The Endocrine System: Structure, Function, and Regulation

The Endocrine System

Introduction to the Endocrine System

The endocrine system is a major regulatory system of the body, working alongside the nervous system to coordinate and integrate the activities of body cells. It achieves this through the secretion of hormones, which are chemical messengers that travel through the bloodstream to target organs.

• Hormones are secreted by endocrine glands and regulate various physiological processes.

• The endocrine system includes glands such as the pituitary, thyroid, parathyroid, adrenal (suprarenal), pineal, and thymus, as well as organs with endocrine functions like the pancreas, ovaries, and testes.

• Hormones are involved in growth, metabolism, stress response, and homeostasis.

Comparison of Nervous and Endocrine Systems

Control Mechanisms

Both the nervous and endocrine systems coordinate body functions, but they differ in their mechanisms and effects.

• Nervous system: Releases neurotransmitters that act on nearby target cells for rapid, short-term responses.

• Endocrine system: Releases hormones into the bloodstream, affecting distant target cells for longer-lasting effects.

• Both systems use chemical messengers that bind to specific receptors on target cells.

Endocrine vs. Exocrine Glands

Types of Glands

• Exocrine glands: Secrete products into ducts (e.g., sweat, oil, digestive glands); their products are not hormones.

• Endocrine glands: Ductless; secrete hormones directly into interstitial fluid, which then diffuse into the bloodstream.

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

• Other hormone-secreting organs: hypothalamus, thymus, pancreas, ovaries, testes, kidneys, stomach, liver, small intestine, skin, heart, adipose tissue, placenta.

Hormone Activity

Hormone-Receptor Interactions

• Hormones affect only target cells with specific receptors.

• Receptors can be down-regulated (decreased) in response to high hormone levels or up-regulated (increased) when hormone levels are low.

• Local hormones (paracrines) act on neighboring cells; autocrines act on the same cell that secreted them.

Chemical Classes of Hormones

• Lipid-soluble hormones: Steroid hormones, thyroid hormones, nitric oxide; circulate bound to transport proteins.

• Water-soluble hormones: Amine, peptide, protein, and eicosanoid hormones; circulate freely in plasma.

Mechanisms of Hormone Action

Cellular Responses

• Hormones can cause synthesis of new molecules, alter membrane permeability, stimulate transport, change metabolic rates, or cause muscle contraction.

• Lipid-soluble hormones bind to intracellular receptors; water-soluble hormones bind to receptors on the cell surface.

• Hormonal effects depend on hormone concentration, receptor number, and interactions with other hormones (synergistic or antagonistic effects).

Control of Hormone Secretion

Regulation Mechanisms

• Hormones are secreted in short bursts as needed.

• Regulated by nervous signals, blood chemistry, or other hormones.

• Most hormone regulation is via negative feedback; a few use positive feedback.

Hypothalamus and Pituitary Gland

Structure and Function

• The hypothalamus and pituitary gland (connected by the infundibulum) control other endocrine glands.

• The anterior pituitary (adenohypophysis) produces 7 hormones; the posterior pituitary (neurohypophysis) stores and releases 2 hormones made by the hypothalamus.

• Hypothalamic hormones reach the anterior pituitary via the hypophyseal portal system.

Anterior Pituitary Hormones

Hormone	Main Actions
Human Growth Hormone (hGH)	Stimulates growth, protein synthesis, and tissue repair
Thyroid-Stimulating Hormone (TSH)	Stimulates thyroid hormone secretion
Follicle-Stimulating Hormone (FSH)	Stimulates gamete production
Luteinizing Hormone (LH)	Stimulates sex hormone production
Prolactin (PRL)	Stimulates milk production
Adrenocorticotropic Hormone (ACTH)	Stimulates adrenal cortex hormone secretion
Melanocyte-Stimulating Hormone (MSH)	Influences skin pigmentation

Posterior Pituitary Hormones

Hormone	Main Actions
Oxytocin (OT)	Stimulates uterine contractions and milk ejection
Antidiuretic Hormone (ADH)	Reduces urine output, regulates water balance

• ADH secretion is regulated by blood osmotic pressure via osmoreceptors in the hypothalamus.

Thyroid Gland

Structure and Hormones

• Located inferior to the larynx, anterior to the trachea; consists of right and left lobes connected by an isthmus.

• Follicular cells produce thyroxine (T4) and triiodothyronine (T3) under TSH stimulation.

