Chapter 16: The Endocrine System

Overview of the Endocrine System

The endocrine system is a network of glands that secrete chemical messengers called hormones into the bloodstream. These hormones regulate various physiological processes by acting on target cells with specific receptors. The endocrine system works closely with the nervous system to maintain homeostasis.

• Endocrine glands: Ductless glands that release hormones directly into the blood.

• Exocrine glands: Glands with ducts that secrete substances onto epithelial surfaces (e.g., sweat, saliva).

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

• Paracrines (local hormones): Chemical messengers that act on nearby cells (e.g., histamine, some pancreatic and GI tract hormones).

Major Endocrine Organs

The major endocrine glands include the hypothalamus, pituitary gland, thyroid gland, parathyroid glands, adrenal glands, pancreas, ovaries (in females), and testes (in males). Each gland produces specific hormones that regulate body functions.

Hypothalamus and Pituitary Gland

Hypothalamus

The hypothalamus is a region of the brain that forms the floor and walls of the third ventricle. It is a key regulator of the endocrine system, controlling the pituitary gland and integrating signals from the nervous system.

Pituitary Gland (Hypophysis)

The pituitary gland is suspended from the hypothalamus by a stalk called the infundibulum. It consists of two main parts:

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

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

Hypothalamic Control of the Anterior Pituitary

The hypothalamus produces releasing and inhibitory factors that travel through the hypophyseal portal system to regulate hormone secretion from the anterior pituitary. Many anterior pituitary hormones are tropic hormones, meaning they stimulate other endocrine glands.

Anterior Pituitary Hormones

The anterior pituitary secretes several key hormones, each with specific target organs and actions. Disorders can result from either hypersecretion (excess) or hyposecretion (deficiency) of these hormones.

Follicle Stimulating Hormone (FSH)

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

• In males: Stimulates sperm production.

• Disorders: Hypersecretion causes precocious puberty or early menopause; hyposecretion leads to failure of sexual maturity or infertility.

Luteinizing Hormone (LH)

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

• In males: Stimulates testosterone production.

• Disorders: Hypersecretion leads to premature sexual maturity; hyposecretion causes inhibited sexual maturation and infertility.

Thyroid Stimulating Hormone (TSH)

TSH stimulates the thyroid gland, the largest endocrine organ, to release thyroid hormones (T3 and T4), which require iodine for synthesis. Calcitonin is also produced by the thyroid and helps regulate blood calcium levels.

• Thyroid hormones: Increase basal metabolic rate, oxygen consumption, respiration rate, heart rate, and body temperature; stimulate appetite and growth.

• Disorders: Hyposecretion causes congenital hypothyroidism, cretinism, or myxedema; hypersecretion leads to goiter or Grave’s disease.

Adrenocorticotropic Hormone (ACTH)

ACTH stimulates the adrenal cortex to produce glucocorticoids, mainly cortisol, which is known as the "stress hormone." The adrenal gland consists of an outer cortex (producing steroid hormones) and an inner medulla (producing catecholamines).

• Mineralocorticoids (aldosterone): Regulate electrolyte balance.

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

• Sex hormones: Androgens and estrogens.

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

ACTH Disorders

• Hypersecretion (Cushing Syndrome): Excess cortisol, muscle and bone loss, edema, fat redistribution (moon face, buffalo hump), hyperglycemia, hypertension, and masculinization (adrenogenital syndrome).

• Hyposecretion (Addison’s Disease): Bronzed skin, muscle weakness, low blood pressure, dehydration, and inability to handle stress.

Prolactin (PRL)

• Females: Stimulates milk production in estrogen-primed mammary glands; associated with maternal behavior.

• Disorders: Hypersecretion causes inappropriate milk production and reproductive issues; hyposecretion leads to lack of milk production.

Growth Hormone (hGH)

Growth hormone promotes growth of bone, muscle, and other tissues, stimulates protein synthesis, and increases blood glucose levels. It is most active during childhood and adolescence.

• Hypersecretion in childhood: Gigantism (excessive growth).

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

• Hyposecretion: Pituitary dwarfism (proportionate short stature).

Posterior Pituitary Hormones (Neurohypophysis)

The posterior pituitary stores and releases hormones produced by the hypothalamus: oxytocin (OT) and antidiuretic hormone (ADH).

Oxytocin (OT)

• Target organs: Uterus and breasts.

• Major actions: Stimulates uterine contractions during labor, milk ejection during breastfeeding, and may play a role in emotional bonding.

• Disorders: Hypersecretion causes early labor and overproduction of milk; hyposecretion leads to non-progression of labor and lack of milk let-down.

Antidiuretic Hormone (ADH)

• Major actions: Promotes water reabsorption in the kidneys, reduces urine output, constricts arterioles to raise blood pressure, and inhibits sweat glands.

• Disorders: Hyposecretion causes diabetes insipidus (excessive urination and thirst); hypersecretion leads to water retention and concentrated urine.

Regulation of the Pituitary: Feedback Mechanisms

Hormone secretion is regulated by feedback from target organs:

• Negative feedback: Increased hormone levels from target organs inhibit further release of tropic hormones (e.g., thyroid hormone inhibits TSH release).

• Positive feedback: Rare; e.g., uterine stretching increases oxytocin release, which further increases uterine contractions until delivery.

Additional info: This guide covers the structure and function of the endocrine system, major glands and hormones, and common disorders associated with hormone imbalances. It is suitable for college-level Anatomy & Physiology students preparing for exams on the endocrine system.