Endocrine System: Hormones and Gland Functions (Marieb Human Anatomy & Physiology, Chapter 16)

Endocrine System Overview

Introduction to Hormones

The endocrine system is a network of glands that produce and secrete hormones, which are chemical messengers that regulate various physiological processes throughout the body. Hormones travel through the bloodstream to target cells, which have specific receptors for each hormone.

• Endocrine glands: Produce hormones and release them directly into the bloodstream (no ducts).

• Exocrine glands: Release substances through ducts to the outside of the body or into the digestive tract.

• Target cells: Cells with specific receptors for a hormone, allowing them to respond to hormonal signals.

• Paracrines (local hormones): Act on nearby cells within the same tissue (e.g., histamine, hormones in pancreas & GI tract).

Major Endocrine Glands

The major glands of the endocrine system include:

• Pineal gland

• Hypothalamus

• Pituitary gland

• Thyroid gland

• Parathyroid glands

• Thymus

• Adrenal glands

• Pancreas

• Ovary (female)

• Testis (male)

Hypothalamus and Pituitary Gland

Hypothalamus

The hypothalamus is located in the brain, forming the floor and walls of the third ventricle. It regulates many bodily functions, primarily through its control of the pituitary gland.

Pituitary Gland (Hypophysis)

The pituitary gland is suspended from the hypothalamus by the infundibulum (stalk) and is divided into two parts:

• Adenohypophysis (anterior pituitary): Arises from the hypophyseal pouch (outgrowth of pharynx).

• Neurohypophysis (posterior pituitary): Arises from the brain.

Regulation of Adenohypophysis

Hormones produced in the hypothalamus travel via the portal system to the anterior pituitary, where they regulate the secretion of other hormones:

• Releasing factors: Stimulate hormone release.

• Inhibitory factors: Suppress hormone release.

• Tropic hormones: Hormones that influence other endocrine tissues.

Anterior Pituitary Hormones

Follicle Stimulating Hormone (FSH)

FSH is responsible for sexual maturation at puberty and has different actions in males and females:

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

• Males: Stimulates production of sperm.

Disorders

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

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

Luteinizing Hormone (LH)

• Females: Stimulates ovaries to produce estrogen and progesterone; triggers ovulation (LH surge).

• Males: Stimulates testes to produce testosterone.

Disorders

• Hypersecretion: Premature sexual maturity.

• Hyposecretion: Inhibited sexual maturation, infertility.

Thyroid Stimulating Hormone (TSH)

TSH stimulates the thyroid gland to release thyroid hormones (T3 and T4), which require iodine for synthesis. Calcitonin is also secreted by the thyroid when blood calcium rises.

• Functions: Increases basal metabolic rate, O2 consumption, respiration rate, body temperature, appetite, breakdown of lipids/carbohydrates/proteins, heart rate, blood pressure, and accelerates body growth.

Disorders

• Hyposecretion: Congenital hypothyroidism (cretinism), myxedema in adults (swollen tissue, low metabolism).

• Hypersecretion: Endemic goiter (iodine deficiency), Grave's disease (autoantibodies mimic TSH).

Adrenocorticotropic Hormone (ACTH)

ACTH stimulates the adrenal cortex to produce glucocorticoids, mainly cortisol (the "stress hormone").

• Functions: Increases blood pressure and sugar, stimulates fat/protein catabolism, suppresses immune system.

Disorders

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

• Hyposecretion: Addison's disease (bronzed skin, muscle weakness, low blood pressure).

Prolactin (PRL)

• Females: Acts on mammary glands to cause milk production; nesting instinct. PRL levels peak during pregnancy.

Disorders

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

• Hyposecretion: Lack of milk production.

Growth Hormone (hGH)

• Promotes growth of bone, cartilage, muscle, and fat.

• Stimulates protein synthesis and lipolysis.

• Target organs (e.g., liver) produce insulin-like growth factors.

