Endocrine Glands and Hormone Functions: Stimuli, Sources, and Actions

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Endocrine Glands and Hormone Functions

Overview of Hormonal Regulation

The endocrine system regulates physiological processes through the secretion of hormones from specialized glands. Each gland responds to specific stimuli and releases hormones that act on target tissues to maintain homeostasis.

Hormones are chemical messengers secreted into the bloodstream, affecting distant organs.
Hormone release is often triggered by changes in blood composition, neural input, or other hormones.
Understanding the source, stimulus, and action of each hormone is essential for grasping endocrine physiology.

Pituitary Gland

Anterior Pituitary (Adenohypophysis)

The anterior pituitary produces several key hormones in response to hypothalamic releasing hormones.

Growth Hormone (GH): Stimulated by Growth Hormone-Releasing Hormone (GHRH) from the hypothalamus; promotes growth of tissues and increases blood glucose.
Thyroid-Stimulating Hormone (TSH): Released in response to Thyrotropin-Releasing Hormone (TRH); stimulates the thyroid gland to produce thyroid hormones.
Adrenocorticotropic Hormone (ACTH): Triggered by Corticotropin-Releasing Hormone (CRH); stimulates adrenal cortex to release cortisol.
Follicle-Stimulating Hormone (FSH) & Luteinizing Hormone (LH): Released in response to Gonadotropin-Releasing Hormone (GnRH); regulate reproductive processes.
Prolactin (PRL): Stimulated by Prolactin-Releasing Hormone (PRH); promotes milk production.

Posterior Pituitary (Neurohypophysis)

The posterior pituitary stores and releases hormones produced by the hypothalamus.

Antidiuretic Hormone (ADH): Released in response to increased blood osmolality; promotes water reabsorption in kidneys.
Oxytocin: Released in response to uterine stretching or suckling; stimulates uterine contractions and milk ejection.

Adrenal Glands

Adrenal Cortex

The adrenal cortex produces steroid hormones in response to ACTH or changes in blood composition.

Cortisol (Glucocorticoid): Released in response to ACTH; increases blood glucose and suppresses immune response.
Aldosterone (Mineralocorticoid): Secreted in response to low blood sodium or high potassium; increases sodium reabsorption in kidneys.
Androgens: Minor role in adults; contribute to secondary sex characteristics.

Adrenal Medulla

The adrenal medulla releases catecholamines in response to sympathetic nervous system stimulation.

Epinephrine and Norepinephrine: Released during stress ("fight or flight"); increase heart rate, blood pressure, and blood glucose.

Thyroid Gland

Follicular Cells

Follicular cells produce thyroid hormones in response to TSH from the anterior pituitary.

Thyroxine (T4) and Triiodothyronine (T3): Increase metabolic rate, regulate growth and development.

Parafollicular (C) Cells

Parafollicular cells secrete calcitonin in response to high blood calcium levels.

Calcitonin: Lowers blood calcium by inhibiting osteoclast activity and promoting calcium deposition in bone.

Parathyroid Glands

The parathyroid glands secrete parathyroid hormone (PTH) in response to low blood calcium.

Parathyroid Hormone (PTH): Increases blood calcium by stimulating osteoclasts, increasing intestinal absorption, and promoting renal reabsorption of calcium.
Example: When blood calcium drops, PTH is released, leading to bone resorption and increased calcium levels in the blood.

Pancreas (Islets of Langerhans)

The pancreas contains clusters of endocrine cells that regulate blood glucose.

Alpha Cells: Secrete glucagon in response to low blood glucose; stimulates glycogen breakdown in the liver to raise blood glucose.
Beta Cells: Secrete insulin in response to high blood glucose; promotes glucose uptake by cells and lowers blood glucose.

Summary Table: Major Endocrine Glands, Hormones, Stimuli, and Actions

Gland/Cell Type	Hormone	Stimulus for Release	Main Action
Anterior Pituitary	GH, TSH, ACTH, FSH, LH, PRL	Hypothalamic releasing hormones	Growth, metabolism, stress response, reproduction, lactation
Posterior Pituitary	ADH, Oxytocin	Neural signals (osmolality, stretch, suckling)	Water balance, uterine contraction, milk ejection
Adrenal Cortex	Cortisol, Aldosterone, Androgens	ACTH, electrolyte changes	Stress response, sodium balance, sex characteristics
Adrenal Medulla	Epinephrine, Norepinephrine	Sympathetic stimulation	"Fight or flight" response
Thyroid (Follicular)	T3, T4	TSH	Increase metabolism
Thyroid (Parafollicular)	Calcitonin	High blood calcium	Lowers blood calcium
Parathyroid	PTH	Low blood calcium	Raises blood calcium
Pancreas (Alpha)	Glucagon	Low blood glucose	Raises blood glucose
Pancreas (Beta)	Insulin	High blood glucose	Lowers blood glucose

Additional info:

Hormonal feedback mechanisms often involve negative feedback loops to maintain homeostasis.
Disorders can arise from hypo- or hypersecretion of these hormones, leading to clinical conditions such as diabetes mellitus, hypothyroidism, or Cushing's syndrome.