Endocrine System Overview

Definition and Function

The endocrine system consists of glands that secrete hormones directly into the bloodstream, regulating various physiological processes throughout the body. These hormones act as chemical messengers, influencing growth, metabolism, and homeostasis.

• Endocrine gland: An organ that produces and secretes hormones into surrounding tissue fluid (ductless glands).

• Hormones are distributed throughout the body via the blood.

Major Endocrine Tissues and Organs

• Pituitary, pineal, thyroid, parathyroids, adrenals, thymus, pancreas, gonads

• The hypothalamus has both neural and endocrine functions.

• Other tissues and organs that produce hormones: adipose cells, pockets of cells in the walls of the small intestine, stomach, kidneys, and heart.

Hormone Chemistry and Action

Hormones: Chemical Messengers

Hormones are chemical messengers secreted by cells into the extracellular fluids. They regulate metabolic functions of other cells and tend to have prolonged effects.

• Secreted by endocrine glands.

• Act on target cells to regulate physiological processes.

• Effects can last from seconds to hours.

Chemical Makeup of Hormones

• Amino acid-based hormones: Most hormones belong to this class.

• Amines: e.g., thyroxine (synthesized from amino acid tyrosine).

• Peptides: Short chains of amino acids.

• Proteins: Long chains of amino acids.

• Steroids: Made from cholesterol.

• Eicosanoids: Include paracrine and autocrine signaling molecules.

Mechanism of Hormone Action

Hormones produce effects by binding to receptors on or in target cells, triggering cellular changes.

• Alter plasma membrane permeability (may lead to muscle fiber depolarization and contraction).

• Stimulate protein synthesis.

• Activate or deactivate enzymes.

• Induce secretory activity.

• Stimulate mitosis.

Hormone Receptors and Second Messengers

Hormones act at receptors in one of two ways:

• Water-soluble (hydrophilic) hormones: All amino acid-based hormones except thyroid hormone. These hormones cannot cross the plasma membrane and act via second messengers.

• Lipid-soluble (hydrophobic) hormones: Steroid and thyroid hormones. These can diffuse through the plasma membrane and act directly on intracellular receptors.

Second Messengers

• Most protein-based hormones use second messengers because they cannot cross the plasma membrane.

• Hormone binds to a receptor on the cell membrane, leading to a cascade of chemical events that amplify the hormone's effect.

Adenylyl Cyclase–cAMP Pathway

Example of a second messenger system:

• Hormone (first messenger) binds to a receptor on the cell surface.

• G protein is activated and converts adenylyl cyclase to its active form.

• Adenylyl cyclase catalyzes the reaction to generate cAMP (second messenger).

• cAMP activates protein kinases, which phosphorylate proteins, leading to cellular responses.

Amplification effect: One activated adenylyl cyclase generates large amounts of cAMP, which activates many protein kinase molecules, resulting in hundreds of reactions.

Direct Gene Activation

Steroid hormones and thyroid hormone act by direct gene activation:

• Steroids are lipid-soluble and diffuse through cell membranes.

• Bind to intracellular receptors, forming a hormone-receptor complex.

• The complex binds to DNA, activating specific genes and leading to protein synthesis.

Control of Hormone Release

Types of Stimuli

• Humoral: Changes in blood levels of ions or nutrients stimulate hormone release.

• Neural: Nerve fibers stimulate hormone release.

• Hormonal: Hormones stimulate other endocrine glands to release hormones.

Humoral Control Example

Regulation of blood calcium levels:

• Low blood Ca2+ stimulates the parathyroid glands to secrete parathyroid hormone (PTH).

• PTH stimulates osteoclast activity, increasing blood Ca2+ levels.

• When blood Ca2+ reaches the set point, PTH release is inhibited.

Hormonal Control Example

• Hypothalamic releasing and inhibiting hormones regulate anterior pituitary hormone release.

• Thyroid stimulating hormone (TSH) released by the anterior pituitary targets follicular cells of the thyroid gland, which respond by increasing their release of thyroid hormones.

Pituitary Gland and Hypothalamic Control

Pituitary Gland Structure and Function

The pituitary gland is a two-lobed organ that secretes eight major hormones and is connected to the hypothalamus by the infundibulum.

• Neurohypophysis: Posterior lobe (neural tissue), connected via hypothalamic-hypophyseal tract. Receives, stores, and releases hormones from the hypothalamus.

• Adenohypophysis: Anterior lobe (glandular tissue), connected via hypothyseal portal system. Synthesizes and secretes a number of hormones.

Hypothalamic-Hypophyseal Portal System

The portal system consists of capillary plexuses and veins connecting the hypothalamus and anterior pituitary, allowing releasing and inhibiting hormones from the hypothalamus to regulate anterior pituitary hormone secretion.

Anterior Pituitary Hormones

• Growth Hormone (GH)

• Thyroid Stimulating Hormone (TSH)

• Adrenocorticotropic Hormone (ACTH)

• Follicle Stimulating Hormone (FSH)

• Luteinizing Hormone (LH)

• Prolactin (PRL)

Major Endocrine Glands and Their Hormones

Summary Table: Major Endocrine Glands and Hormones

Homeostatic Imbalances of Endocrine Function

Pituitary Gland

• Hyposecretion of GH: Pituitary dwarfism (in children).

• Hypersecretion of GH: Gigantism (in children), acromegaly (in adults).

• Hyposecretion of ADH: Diabetes insipidus (excessive urination, dehydration).

Thyroid Gland

• Hypothyroidism: Symptoms include mental and physical sluggishness, decreased body temperature, constipation, edema, and dry skin. Causes include iodine deficiency, surgical removal, or autoimmune destruction.

• Hyperthyroidism (Graves' disease): Excess thyroid hormone release, symptoms include elevated metabolic rate, sweating, rapid heartbeat, nervousness, and weight loss.

Parathyroid Glands

• Hypoparathyroidism: PTH deficiency leads to muscle spasms, twitches, and tetany.

• Hyperparathyroidism: Excess PTH causes bone resorption, leading to softening of bones and fractures.

Adrenal Glands

• Hyposecretion of Cortisol & Aldosterone (Addison's disease): Symptoms include weight loss, sodium and water loss, severe dehydration, and hypotension.

• Hypersecretion of Cortisol (Cushing's syndrome): Symptoms include persistent hyperglycemia, loss of muscle and bone protein, hypertension, and edema.

Key Equations and Concepts

Hormone-Receptor Interaction

The effect of a hormone depends on:

• Blood levels of the hormone

• Number of receptors on the target cell

• Affinity of the binding between hormone and receptor

Example Equation: Amplification in Second Messenger Systems

Amplification can be described as:

Negative Feedback Regulation

Most hormone release is regulated by negative feedback:

Additional info:

• Some details and examples were expanded for clarity and completeness.

• Table entries were inferred and grouped for academic context.