Endocrine System: Structure, Function, and Hormonal Regulation

Endocrine System Overview

Definition and Components

The endocrine system consists of cells, tissues, and organs called endocrine glands that secrete chemical messengers known as hormones. These hormones regulate various physiological processes by acting on distant target organs.

• Major endocrine glands: pituitary, thyroid, parathyroid, adrenal, and pineal glands.

• Hormone-secreting organs/tissues: heart, stomach, small intestine, kidneys, adipose tissue, and pancreas.

• Pancreas: has both endocrine (insulin, glucagon) and exocrine (digestive enzymes) functions.

Key Point: The endocrine and nervous systems work together to coordinate body functions, but hormones generally act more slowly and have longer-lasting effects than neurotransmitters.

Endocrine vs. Exocrine Glands

• Exocrine glands: Secrete products via ducts to an epithelial surface (e.g., sweat, digestive enzymes).

• Endocrine glands: Ductless; secrete hormones directly into the bloodstream.

Hormone Action and Receptors

• Hormones bind to specific receptors on or in target cells.

• Both hormones and neurotransmitters depend on receptor binding for their effects.

• Some chemicals, such as oxytocin and epinephrine, can act as both hormones and neurotransmitters.

Classification of Hormones

Types of Hormones

• Steroid hormones: Derived from cholesterol; include sex hormones (estrogen, progesterone, testosterone) and corticosteroids (cortisol).

• Monoamines (biogenic amines): Derived from amino acids; include dopamine, epinephrine, norepinephrine, melatonin, and thyroid hormone.

• Peptide (protein) hormones: Chains of 3 to 200+ amino acids; most anterior pituitary hormones are polypeptides.

Example: Dopamine, epinephrine, and norepinephrine are called catecholamines.

Hormone Mechanisms

• Hormones only affect cells with the appropriate receptor.

• Target cell response depends on hormone concentration and receptor abundance.

• Hormones can cause target cells to synthesize new molecules, change membrane permeability, or alter metabolic activity.

Control of Hormonal Secretions

Feedback Mechanisms

• Negative feedback: Most common; hormone inhibits further secretion (e.g., thyroid hormone inhibits TSH release).

• Positive feedback: Hormone stimulates further secretion (e.g., oxytocin during childbirth).

• Disorders can result from hypo- or hypersecretion of hormones.

Regulation of Hormone Release

• Humoral stimuli: Changes in blood levels of ions/nutrients trigger hormone release.

• Neural stimuli: Nerve fibers stimulate hormone release (e.g., adrenal medulla).

• Hormonal stimuli: Hormones stimulate other endocrine glands to release hormones (e.g., hypothalamic hormones stimulate pituitary).

Hypothalamus and Pituitary Gland

Structure and Function

The hypothalamus is the major link between the nervous and endocrine systems. It regulates the pituitary gland, which is often called the master gland because it controls other endocrine glands.

• The pituitary is suspended from the hypothalamus by the infundibulum and sits in the sella turcica of the sphenoid bone.

• It has two lobes: anterior (adenohypophysis) and posterior (neurohypophysis).

Anterior Pituitary (Adenohypophysis)

• Controlled by hypothalamic releasing/inhibiting hormones via the hypophyseal portal system (network of capillaries).

• Secretes several key hormones that regulate growth, metabolism, and reproduction.

Posterior Pituitary (Neurohypophysis)

• Not a true gland; stores and releases hormones synthesized by the hypothalamus (oxytocin and ADH).

• Release is controlled by neuroendocrine reflexes in response to nervous system signals.

Anterior Pituitary Hormones

Growth Hormone (GH)

• Stimulates secretion of insulin-like growth factors (IGFs) that promote growth, protein synthesis, and fat breakdown.

• GH levels decline with age; excess in childhood causes gigantism, in adults causes acromegaly; deficiency causes pituitary dwarfism.

Thyroid-Stimulating Hormone (TSH)

• Stimulates growth of the thyroid gland and synthesis/secretion of thyroid hormones, which regulate metabolic rate.

Gonadotropins

• Follicle-stimulating hormone (FSH): Stimulates ovarian estrogen secretion and follicle development in females; sperm production in males.

• Luteinizing hormone (LH): Triggers ovulation and corpus luteum formation in females; stimulates testosterone secretion in males.

Prolactin (PRL)

• Stimulates milk synthesis in mammary glands after childbirth.

• In males, increases sensitivity of testes to LH, enhancing testosterone secretion.

Adrenocorticotropic Hormone (ACTH)

• Stimulates adrenal cortex to secrete glucocorticoids (e.g., cortisol), which regulate metabolism and stress response.

Posterior Pituitary Hormones

Oxytocin (OT)

• Stimulates uterine contractions during childbirth (positive feedback) and milk ejection during breastfeeding.

Antidiuretic Hormone (ADH, Vasopressin)

• Promotes water retention by kidneys, reducing urine volume and helping maintain blood pressure.

• Secretion increases in response to dehydration or high blood osmolarity.

Thyroid and Parathyroid Glands

Thyroid Gland

• Located below the larynx; has two lateral lobes connected by an isthmus.

• Produces thyroid hormones (T3 and T4) that regulate metabolism, growth, and development.

• Secretes calcitonin, which lowers blood calcium levels.

Parathyroid Glands

• Four small glands embedded in the posterior thyroid.

• Secrete parathyroid hormone (PTH), which increases blood calcium by stimulating bone resorption, increasing intestinal absorption, and promoting kidney reabsorption of calcium.

Adrenal Glands

Structure and Hormones

• Located atop each kidney; consist of an outer cortex and inner medulla.

• Adrenal cortex: Produces corticosteroids (mineralocorticoids, glucocorticoids, and androgens).

• Adrenal medulla: Produces catecholamines (epinephrine and norepinephrine) in response to stress.

Adrenal Cortex Hormones

Zone	Main Hormone(s)	Function
Zona glomerulosa	Aldosterone	Regulates sodium and potassium balance
Zona fasciculata	Cortisol	Regulates metabolism, stress response
Zona reticularis	Androgens (DHEA)	Sex hormone precursors

Pancreas

Endocrine Function

• Contains islets of Langerhans with alpha cells (secrete glucagon) and beta cells (secrete insulin).

• Insulin: Lowers blood glucose by promoting cellular uptake.

• Glucagon: Raises blood glucose by stimulating glycogen breakdown and gluconeogenesis.

Diabetes Mellitus

• Disorder of carbohydrate, fat, and protein metabolism due to insulin deficiency or resistance.

• Type 1: Insulin-dependent; usually onset in childhood.

• Type 2: Non-insulin-dependent; often associated with obesity and adult onset.

Other Endocrine Organs

Gonads

• Ovaries: Produce estrogens and progesterone; regulate female reproductive cycle and secondary sex characteristics.

• Testes: Produce testosterone; regulates male reproductive development and secondary sex characteristics.

Pineal Gland

• Secretes melatonin, which regulates circadian rhythms and reproductive timing.

Thymus

• Secretes hormones that promote development of T lymphocytes (immune function).

Aging and the Endocrine System

• Hormone secretion generally declines with age.

• Decreased hormone levels can affect metabolism, bone density, and immune function.

• Increased risk of endocrine disorders such as diabetes and osteoporosis in the elderly.