• Parafollicular cells produce calcitonin, which lowers blood calcium levels.

Functions of Thyroid Hormones

• Increase basal metabolic rate (BMR)

• Regulate body temperature

• Stimulate protein synthesis and energy production

• Promote growth with hGH and insulin

Parathyroid Glands

Structure and Hormones

• Located on the posterior surface of the thyroid gland; usually four glands.

• Chief cells secrete parathyroid hormone (PTH), which increases blood calcium levels.

• Oxyphil cells have unknown function in normal glands but may secrete excess PTH in cancer.

• Calcitonin (thyroid), PTH (parathyroid), and calcitriol (kidney) regulate calcium homeostasis.

Adrenal (Suprarenal) Glands

Structure and Regions

• Located atop each kidney; covered by a connective tissue capsule.

• Divided into outer cortex (three zones) and inner medulla.

Zone	Hormones	Main Actions
Zona glomerulosa	Mineralocorticoids (e.g., aldosterone)	Regulate sodium and potassium balance
Zona fasciculata	Glucocorticoids (e.g., cortisol)	Regulate metabolism, stress response, inflammation
Zona reticularis	Androgens (e.g., DHEA)	Contribute to sex characteristics, libido

• The adrenal medulla secretes epinephrine and norepinephrine, mediating the fight-or-flight response.

Pancreatic Islets

Structure and Hormones

• The pancreas is both an exocrine and endocrine gland.

• Endocrine portion consists of islets of Langerhans with four cell types:

Cell Type	Hormone	Main Actions
Alpha (A)	Glucagon	Raises blood glucose
Beta (B)	Insulin	Lowers blood glucose
Delta (D)	Somatostatin	Inhibits insulin and glucagon secretion
F cells	Pancreatic polypeptide	Regulates pancreatic secretions

• Insulin and glucagon secretion are regulated by negative feedback.

Gonads: Ovaries and Testes

Hormones and Functions

• Ovaries produce estrogens (estradiol, estrone), progesterone, relaxin, and inhibin; regulate female reproductive cycle and secondary sex characteristics.

• Testes produce testosterone; regulate sperm production and male secondary sex characteristics.

Pineal Gland and Thymus

Hormones and Functions

• Pineal gland secretes melatonin, regulating circadian rhythms (biological clock).

• Thymus produces thymosin, THF, TF, and thymopoietin; promote maturation of T cells for immune function.

Other Endocrine Tissues and Organs

Hormones from Non-Endocrine Organs

• Skin, gastrointestinal tract, placenta, kidneys, heart, and adipose tissue produce hormones affecting various physiological processes.

• Example: Kidneys produce erythropoietin (stimulates red blood cell production).

Eicosanoids and Growth Factors

Locally Acting Hormones

• Eicosanoids are derived from arachidonic acid and act locally (paracrine/autocrine).

• Growth factors are hormones that stimulate cell growth, division, and tissue repair.

The Stress Response

Stages of Stress

• Eustress: Beneficial, prepares body for challenges.

• Distress: Harmful, may damage health.

• Fight-or-flight response: Immediate, short-term response to stress.

• Resistance reaction: Longer-term adaptation to stress; prolonged stress leads to exhaustion.

Development and Aging of the Endocrine System

Development

• Endocrine glands develop from all three primary germ layers during embryogenesis.

Aging

• Hormone levels change with age; some increase, some decrease, others remain stable.

• Endocrine glands often decrease in size and become more fibrous with age.

Endocrine Disorders

Pituitary Gland Disorders

• Pituitary gigantism and acromegaly: Excess growth hormone secretion.

• Goiter: Thyroid hormone deficiency.

• Graves disease: Excess thyroid hormone, often with exophthalmos (eye bulging).

Adrenal Gland Disorders

• Cushing’s syndrome: Excess glucocorticoids.

• Addison’s disease: Deficiency of glucocorticoids and aldosterone.

• Pheochromocytomas: Benign tumors causing excess epinephrine/norepinephrine.

Pancreatic Islet Disorders

• Diabetes mellitus: Inability to produce or use insulin.

• Type I diabetes: Autoimmune destruction of beta cells.

• Type II diabetes: Insulin resistance, often linked to obesity and inactivity.

Additional info: For more detailed mechanisms, refer to the feedback regulation of hormone secretion and the specific pathways (e.g., renin-angiotensin-aldosterone system for aldosterone regulation).