The Endocrine System

Overview of the Endocrine System

The endocrine system is a network of glands that secrete chemical messengers called hormones directly into the bloodstream. These hormones regulate various physiological processes by acting on specific target cells that possess the appropriate receptors. The endocrine system works in concert with the nervous system to maintain homeostasis, but its effects are generally slower and longer-lasting.

• Endocrine glands: Ductless glands that release hormones into the blood (e.g., pituitary, thyroid, adrenal glands).

• Exocrine glands: Secrete substances through ducts to an epithelial surface (e.g., sweat, salivary glands).

• Paracrines: Local hormones that act on nearby cells (e.g., histamine in tissues, hormones in the pancreas and GI tract).

• Target cells: Cells with specific receptors for a given hormone.

Major Endocrine Organs and Their Functions

Hypothalamus and Pituitary Gland

The hypothalamus is a region of the brain that forms the floor and walls of the third ventricle. It is a crucial link between the nervous and endocrine systems, controlling many bodily functions through its influence on the pituitary gland (hypophysis). The pituitary gland is suspended from the hypothalamus by a stalk called the infundibulum and consists of two main parts:

• Adenohypophysis (anterior pituitary): Arises from the pharynx and produces several key hormones.

• Neurohypophysis (posterior pituitary): Arises from the brain and stores hormones produced by the hypothalamus.

Regulation of the Adenohypophysis (Anterior Pituitary)

The anterior pituitary is regulated by releasing and inhibitory factors from the hypothalamus, which travel through the hypophyseal portal system. The anterior pituitary secretes several tropic hormones that influence other endocrine glands.

• Tropic hormones: Hormones that stimulate other endocrine glands to secrete their hormones (e.g., TSH, ACTH, FSH, LH).

Anterior Pituitary Hormones

Follicle Stimulating Hormone (FSH)

FSH is essential for sexual maturation and reproductive function in both sexes.

• In females: Stimulates development of ovarian follicles and egg cells; promotes estrogen production and secondary sex characteristics.

• In males: Stimulates sperm production in the testes.

• Hypersecretion: Precocious puberty in children, early menopause in women.

• Hyposecretion: Failure of sexual maturity in children, infertility in adults.

Luteinizing Hormone (LH)

LH is involved in regulating the function of gonads in both sexes.

• In females: Stimulates ovulation and production of estrogen and progesterone.

• In males: Stimulates testosterone production in the testes.

• Hypersecretion: Premature sexual maturity.

• Hyposecretion: Inhibited sexual maturation, infertility.

Thyroid Stimulating Hormone (TSH)

TSH stimulates the thyroid gland, the largest endocrine organ located below the larynx, to release thyroid hormones.

• T3 (triiodothyronine) and T4 (thyroxine): Increase basal metabolic rate, oxygen consumption, heart rate, and body temperature; stimulate appetite and growth.

• Calcitonin: Secreted when blood calcium rises, promoting bone formation.

• Hyposecretion: Causes congenital hypothyroidism (cretinism) in children and myxedema in adults.

• Hypersecretion: Causes endemic goiter (iodine deficiency) and Grave’s disease (autoimmune hyperthyroidism).

Adrenocorticotropic Hormone (ACTH)

ACTH stimulates the adrenal cortex to produce glucocorticoids, mainly cortisol, which is known as the "stress hormone." The adrenal glands are located atop the kidneys and consist of a cortex (producing steroid hormones) and a medulla (producing catecholamines).

• Mineralocorticoids (aldosterone): Regulate electrolyte balance.

• Glucocorticoids (cortisol): Involved in fat and protein catabolism, anti-inflammatory effects, and stress response.

• Sex hormones: Androgens and estrogens.

• Medulla: Produces epinephrine and norepinephrine for the fight-or-flight response.

• Hypersecretion: Cushing syndrome (excess cortisol), adrenogenital syndrome (androgen excess).

• Hyposecretion: Addison’s disease (insufficient cortisol and aldosterone).

Prolactin (PRL)

Prolactin primarily acts on the mammary glands to stimulate milk production, especially during pregnancy. It also plays a role in the nesting instinct and may affect reproductive functions in males.

• Hypersecretion: Inappropriate milk production, amenorrhea, impotence in males.

• Hyposecretion: Lack of milk production.

Growth Hormone (hGH)

Growth hormone is essential for growth and metabolism. It promotes protein synthesis, lipolysis, and the growth of bone and soft tissues. Its effects are mediated in part by insulin-like growth factors produced by the liver.

• In children: Stimulates growth at epiphyseal plates (bone, cartilage, muscle growth).

• In adults: Increases osteoblastic activity; levels decrease with age.

• Hypersecretion in childhood: Gigantism.

• Hypersecretion in adulthood: Acromegaly (thickening of jaw, brow, hands, feet).

• Hyposecretion: Pituitary dwarfism.

Posterior Pituitary Hormones (Neurohypophysis)

Oxytocin (OT)

Oxytocin is produced in the hypothalamus and stored in the posterior pituitary. It acts on the uterus and breasts, causing uterine contractions during labor and milk let-down during lactation. It may also play a role in emotional bonding and sperm transport.

• Hypersecretion: Early onset of labor, overproduction of milk.

• Hyposecretion: Non-progression of labor, no milk let-down (may require synthetic oxytocin, e.g., pitocin).

Antidiuretic Hormone (ADH)

ADH (also called vasopressin) is produced in the hypothalamus and stored in the posterior pituitary. It acts on the kidneys to promote water reabsorption, reduce urine output, and increase blood pressure. It also constricts arterioles and inhibits sweat glands.

• Hyposecretion: Diabetes insipidus (increased urine output, dehydration, thirst).

• Hypersecretion: Water retention and concentrated urine.

Control of Pituitary: Feedback from Target Organs

Feedback Mechanisms

The secretion of pituitary hormones is tightly regulated by feedback from target organs:

• Negative feedback: Increased hormone levels from target organs inhibit further release of tropic hormones from the pituitary.

• Positive feedback: For example, stretching of the uterus increases oxytocin release, which causes more uterine stretching until delivery.

Summary Table: Major Anterior Pituitary Hormones