The Endocrine System: Structure, Function, and Regulation

Endocrine System Overview

General Function and Organization

The endocrine system works alongside the nervous system to coordinate and integrate the activities of body cells. It influences metabolic activities by means of hormones transported in the blood. Endocrine responses are generally slower but longer-lasting than nervous system responses, which use action potentials.

• Hormones: Chemical messengers that travel through the blood to regulate processes such as growth, metabolism, and reproduction.

• Some organs, such as the pancreas and gonads, have both endocrine (hormone-secreting, ductless) and exocrine (duct-containing, non-hormonal) functions.

• The hypothalamus has both neural and endocrine roles, acting as a neuroendocrine organ.

Chemical Messengers of the Endocrine System

Types of Chemical Messengers

Hormones are long-distance chemical signals, but other local messengers exist:

• Autocrines: Act on the same cells that secrete them (e.g., prostaglandins).

• Paracrines: Affect nearby cells (e.g., histamine, serotonin in the stomach).

• Only hormones are considered part of the endocrine system; autocrines and paracrines are not.

Chemistry of Hormones

Hormones are classified into two main groups based on their chemical structure:

• Amino acid-based hormones: Includes amines, thyroxine, peptides, and proteins.

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

Mechanisms of Hormone Action

How Hormones Affect Target Cells

Hormones act on target cells that have specific receptors, altering cell activity by:

• Changing membrane permeability or potential (e.g., opening/closing ion channels)

• Stimulating protein or enzyme synthesis

• Activating or deactivating enzymes

• Inducing secretory activity

• Stimulating mitosis

Water-Soluble vs. Lipid-Soluble Hormones

• Water-soluble hormones (all amino acid-based except thyroid hormone): Cannot enter target cells; act on plasma membrane receptors and use G proteins and second messengers (e.g., cAMP).

• Lipid-soluble hormones (steroids and thyroid hormone): Pass through the plasma membrane, bind to intracellular receptors, and directly activate genes.

Target Cell Specificity and Activation

Specificity of Hormone Action

Only cells with the appropriate receptors respond to a given hormone. For example, ACTH receptors are found only on certain cells, while thyroxine receptors are widespread.

Factors Affecting Target Cell Activation

• Number of receptors on/in the target cell

• Blood levels of the hormone

• Affinity (strength) of binding between hormone and receptor

Cells can adjust their sensitivity to hormones:

• Up-regulation: Increase in receptor number in response to low hormone levels

• Down-regulation: Decrease in receptor number in response to high hormone levels

Hormone Levels and Regulation

Transport and Removal

• Steroid and thyroid hormones circulate bound to plasma proteins; others circulate freely.

• Hormone levels depend on rate of release and speed of inactivation/removal (by enzymes, kidneys, or liver).

• Half-life: Time required for hormone blood level to decrease by half; water-soluble hormones have the shortest half-lives.

Control of Hormone Release

Hormone secretion is primarily regulated by negative feedback, maintaining levels within a narrow range. Three main types of stimuli control hormone release:

• Humoral stimuli: Changes in blood levels of ions/nutrients (e.g., low Ca2+ stimulates PTH release)

• Neural stimuli: Nerve fibers stimulate hormone release (e.g., sympathetic stimulation of adrenal medulla)

• Hormonal stimuli: Hormones stimulate other endocrine glands (e.g., hypothalamic hormones stimulate pituitary hormones)

Interaction of Hormones at Target Cells

Types of Hormone Interactions

• Permissiveness: One hormone requires another to exert its effect (e.g., reproductive hormones and thyroid hormone).

• Synergism: Multiple hormones produce the same effect, amplifying the response (e.g., glucagon and epinephrine on glucose release).

• Antagonism: One hormone opposes the action of another (e.g., insulin lowers blood glucose, epinephrine raises it).

Pituitary Gland and Hypothalamus

Structure and Function

The pituitary gland (hypophysis) has two lobes:

• Posterior pituitary: Neural tissue; stores and releases hormones made by the hypothalamus (oxytocin and ADH).

• Anterior pituitary: Glandular tissue; produces and releases its own hormones under hypothalamic control.

Pituitary-Hypothalamic Relationships

• Posterior lobe: Down-growth of hypothalamic neural tissue; stores oxytocin and ADH made in the hypothalamus.

• Anterior lobe: Originates from oral mucosa; connected via the hypophyseal portal system, which carries releasing and inhibiting hormones from the hypothalamus.

Anterior Pituitary Hormones

Major Hormones and Their Functions

• Growth hormone (GH): Stimulates growth, especially in bones and muscles; promotes protein synthesis and fat utilization.

• Thyroid-stimulating hormone (TSH): Stimulates thyroid gland activity.

• Adrenocorticotropic hormone (ACTH): Stimulates adrenal cortex to release corticosteroids.

• Follicle-stimulating hormone (FSH): Stimulates gamete production.

• Luteinizing hormone (LH): Promotes production of gonadal hormones.

• Prolactin (PRL): Stimulates milk production.

All are proteins; all except GH use cAMP second-messenger systems. TSH, ACTH, FSH, and LH are tropic hormones (regulate other endocrine glands).

