The Endocrine System: Structure, Function, and Disorders

The Endocrine System: Overview

Introduction to the Endocrine System

The endocrine system works alongside the nervous system to coordinate and integrate the activity of cells throughout the body. It influences metabolic activities by means of hormones transported in the blood. Endocrine responses occur more slowly than nervous responses but tend to last longer. Hormones are produced as needed by various glands and tissues.

• Hormones: Chemical messengers that regulate physiological activities and maintain homeostasis.

• Some organs, such as the pancreas and gonads, produce both hormones and exocrine products.

• The hypothalamus has both neural and endocrine functions.

• Other hormone-producing tissues include adipose cells, thymus, small intestine, stomach, kidneys, and heart.

Endocrine vs. Nervous System

Comparison of Control Systems

The endocrine system and nervous system are both control systems but differ in their mechanisms and speed of action.

• Nervous system: Uses electrochemical impulses delivered by neurons; responses occur within milliseconds.

• Endocrine system: Uses chemical messengers (hormones) carried in the blood; responses are slower but longer-lasting.

Endocrine Glands vs. Exocrine Glands

Types of Glands

There are two main types of glands in the body:

• Exocrine glands: Have ducts; secrete non-hormonal products to membrane surfaces (e.g., sweat, sebaceous, mucous, digestive glands).

• Endocrine glands: Ductless; release hormones directly into blood or lymph. These glands have rich vascular and lymphatic drainage.

Major endocrine glands include the pituitary, thyroid, parathyroid, adrenal, pineal, and thymus. Some organs (e.g., pancreas, gonads) contain endocrine tissue but are not exclusively endocrine glands.

Chemical Messengers and Hormone Action

Hormones and Their Effects

Hormones are long-distance chemical signals that travel in the blood or lymph to regulate metabolic functions of other cells. They serve to maintain homeostasis by altering the rate of physiological activities in target cells.

• Hormones can alter plasma membrane permeability, stimulate protein synthesis, activate or deactivate enzymes, induce secretory activity, and stimulate mitosis.

Chemistry of Hormones

Hormones are classified into two main groups:

• Amino acid-based hormones: Most hormones; water-soluble; act via second-messenger systems and bind to cell surface receptors.

• Steroid hormones: Synthesized from cholesterol; lipid-soluble; act via direct gene activation and bind to intracellular receptors.

Second-Messenger Systems

Most amino acid-based hormones use a second-messenger system:

• The hormone (first messenger) binds to a receptor on the cell membrane.

• This triggers the synthesis of cyclic AMP (cAMP) inside the cell.

• cAMP (second messenger) activates enzymes that induce cellular changes.

Intracellular Receptors and Direct Gene Activation

Steroid and thyroid hormones diffuse into target cells and bind to intracellular receptors. The receptor-hormone complex interacts with DNA to initiate transcription and protein synthesis.

Target Cell Specificity and Activation

Target Cell Specificity

Only cells with specific receptors for a hormone will respond to it. The effect of a hormone depends on:

• Blood levels of the hormone

• Number of receptors on/in the target cell

• Affinity (strength) of binding between hormone and receptor

Hormones are inactivated by enzymes or removed by the kidneys and liver.

Control of Hormone Release

Stimuli for Hormone Release

Hormones are released in response to three types of stimuli:

• 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).

The Pituitary Gland and Hypothalamus

Structure and Function

The pituitary gland (hypophysis) is located in the sella turcica of the sphenoid bone and is attached to the hypothalamus by the infundibulum. It has two major lobes:

• Posterior lobe (neurohypophysis): Composed of nervous tissue; stores and releases oxytocin and antidiuretic hormone (ADH) made by the hypothalamus.

• Anterior lobe (adenohypophysis): Composed of glandular tissue; manufactures and releases its own hormones in response to hypothalamic stimulation.

Anterior Pituitary Hormones

• TSH (Thyroid Stimulating Hormone): Stimulates thyroid hormone secretion.

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

• FSH (Follicle Stimulating Hormone): Stimulates gamete production.

• LH (Luteinizing Hormone): Promotes production of estrogen and testosterone.

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

• Prolactin: Stimulates milk production.

Posterior Pituitary Hormones

• ADH (Antidiuretic Hormone): Promotes water reabsorption in kidneys; inhibited by alcohol.

• Oxytocin: Stimulates uterine contractions during childbirth and milk ejection during breastfeeding.

Thyroid and Parathyroid Glands

Thyroid Gland Structure and Function

The thyroid gland is located in the anterior neck and consists of two lobes connected by an isthmus. It produces thyroid hormone (TH) and calcitonin.

Thyroid Hormone (TH)

• Major metabolic hormone; affects nearly every cell in the body.

• Composed of T4 (thyroxine) and T3 (triiodothyronine).

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

Calcitonin

• Produced by parafollicular (C) cells of the thyroid.

• Released in response to high blood calcium; lowers blood calcium by stimulating bone uptake.

Parathyroid Glands and Hormone

The parathyroid glands are small glands on the posterior thyroid. They secrete parathyroid hormone (PTH), the main regulator of blood calcium.

• PTH increases blood calcium by stimulating bone resorption, increasing kidney reabsorption, and enhancing intestinal absorption (via vitamin D activation).

Adrenal (Suprarenal) Glands

Structure and Hormones

The adrenal glands sit atop the kidneys and consist of two regions:

• Adrenal cortex: Produces corticosteroids (mineralocorticoids, glucocorticoids, gonadocorticoids).

• Adrenal medulla: Produces catecholamines (epinephrine and norepinephrine).

Adrenal Cortex Hormones

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

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

• Gonadocorticoids: Sex hormones; contribute to puberty and secondary sex characteristics.

Adrenal Medulla Hormones

• Secretes epinephrine and norepinephrine in response to stress; increases heart rate, blood pressure, and blood glucose.

Pineal Gland

Melatonin and Circadian Rhythms

The pineal gland secretes melatonin, which regulates sleep-wake cycles and other rhythmic physiological processes.

Pancreas

Structure and Function

The pancreas has both exocrine (digestive enzyme production) and endocrine (hormone production) functions. The islets of Langerhans contain alpha cells (produce glucagon) and beta cells (produce insulin).

Glucagon and Insulin

• Glucagon: Released when blood glucose is low; stimulates glycogen breakdown and glucose synthesis in the liver.

• Insulin: Released when blood glucose is high; promotes glucose uptake by cells, glycogen formation, and fat storage.

Gonads

Sex Hormones

• Ovaries: Produce estrogens and progesterone; regulate female reproductive development and function.

• Testes: Produce testosterone; regulate male reproductive development and function.

Thymus

Immune Function

The thymus is involved in the development of T lymphocytes and the immune response, especially during youth.

Selected Endocrine Disorders

Growth Hormone Disorders

• Gigantism: Hypersecretion of GH in children; excessive growth.

• Acromegaly: Hypersecretion of GH in adults; overgrowth of hands, feet, face.

• Pituitary dwarfism: Hyposecretion of GH in children; short stature.

Diabetes Insipidus and Mellitus

• Diabetes insipidus: ADH deficiency; excessive urine output and thirst.

• Diabetes mellitus (DM): Insulin deficiency or inactivity; high blood glucose, polyuria, polydipsia, polyphagia.

Other Disorders

• Myxedema: Hypothyroidism; low metabolic rate, edema.

• Graves' disease: Hyperthyroidism; elevated metabolism, weight loss.

• Cushing's syndrome: Excess glucocorticoids; high blood glucose, muscle/bone loss.

• Addison's disease: Deficits in glucocorticoids/mineralocorticoids; weight loss, dehydration, low blood glucose.