Endocrine System Notes (Chapter 17)

Endocrine system: ductless glands secrete hormones into blood for distant target cells with specific receptors.
Hormones travel via bloodstream to target cells, where receptors determine specificity.

Endocrine glands (e.g., pituitary, thyroid) are ductless; some endocrine cells are in other organs (e.g., hypothalamus, pancreas).

Endocrine and nervous systems both use chemical messengers (ligands) that bind to receptors.
Differences: Endocrine system is widespread, slower, and has longer-lasting effects via blood transport. Nervous system is localized, rapid, and precise via nerve signals.

Major glands: pituitary, pineal, thyroid, parathyroid, adrenal.
Hormone secretion is regulated by three reflex mechanisms: hormonal, humoral, or nervous stimulation.

Circulating Hormones:
1. Steroids: Lipid-soluble, from cholesterol (e.g., estrogen, cortisol).
2. Biogenic amines: Moderately lipid-soluble to water-soluble (e.g., catecholamines, thyroid hormone [lipid-soluble]).
3. Proteins: Water-soluble polypeptide chains; most hormones.
Local Hormones: Do not circulate; act on releasing cells (autocrine) or neighboring cells (paracrine). Eicosanoids (e.g., prostaglandins) are derived from fatty acids.

Lipid-soluble hormones: Require carrier proteins in blood; only unbound hormone is active; generally have long half-lives.
Water-soluble hormones: Travel freely in blood; generally have short half-lives.
Hormone concentration depends on synthesis and elimination rates.
Half-life ( $t<br>_{1/2}$ ) is the time to reduce hormone concentration by half.

Lipid-soluble hormones: Diffuse across membrane, bind to intracellular receptors (cytosol/nucleus), form hormone-receptor complex, which binds DNA (HRE) to stimulate gene transcription and protein synthesis.
Water-soluble hormones: Bind to membrane receptors, initiate signal transduction pathways via G-proteins and second messengers (e.g., cAMP, IP3/DAG/Ca2+), resulting in amplified cellular responses.

Cellular response depends on receptor density and hormone interactions.
Receptor regulation: Up-regulation (increased receptors, sensitivity) and down-regulation (decreased receptors, sensitivity).
Hormone interactions: Synergistic (greater combined effect), permissive (one enables another), antagonistic (one opposes another).

Pituitary gland (anterior/posterior) is connected to hypothalamus by infundibulum.
Posterior pituitary (neurohypophysis): Stores and releases ADH (decreases urine and vasoconstricts) and Oxytocin (uterine contractions, milk ejection) produced by hypothalamus.
Anterior pituitary (adenohypophysis): Regulated by hypothalamic releasing and inhibiting hormones (e.g., TRH, GHRH, PIH) via the hypophyseal portal system. Secretes TSH, PRL, ACTH, FSH, LH, GH.

Regulated by GHRH (stimulates) and GHIH (inhibits).
Stimulates liver to release IGFs; promotes protein synthesis, cell division, and mobilizes nutrients (e.g., increases blood glucose, stimulates lipolysis).
Disorders: pituitary dwarfism (deficiency), gigantism/acromegaly (excess).

Located inferior to larynx; has follicular cells (produce TH: T3/T4) and parafollicular (C) cells (produce calcitonin).
Thyroid hormone (TH): Regulated by HPT axis (TRH → TSH → TH); increases metabolic rate, thermogenesis, and supports growth/development.
Calcitonin: Lowers blood calcium by inhibiting osteoclasts and promoting renal excretion.

Located atop kidneys.
Adrenal medulla: Inner core, neural tissue, releases epinephrine/norepinephrine.
Adrenal cortex: Outer region, synthesizes corticosteroids in three zones.
Cortisol: Regulated by HPA axis (CRH → ACTH → Cortisol); mobilizes nutrients, increases blood glucose, anti-inflammatory, immune suppressive at high doses.
Disorders: Cushing syndrome (excess), Addison disease (insufficiency).

Located posterior to stomach; has exocrine (digestive) and endocrine (pancreatic islets) functions.
Pancreatic islets: Alpha cells (glucagon), Beta cells (insulin), Delta cells (somatostatin), F cells (pancreatic polypeptide).
Insulin (beta cells): Lowers blood glucose by promoting uptake, glycogenesis, lipogenesis, and protein synthesis.
Glucagon (alpha cells): Raises blood glucose by promoting glycogenolysis and gluconeogenesis; stimulates lipolysis.
Normal fasting blood glucose: $70\text{ to }110\frac{\text{mg}}{\text{dL}}$ .

Pineal gland: Secretes melatonin (circadian rhythm).
Parathyroid glands: Chief cells secrete PTH, which increases blood calcium by bone resorption, decreasing renal excretion, and activating calcitriol.
Other organs with endocrine function: Thymus (thymic hormones), Heart (ANP), Kidneys (EPO, calcitriol), Liver (IGFs, angiotensinogen), Stomach (gastrin), Small intestine (secretin, CCK), Skin (vitamin D3), Adipose tissue (leptin).

Endocrine activity generally declines with age, leading to reduced hormone levels, loss of body mass, and altered metabolism.