pregnancy → Corpus luteum degenerates → Progesterone & estrogen
drop → Menstruation begins.
6. Hormonal Regulation of Stress Response
Hypothalamus releases CRH → Stimulates pituitary to release ACTH → Stimulates
adrenal cortex to release cortisol.
Effects of Cortisol
Increases blood sugar (gluconeogenesis).
Suppresses immune function.
Enhances metabolism during stress.
Final Notes
The endocrine system regulates nearly all physiological processes using hormonal
signaling.
Hormones control growth, metabolism, reproduction, and stress responses.
Feedback mechanisms maintain homeostasis (e.g., negative feedback for cortisol,
insulin, and thyroid hormones).