Growth Hormone and Aging

• Childhood/adolescence: Stimulates growth at epiphyseal plates.

• Adulthood: Increases osteoblastic activity; blood concentration decreases with age.

• Levels fluctuate: Higher during deep sleep, after high protein meals, vigorous exercise; lower after high carbohydrate meals.

Disorders

• Hypersecretion (childhood): Gigantism.

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

• Hyposecretion: Pituitary dwarfism.

Posterior Pituitary Hormones (Neurohypophysis)

Oxytocin (OT)

• Produced in hypothalamus, stored/released by posterior pituitary.

• Major actions: Uterine contractions during labor, milk let-down, possible role in sperm transport and emotional bonding.

• Target organs: Uterus and breasts.

Disorders

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

• Hyposecretion: Non-progression of labor, no milk let-down.

Antidiuretic Hormone (ADH)

• Produced in hypothalamus, stored/released by posterior pituitary.

• Major actions: Water reabsorption from kidney tubules, water retention, reduced urine output, constricts arterioles to raise blood pressure, inhibits sweat glands.

• Target organ: Kidney.

Disorders

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

• Hypersecretion: Water retention, concentrated urine.

Control of Pituitary: Feedback Mechanisms

Negative Feedback

• High levels of target organ hormones inhibit release of tropic hormones from the pituitary.

Positive Feedback

• Stretching of uterus increases oxytocin release, which causes more stretching until delivery.

Adrenal Glands

Cortex

• Mineralocorticoids: Aldosterone (regulates electrolyte balance).

• Glucocorticoids: Cortisol (involved in fat/protein catabolism, anti-inflammatory).

• Sex hormones: Androgens, estrogens.

Medulla

• Neuroendocrine catecholamines: Epinephrine (adrenaline), norepinephrine (fight or flight response).

Summary Table: Major Pituitary Hormones

Hormone	Source	Target Organ	Main Action	Hypersecretion	Hyposecretion
FSH	Anterior Pituitary	Ovary/Testis	Sexual maturation, gamete production	Precocious puberty, early menopause	Infertility, failure of sexual maturity
LH	Anterior Pituitary	Ovary/Testis	Estrogen/progesterone/testosterone production, ovulation	Premature sexual maturity	Infertility, inhibited sexual maturation
TSH	Anterior Pituitary	Thyroid	Stimulates thyroid hormone release	Goiter, Grave's disease	Hypothyroidism, myxedema
ACTH	Anterior Pituitary	Adrenal Cortex	Stimulates cortisol production	Cushing syndrome	Addison's disease
PRL	Anterior Pituitary	Mammary glands	Milk production	Inappropriate milk production	Lack of milk production
hGH	Anterior Pituitary	Bone, muscle, liver	Growth, protein synthesis	Gigantism, acromegaly	Pituitary dwarfism
OT	Posterior Pituitary	Uterus, breasts	Labor contractions, milk let-down	Early labor, excess milk	No labor progression, no milk let-down
ADH	Posterior Pituitary	Kidney	Water retention, blood pressure	Water retention	Diabetes insipidus

Key Equations and Concepts

• Hormone feedback regulation: $\text{Negative feedback:} \quad \uparrow \text{Target hormone} \rightarrow \downarrow \text{Pituitary tropic hormone}$ $\text{Positive feedback:} \quad \text{Stimulus} \rightarrow \uparrow \text{Hormone release} \rightarrow \uparrow \text{Stimulus}$

• Thyroid hormone synthesis: $\text{T3 (triiodothyronine)}$ and $\text{T4 (thyroxine)}$ require iodine for synthesis.

• Adrenal cortex hormone synthesis: $\text{Cholesterol} \rightarrow \text{Corticosteroids (mineralocorticoids, glucocorticoids, sex hormones)}$

Summary

The endocrine system is essential for regulating growth, metabolism, reproduction, and stress responses. Disorders of hormone secretion can lead to significant physiological changes, highlighting the importance of feedback mechanisms and hormonal balance.