The Endocrine System

Introduction to the Endocrine System

The endocrine system is one of the body’s two major control systems, working alongside the nervous system to coordinate and integrate the activity of body cells. It uses hormones—chemical messengers transported in the blood—to influence metabolic activities throughout the body. Endocrine responses are generally slower but longer-lasting than those of the nervous system. The study of hormones and endocrine organs is known as endocrinology.

• Hormones: Chemical messengers secreted by endocrine glands, transported by blood to target cells.

• Endocrine glands: Ductless glands that secrete hormones directly into the extracellular fluid (e.g., pituitary, thyroid, parathyroid, adrenal, pineal glands).

• Exocrine glands: Glands with ducts that secrete nonhormonal substances (e.g., sweat, saliva).

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

Comparison of Nervous and Endocrine Systems

Major Processes Controlled by the Endocrine System

• Reproduction

• Growth and development

• Maintenance of electrolyte, water, and nutrient balance

• Regulation of cellular metabolism and energy balance

• Mobilization of body defenses

Chemical Messengers

• Hormones: Long-distance chemical signals in the blood.

• Autocrines: Chemicals that exert effects on the same cells that secrete them (local action).

• Paracrines: Chemicals that affect neighboring cells (local action).

• Autocrines and paracrines are not considered part of the endocrine system due to their local effects.

Chemical Structure and Classification of Hormones

Hormone Structure and Solubility

The chemical structure of a hormone determines its solubility in water, which affects its transport in the blood, duration of action, and receptor location.

• Amino acid–based hormones: Most hormones; include amino acid derivatives, peptides, and proteins. They are water-soluble (except thyroxine) and cannot cross the plasma membrane.

• Steroid hormones: Synthesized from cholesterol; lipid-soluble and can cross the plasma membrane. Includes gonadal and adrenocortical hormones.

• Eicosanoids: Sometimes considered hormones, but mostly act as local paracrines and autocrines.

Mechanisms of Hormone Action

Target Cells and Effects

Hormones affect only target cells that have specific receptors for them. They can alter target cell activity by increasing or decreasing the rates of normal cellular processes.

• Alter membrane permeability or membrane potential by opening/closing ion channels

• Stimulate synthesis of enzymes or proteins

• Activate or deactivate enzymes

• Induce secretory activity

• Stimulate mitosis

Hormone Action Pathways

• Water-soluble hormones: Act on plasma membrane receptors; use second messenger systems (e.g., cAMP, PIP2-calcium). Cannot enter the cell.

• Lipid-soluble hormones: Act on intracellular receptors; directly activate genes. Can diffuse through the plasma membrane.

Cyclic AMP (cAMP) Second Messenger System

Most amino acid–based hormones (except thyroid hormone) use the cAMP second messenger system to exert their effects.

1. Hormone (first messenger) binds to receptor.

2. Receptor activates a G protein.

3. G protein activates (or inhibits) adenylate cyclase.

4. Adenylate cyclase converts ATP to cAMP (second messenger).

5. cAMP activates protein kinases, which phosphorylate other proteins.

PIP2-Calcium Signaling Mechanism

Some hormones use the PIP2-calcium pathway, where hormone-activated G proteins activate phospholipase C, splitting PIP2 into DAG and IP3. DAG activates protein kinases, while IP3 triggers Ca2+ release from intracellular stores, amplifying the cellular response.

Direct Gene Activation by Lipid-Soluble Hormones

Lipid-soluble hormones (steroids and thyroid hormone) diffuse into target cells, bind to intracellular receptors, and the hormone-receptor complex enters the nucleus to bind specific DNA regions, initiating transcription and protein synthesis.

1. Steroid hormone diffuses through plasma membrane and binds to intracellular receptor.

2. Receptor-hormone complex enters the nucleus.

3. Complex binds to a specific DNA region.

4. Binding initiates transcription of the gene to mRNA.

5. mRNA directs protein synthesis.