Growth Hormone (GH) Regulation and Imbalances

• Regulated by growth hormone–releasing hormone (GHRH) and growth hormone–inhibiting hormone (GHIH, somatostatin) from the hypothalamus.

• Hypersecretion: Gigantism in children, acromegaly in adults.

• Hyposecretion: Pituitary dwarfism in children.

Thyroid-Stimulating Hormone (TSH)

• Produced by thyrotrophs; stimulates thyroid gland development and activity.

• Regulated by thyrotropin-releasing hormone (TRH) and negative feedback from thyroid hormones.

Adrenocorticotropic Hormone (ACTH)

• Stimulates adrenal cortex to release corticosteroids.

• Regulated by corticotropin-releasing hormone (CRH); affected by stress, fever, hypoglycemia.

Gonadotropins (FSH and LH)

• FSH: Stimulates gamete production.

• LH: Promotes gonadal hormone production.

• Regulated by gonadotropin-releasing hormone (GnRH); suppressed by gonadal hormones (negative feedback).

Prolactin (PRL)

• Stimulates milk production; regulated by prolactin-inhibiting hormone (PIH, dopamine).

• Blood levels rise toward end of pregnancy; suckling stimulates release.

Posterior Pituitary Hormones

Oxytocin

• Stimulates uterine contractions during childbirth and milk ejection during lactation.

• Involved in sexual arousal and orgasm.

Antidiuretic Hormone (ADH)

• Regulates water balance by increasing water reabsorption in kidneys.

• Released in response to high blood solute concentration; inhibited by alcohol.

• Imbalances: Diabetes insipidus (deficiency), SIADH (excess).

Thyroid Gland and Hormones

Structure and Function

• Located in the neck; consists of follicle cells (produce thyroglobulin) and parafollicular cells (produce calcitonin).

• Thyroid hormone (TH) includes T3 (triiodothyronine) and T4 (thyroxine).

Thyroid Hormone (TH)

• Major metabolic hormone; increases metabolic rate and heat production.

• Essential for normal growth, development, and reproductive function.

• Regulated by negative feedback involving TRH and TSH.

Calcitonin

• Produced by parafollicular cells; lowers blood calcium by inhibiting osteoclasts and stimulating calcium uptake into bone.

• Regulated by blood calcium levels; not essential in humans.

Parathyroid Glands and Hormone

Parathyroid Hormone (PTH)

• Most important hormone for calcium homeostasis.

• Increases blood calcium by stimulating osteoclasts, enhancing kidney reabsorption, and activating vitamin D for intestinal absorption.

• Negative feedback: High blood calcium inhibits PTH release.

Imbalances

• Hyperparathyroidism: Bone softening, kidney stones, nervous system depression.

• Hypoparathyroidism: Tetany, muscle spasms, possible death.

Adrenal Glands

Structure and Hormones

• Located atop the kidneys; consist of adrenal cortex (three layers) and adrenal medulla.

• Cortex produces corticosteroids: mineralocorticoids (aldosterone), glucocorticoids (cortisol), and gonadocorticoids (sex hormones).

• Medulla produces catecholamines: epinephrine and norepinephrine.

Mineralocorticoids (Aldosterone)

• Regulate sodium and potassium balance, blood volume, and pressure.

• Secretion controlled by renin-angiotensin system, potassium levels, ACTH, and ANP.

Glucocorticoids (Cortisol)

• Maintain blood glucose, resist stress, suppress inflammation and immune response.

• Imbalances: Cushing's syndrome (excess), Addison's disease (deficiency).

Gonadocorticoids

• Mostly androgens; contribute to puberty, secondary sex characteristics, and sex drive.

Adrenal Medulla

• Secretes epinephrine and norepinephrine for fight-or-flight response: increases blood glucose, heart rate, and blood pressure.

Pineal Gland

• Secretes melatonin, which regulates sleep-wake cycles and may influence puberty and reproductive timing.

Pancreas

Structure and Hormones

• Both exocrine (digestive enzymes) and endocrine (hormones) functions.

• Alpha cells produce glucagon (raises blood glucose); beta cells produce insulin (lowers blood glucose).

Glucagon

• Targets the liver to promote glycogenolysis and gluconeogenesis, increasing blood glucose.

Insulin

• Lowers blood glucose by enhancing cellular uptake and inhibiting glycogenolysis and gluconeogenesis.

• Imbalances: Diabetes mellitus (deficiency or inactivity), hyperinsulinism (excess).

Gonads and Placenta

Ovaries

• Produce estrogens and progesterone for female reproductive development and function.

Testes

• Produce testosterone for male reproductive development, secondary sex characteristics, and sperm production.

Placenta

• Secretes estrogens, progesterone, and hCG during pregnancy.

Other Hormone-Producing Structures

• Heart: Atrial natriuretic peptide (ANP) lowers blood pressure.

• Kidneys: Erythropoietin (EPO) stimulates red blood cell production; renin regulates blood pressure.

• Skin: Produces cholecalciferol (vitamin D precursor).

• Adipose tissue: Leptin regulates appetite and energy expenditure.

• Skeleton: Osteocalcin enhances insulin secretion and sensitivity.

• Thymus: Thymulin, thymopoietins, and thymosins are involved in T lymphocyte development.