The Endocrine System: Structure, Function, and Disorders

The Endocrine System Overview

Endocrine vs. Exocrine Glands

The human body contains both endocrine and exocrine glands, which differ in their method of secretion and function.

• Exocrine glands: Secrete products into ducts that empty onto surfaces or into cavities; not part of the endocrine system.

• Endocrine glands: Ductless; secrete hormones directly into the bloodstream, which regulate the activity of target cells or organs.

Major Endocrine Glands and Their Locations

• Pineal

• Hypothalamus

• Pituitary

• Parathyroids

• Thyroid

• Thymus

• Adrenals

• Pancreatic islets

• Ovaries (female)

• Testes (male)

Hormones and Their Mechanisms of Action

Hormone Function and Target Cells

Hormones are chemical messengers that provide communication and control within the body, typically slower and longer-lasting than nerve impulses. Cells affected by hormones are called target cells, located within target organs.

Types of Hormones: Steroid vs. Nonsteroid

• Nonsteroid hormones: Bind to receptors on the cell membrane, acting as "first messengers" and triggering "second messengers" inside the cell to affect cellular activity.

• Steroid hormones: Enter the cell and bind to receptors within the nucleus, directly influencing gene expression and protein synthesis.

Regulation of Hormone Secretion

Feedback Mechanisms

Hormone secretion is regulated by homeostatic feedback mechanisms:

• Negative feedback: Reverses the direction of a change in a physiological system, maintaining stability.

• Positive feedback: Amplifies physiological changes; less common in endocrine regulation.

Mechanisms of Endocrine Disease

Types of Endocrine Disorders

• Hypersecretion: Excess hormone production.

• Hyposecretion: Insufficient hormone production.

• Polyendocrine disorders: Involvement of more than one hormone.

• Target cell insensitivity: Cells fail to respond to hormones, mimicking hyposecretion.

Prostaglandins (PGs)

Definition and Function

Prostaglandins are locally acting substances produced in various tissues, influencing functions such as respiration, blood pressure, gastrointestinal secretions, and reproduction. Classes include PGA, PGE, and PGF.

Major Endocrine Glands and Their Hormones

Pituitary Gland

• Anterior pituitary (adenohypophysis): Produces TSH, ACTH, FSH, LH, GH, and PRL.

• Posterior pituitary (neurohypophysis): Releases ADH and OT, produced in the hypothalamus.

Functions of Major Anterior Pituitary Hormones

• TSH: Stimulates thyroid growth and hormone secretion.

• ACTH: Stimulates adrenal cortex growth and glucocorticoid secretion.

• FSH: Initiates ovarian follicle growth and maturation.

• LH: Stimulates estrogen secretion, ovulation, and progesterone production.

• GH: Stimulates growth, protein anabolism, fat catabolism, and increases blood glucose.

• PRL: Stimulates breast development and milk secretion.

Growth Hormone Abnormalities

• Gigantism: Hypersecretion in childhood.

• Acromegaly: Hypersecretion in adulthood.

• Pituitary dwarfism: Hyposecretion in childhood.

Posterior Pituitary Hormones

• ADH: Promotes water reabsorption in kidneys, reducing urine output; hyposecretion causes diabetes insipidus.

• Oxytocin: Stimulates uterine contractions and milk release.

Hypothalamus

• Produces ADH and oxytocin, which are transported to the pituitary for release.

• Controls homeostasis (temperature, appetite, thirst).

Thyroid Gland

• Thyroid hormones (T3, T4): Increase metabolic rate.

• Calcitonin: Lowers blood calcium by inhibiting bone breakdown.

Thyroid Disorders

• Hyperthyroidism: Increased metabolic rate, restlessness, exophthalmos (Graves disease).

• Hypothyroidism: Goiter (iodine deficiency), cretinism (developmental retardation), myxedema (edema, sluggishness).

Parathyroid Glands

• Parathyroid hormone (PTH): Increases blood calcium by promoting bone breakdown.

Adrenal Glands

Adrenal Cortex

• Glucocorticoids (cortisol): Increase gluconeogenesis, maintain blood pressure, anti-inflammatory, anti-immunity, stress response.

• Mineralocorticoids (aldosterone): Increase sodium reabsorption, decrease potassium.

• Sex hormones: Small amounts of androgens.

Glucocorticoid Stress Responses

• High glucocorticoid levels mobilize fats/proteins, decrease immunity, decrease allergic responses, inhibit inflammation.

Adrenal Medulla

• Epinephrine (adrenaline) and norepinephrine: Intensify and prolong sympathetic stimulation, help resist stress.

Adrenal Disorders

• Cushing syndrome: Hypersecretion of glucocorticoids.

• Virilizing tumor: Hypersecretion of adrenal androgens, masculinization in women.

• Addison disease: Hyposecretion of cortical hormones.

Pancreatic Islets

• Glucagon: Increases blood glucose by promoting glycogen breakdown in the liver.

• Insulin: Decreases blood glucose by promoting cellular uptake and metabolism.

Diabetes Mellitus

• Type 1: Hyposecretion of insulin.

• Type 2: Target cell insensitivity to insulin.

• Results in high blood glucose and glycosuria.

Sex Glands

• Ovaries: Secrete estrogen (development of female characteristics, menstrual cycle) and progesterone (pregnancy maintenance).

• Testes: Secrete testosterone (male characteristics, puberty changes).

Thymus

• Thymosin: Essential for immune system development and function.

Placenta

• Secretes chorionic gonadotropins, estrogens, and progesterone to maintain pregnancy.

Pineal Gland

• Secretes melatonin, which regulates the internal clock and inhibits ovarian activity; abnormal secretion may cause seasonal affective disorder (SAD).

Other Endocrine Structures

• Ghrelin: Secreted by stomach, boosts appetite, slows metabolism.

• Atrial natriuretic hormone (ANH): Secreted by heart, stimulates sodium loss from kidneys.

• Leptin: Secreted by fat cells, regulates hunger and fullness.

Summary Table: Major Endocrine Glands, Hormones, and Functions

Gland	Hormone(s)	Main Function(s)
Pituitary (anterior)	TSH, ACTH, FSH, LH, GH, PRL	Growth, metabolism, reproduction
Pituitary (posterior)	ADH, OT	Water balance, uterine contraction, milk release
Thyroid	T3, T4, Calcitonin	Metabolic rate, calcium regulation
Parathyroid	PTH	Blood calcium regulation
Adrenal cortex	Cortisol, Aldosterone, Androgens	Stress response, sodium/potassium balance, sex traits
Adrenal medulla	Epinephrine, Norepinephrine	Fight-or-flight response
Pancreatic islets	Insulin, Glucagon	Blood glucose regulation
Ovaries	Estrogen, Progesterone	Female traits, menstrual cycle, pregnancy
Testes	Testosterone	Male traits, puberty
Thymus	Thymosin	Immune system development
Pineal	Melatonin	Internal clock, ovarian inhibition
Placenta	Chorionic gonadotropins, Estrogens, Progesterone	Pregnancy maintenance

Additional info: This guide expands on brief points with academic context, definitions, and examples to ensure completeness and clarity for college-level study.