Regulation of Hormone Release

Negative Feedback Mechanisms

Hormone secretion is primarily regulated by negative feedback mechanisms, maintaining hormone levels within a narrow range. Stimulus triggers hormone release, and as hormone levels rise, target organ effects remove the stimulus, inhibiting further release.

Types of Endocrine Gland Stimuli

• Humoral stimuli: Changing blood levels of ions/nutrients directly stimulate hormone release (e.g., low Ca2+ stimulates PTH release).

• Neural stimuli: Nerve fibers stimulate hormone release (e.g., sympathetic fibers stimulate adrenal medulla to secrete catecholamines).

• Hormonal stimuli: Hormones stimulate other endocrine organs to release their hormones (e.g., hypothalamic hormones regulate anterior pituitary hormones).

Hormone Activity and Target Cell Response

Target Cell Specificity and Regulation

• Target cells must have specific receptors for a hormone to respond.

• Degree of activation depends on hormone blood levels, number of receptors, and receptor affinity.

• Up-regulation: Target cells form more receptors in response to low hormone levels.

• Down-regulation: Target cells lose receptors in response to high hormone levels.

Hormone Half-Life, Onset, and Duration

• Hormones circulate in blood either free or bound to plasma proteins (steroids and thyroid hormone are bound).

• Half-life: Time required for hormone blood level to decrease by half; varies from seconds to a week.

• Onset and duration of hormone action depend on hormone type (water- vs. lipid-soluble).

Comparison of Lipid- and Water-Soluble Hormones

Hormone Interactions at Target Cells

• Permissiveness: One hormone cannot exert its effects without another hormone present (e.g., reproductive hormones need thyroid hormone).

• Synergism: More than one hormone produces the same effect, amplifying the response (e.g., glucagon and epinephrine).

• Antagonism: One or more hormones oppose the action of another (e.g., insulin and glucagon).

The Hypothalamus and Pituitary Gland

Structure and Function

The hypothalamus is connected to the pituitary gland (hypophysis) via the infundibulum. The pituitary has two major lobes: the posterior pituitary (neural tissue, stores and releases neurohormones) and the anterior pituitary (glandular tissue, manufactures and releases hormones).

Pituitary-Hypothalamic Relationships

• Posterior pituitary: Contains axon terminals of hypothalamic neurons; stores and releases oxytocin and antidiuretic hormone (ADH).

• Anterior pituitary: Glandular tissue; connected to hypothalamus via hypophyseal portal system; releases six hormones (GH, TSH, ACTH, FSH, LH, PRL).

Posterior Pituitary Hormones

Oxytocin

• Stimulates uterine contractions during childbirth and milk ejection during breastfeeding (positive feedback mechanisms).

• Acts via PIP2-calcium second messenger system.

• Also acts as a neurotransmitter in the brain.

Antidiuretic Hormone (ADH)

• Released in response to high blood osmolarity or low blood volume.

• Promotes water reabsorption in kidney tubules, reducing urine output and increasing blood volume.

• High concentrations cause vasoconstriction (vasopressin).

• Inhibited by alcohol.

Anterior Pituitary Hormones

Growth Hormone (GH)

• Direct actions: Decreases glucose uptake, increases blood glucose and fatty acids, stimulates protein synthesis.

• Indirect actions: Stimulates liver, skeletal muscle, and bone to produce insulin-like growth factors (IGFs), promoting cell division and growth.

• Regulation: Stimulated by GHRH, inhibited by GHIH (somatostatin) and negative feedback from IGFs.

Thyroid-Stimulating Hormone (TSH)

• Stimulates normal development and secretory activity of the thyroid gland.

• Regulated by TRH from the hypothalamus; inhibited by rising thyroid hormone levels (negative feedback).

Adrenocorticotropic Hormone (ACTH)

• Stimulates adrenal cortex to release corticosteroids (mainly glucocorticoids).

• Regulated by CRH from the hypothalamus; inhibited by rising glucocorticoid levels.

Gonadotropins (FSH and LH)

• FSH stimulates gamete production; LH stimulates production of gonadal hormones.

• Regulated by GnRH from the hypothalamus; inhibited by rising gonadal hormone levels.

