The Endocrine System

Overview and Functions

The endocrine system is one of the body's two major control systems, interacting with the nervous system to coordinate and integrate the activity of most body cells. It influences metabolic activities via hormones, with responses that are slower but longer lasting than those of the nervous system. The endocrine system controls and integrates reproduction, growth and development, maintenance of electrolyte, water, and nutrient balance of blood, regulation of cellular metabolism and energy balance, and mobilization of body defenses.

• Hormones: Chemical messengers secreted by endocrine glands, traveling through the blood to target cells.

• Endocrine glands: Ductless glands that produce hormones (e.g., pituitary, thyroid, adrenal glands).

• Neuroendocrine organ: The hypothalamus, which integrates neural and endocrine functions.

• Other organs with endocrine tissue: Pancreas, gonads, placenta, stomach, intestine, heart, kidneys, skin, thymus, bone, adipose tissue.

Types of Chemical Messengers

• Hormones: Long-distance chemical signals.

• Autocrines: Local messengers acting on the same cell that secretes them.

• Paracrines: Local messengers acting on nearby cells.

Hormone Structure and Classification

Chemical Structure Determines Function

The chemical structure of a hormone determines its solubility in water, which affects its transport in blood, degradation rate, and receptor interaction.

• Amino acid–based hormones: Includes amino acid derivatives, peptides, and proteins; generally water-soluble (except thyroxine).

• Steroid hormones: Synthesized from cholesterol; lipid-soluble; includes gonadal and adrenocortical hormones.

• Eicosanoids: Mostly paracrines and autocrines.

Hormone Mechanisms of Action

Second Messenger Systems

Hormones act through second messengers or by activating specific genes. Only cells with receptors for a hormone are affected (target cells). Water-soluble hormones act on plasma membrane receptors, often via G proteins and second messengers such as cyclic AMP (cAMP).

• cAMP Signaling Mechanism: Hormone binds to receptor → G protein activation → adenylate cyclase activation → ATP converted to cAMP → cAMP activates protein kinases → cellular response.

Direct Gene Activation

Lipid-soluble hormones (steroid and thyroid hormones) act on intracellular receptors, directly activating genes to initiate protein synthesis.

Regulation of Hormone Release

Types of Stimuli

Hormone release is controlled by negative feedback mechanisms and triggered by three types of stimuli:

• Humoral stimuli: Changing blood levels of ions and nutrients directly stimulate hormone release.

• Neural stimuli: Nerve fibers stimulate hormone release.

• Hormonal stimuli: Hormones stimulate other endocrine organs to release their hormones.

Target Cell Activation and Receptor Regulation

Factors Affecting Activation

Target cell activation depends on blood levels of hormone, number of receptors, and affinity of binding. Cells can regulate their sensitivity by up-regulation (adding receptors) or down-regulation (removing receptors).

Hypothalamus and Pituitary Gland

Structural Relationships

The hypothalamus controls release of hormones from the pituitary gland in two ways. The pituitary has two lobes: posterior (neural tissue, stores/secretes oxytocin and ADH) and anterior (glandular tissue, secretes six hormones).

Anterior Pituitary Hormones

Growth Hormone (GH)

GH has direct actions on metabolism and indirect growth-promoting actions via insulin-like growth factors (IGFs). It is regulated by GHRH and GHIH from the hypothalamus.

• Hypersecretion: Gigantism in children, acromegaly in adults.

• Hyposecretion: Pituitary dwarfism in children.

Thyroid-Stimulating Hormone (TSH)

TSH stimulates the thyroid gland and is regulated by TRH from the hypothalamus and negative feedback from thyroid hormones.

The Thyroid Gland

Structure and Function

The thyroid gland is butterfly-shaped, located anterior to the trachea. It consists of follicles filled with colloid, which contains thyroglobulin and iodine for thyroid hormone synthesis. Parafollicular cells produce calcitonin.

Thyroid Hormone (TH)

TH is the body's major metabolic hormone, produced as T4 (thyroxine) and T3 (triiodothyronine). It increases basal metabolic rate, regulates tissue growth, and is critical for skeletal and nervous system development.

• Regulation: Negative feedback via TSH and TRH.

Parathyroid Glands

Structure and Function

Parathyroid glands are small glands on the posterior thyroid. They secrete parathyroid hormone (PTH), which is the most important hormone in calcium homeostasis.

Adrenal Glands

Structure and Hormones

The adrenal glands are located atop the kidneys and consist of the adrenal cortex (producing corticosteroids) and adrenal medulla (producing catecholamines).

• Mineralocorticoids: Aldosterone regulates sodium and potassium balance.

• Glucocorticoids: Cortisol regulates metabolism and stress response.

• Gonadocorticoids: Androgens contribute to secondary sex characteristics.

• Catecholamines: Epinephrine and norepinephrine mediate fight-or-flight response.

Pineal Gland

Melatonin

The pineal gland secretes melatonin, which regulates sleep-wake cycles and acts as an antioxidant.

Pancreas

Insulin and Glucagon

The pancreas is a mixed gland with both exocrine and endocrine functions. The islets of Langerhans contain alpha cells (secrete glucagon) and beta cells (secrete insulin).

• Glucagon: Raises blood glucose by promoting glycogenolysis and gluconeogenesis.

• Insulin: Lowers blood glucose by promoting cellular uptake and storage.

Gonads and Placenta

Sex Hormones

Ovaries produce estrogens and progesterone; testes produce testosterone. The placenta produces hormones important for pregnancy.

Hormone Secretion by Other Organs

Examples

• Adipose tissue: Leptin, resistin, adiponectin.

• Heart: Atrial natriuretic peptide (ANP).

• Kidneys: Erythropoietin, renin.

• Skeleton: Osteocalcin.

• Skin: Cholecalciferol.

• Thymus: Thymosins, thymulin, thymopoietins.

Developmental Aspects

Endocrine Function Throughout Life

Most endocrine organs operate well until old age. GH, estrogen, and testosterone levels decline with age, affecting muscle mass, reproductive function, and glucose tolerance.