Prolactin (PRL)

• Stimulates milk production in females; role in males is unclear.

• Regulated by PIH (dopamine); levels rise toward the end of pregnancy and with infant suckling.

The Thyroid Gland

Structure and Function

The thyroid gland is a butterfly-shaped organ located on the anterior trachea, just below the larynx. It consists of follicles filled with colloid, where thyroid hormone is produced, and parafollicular cells that produce calcitonin.

Thyroid Hormone (TH)

• Major metabolic hormone; produced as T4 (thyroxine) and T3 (triiodothyronine).

• Increases basal metabolic rate, heat production, and regulates tissue growth and development.

• Permissive for catecholamines (epinephrine, norepinephrine) in maintaining blood pressure.

• Regulated by TSH from the anterior pituitary (negative feedback).

Calcitonin

• Produced by parafollicular (C) cells in response to high blood Ca2+ levels.

• Inhibits osteoclast activity, stimulates Ca2+ uptake into bone matrix.

• Antagonist to parathyroid hormone (PTH).

The Parathyroid Glands

Structure and Function

Four small glands embedded in the posterior aspect of the thyroid gland. They secrete parathyroid hormone (PTH), the most important hormone in Ca2+ homeostasis.

• Stimulates osteoclasts to release Ca2+ from bone.

• Enhances reabsorption of Ca2+ by kidneys.

• Promotes activation of vitamin D, increasing intestinal absorption of Ca2+.

The Adrenal Glands

Structure and Function

Located atop the kidneys, each adrenal gland consists of an outer cortex (produces corticosteroids) and an inner medulla (produces catecholamines).

• Mineralocorticoids (e.g., aldosterone): Regulate Na+ and K+ balance, blood volume, and pressure.

• Glucocorticoids (e.g., cortisol): Influence metabolism, help resist stress, and regulate blood glucose.

• Gonadocorticoids (e.g., androgens): Contribute to secondary sex characteristics and sex drive.

• Catecholamines (epinephrine, norepinephrine): Mediate the fight-or-flight response, increasing heart rate, blood pressure, and blood glucose.

The Pineal Gland

Structure and Function

Located in the diencephalon, the pineal gland secretes melatonin, which regulates sleep-wake cycles and may have antioxidant and antigonadotropic effects.

The Pancreas

Structure and Function

The pancreas is a mixed gland with both exocrine (digestive enzymes) and endocrine (hormones) functions. The endocrine portion consists of pancreatic islets containing alpha cells (glucagon) and beta cells (insulin).

• Glucagon: Raises blood glucose by promoting glycogen breakdown and gluconeogenesis in the liver.

• Insulin: Lowers blood glucose by promoting glucose uptake and storage, and inhibiting glycogen breakdown.

The Gonads and Placenta

Structure and Function

• Ovaries: Produce estrogens and progesterone, regulating female reproductive development and function.

• Testes: Produce testosterone, regulating male reproductive development and function.

• Placenta: Temporary endocrine organ during pregnancy, secretes estrogens, progesterone, and hCG.

Hormone Secretion by Other Organs

• Adipose tissue: Leptin (satiety), resistin (insulin antagonist), adiponectin (insulin sensitizer).

• Gastrointestinal tract: Gastrin, ghrelin, secretin, CCK, GIP (regulate digestion).

• Heart: ANP and BNP (lower blood pressure by increasing Na+ excretion).

• Kidneys: Erythropoietin (stimulates RBC production), renin (regulates blood pressure).

• Skeleton: Osteocalcin (regulates insulin secretion and sensitivity).

• Skin: Cholecalciferol (vitamin D3 precursor).

• Thymus: Thymosins, thymulin, thymopoietins (T cell development).

Developmental Aspects and Environmental Influences

• Hormone-producing glands arise from all three germ layers during development.

• Endocrine function generally declines with age (e.g., GH, estrogen, testosterone, TH).

• Environmental pollutants can disrupt hormone function, affecting sex hormones, thyroid hormone, and glucocorticoids.