The Endocrine System: Structure, Function, and Regulation

The Endocrine System

Overview of the Endocrine System

The endocrine system is a network of glands that secrete hormones to regulate various physiological processes, including metabolism, growth, reproduction, and homeostasis. Hormones are chemical messengers released into the bloodstream, affecting distant target organs.

• Key Glands: Pituitary, thyroid, parathyroid, adrenal, pancreas, gonads, pineal, and others.

• Hormones: Include peptide, steroid, and amino acid-derived hormones.

• Regulation: Often involves negative feedback mechanisms.

Posterior Pituitary and Its Hormones

Oxytocin and Antidiuretic Hormone (ADH)

The posterior pituitary stores and releases oxytocin and ADH, which are produced by hypothalamic neurons. These hormones are released in response to nerve impulses and act via second-messenger systems.

• Oxytocin: Stimulates uterine contractions during childbirth and triggers milk ejection in lactating women. Also involved in sexual arousal and orgasm.

• ADH: Regulates water balance by inhibiting urine formation when blood solute concentration is high. Alcohol inhibits ADH, increasing urine output.

• Homeostatic Imbalances: ADH deficiency causes diabetes insipidus; excess ADH leads to SIADH.

Thyroid Gland

Anatomy and Histology

The thyroid gland consists of two lateral lobes connected by an isthmus. It is composed of follicles filled with colloid, which is the precursor to thyroid hormones. Parafollicular cells produce calcitonin.

• Follicles: Produce thyroglobulin, a glycoprotein.

• Colloid: Contains thyroglobulin and iodine, essential for hormone synthesis.

• Parafollicular cells: Secrete calcitonin.

Thyroid Hormones (TH): T3 and T4

Thyroid hormone consists of two related compounds: T4 (thyroxine) and T3 (triiodothyronine). T4 has four iodine atoms, T3 has three. Both are critical for metabolic regulation.

• Major metabolic hormone: Increases metabolic rate and heat production (calorigenic effect).

• Functions: Maintains blood pressure, regulates tissue growth, supports skeletal and nervous system development, and reproductive capabilities.

Synthesis of Thyroid Hormone

Thyroid hormone synthesis involves several steps, including thyroglobulin production, iodide uptake, iodination of tyrosine, and release of T3 and T4 into the bloodstream.

• Stepwise process: Iodide is actively transported, oxidized to iodine, attached to tyrosine, and then iodinated tyrosines are linked to form T3 and T4.

• Colloid endocytosis: Colloid is taken up and combined with lysosomes, releasing T3 and T4.

Transport and Regulation of Thyroid Hormone

T4 and T3 are transported by thyroxine-binding globulins (TBGs). T3 is more active than T4, and peripheral tissues convert T4 to T3. Release is regulated by negative feedback involving TSH and TRH.

• Negative feedback: Rising TH levels inhibit TSH release.

• TRH: Can override feedback during pregnancy or cold exposure.

Homeostatic Imbalances of Thyroid Hormone

Imbalances in thyroid hormone secretion can lead to various disorders.

• Hyposecretion in adults: Myxedema; endemic goiter if due to iodine deficiency.

• Hyposecretion in infants: Cretinism.

• Hypersecretion: Graves’ disease, an autoimmune disorder causing continuous stimulation of TH release.

Calcitonin

Calcitonin is produced by parafollicular cells and acts as an antagonist to parathyroid hormone (PTH). It inhibits osteoclast activity and promotes Ca2+ uptake into bone.

• Regulation: Humoral negative feedback based on blood Ca2+ concentration.

• Role in humans: Removal of thyroid does not affect Ca2+ homeostasis.

Parathyroid Glands

Anatomy and Histology

The parathyroid glands are small glands located on the posterior aspect of the thyroid. They contain chief cells (secrete PTH) and oxyphil cells (function unknown).

• PTH: Most important hormone in Ca2+ homeostasis.

Parathyroid Hormone (PTH)

PTH regulates calcium levels in the blood by acting on bones, kidneys, and intestines.

• Stimulates osteoclasts: Releases Ca2+ and phosphate from bone.

• Enhances kidney reabsorption: Increases Ca2+ reabsorption and phosphate excretion.

• Activates vitamin D: Promotes Ca2+ absorption from the intestine.

• Negative feedback: Rising blood Ca2+ inhibits PTH release.

Homeostatic Imbalances of PTH

Imbalances in PTH secretion can have serious consequences.

• Hyperparathyroidism: Bones soften, nervous system depression, kidney stones.

• Hypoparathyroidism: Tetany, respiratory paralysis, death.

Adrenal (Suprarenal) Glands

Anatomy and Histology

The adrenal glands are paired, pyramid-shaped organs atop the kidneys. They consist of two regions: the adrenal cortex (glandular tissue) and adrenal medulla (nervous tissue).

• Adrenal cortex: Three layers producing corticosteroids.

• Adrenal medulla: Part of the sympathetic nervous system.

Adrenal Cortex Hormones

The adrenal cortex produces three types of corticosteroids:

• Zona glomerulosa: Mineralocorticoids (e.g., aldosterone).

• Zona fasciculata: Glucocorticoids (e.g., cortisol).

• Zona reticularis: Gonadocorticoids (sex hormones).

Mineralocorticoids (Aldosterone)

Aldosterone regulates electrolytes, especially Na+ and K+, affecting blood volume and pressure.

• Stimulates Na+ reabsorption and water retention by kidneys.

• Regulatory mechanisms: Renin-angiotensin, plasma K+, ACTH, and ANP.

Glucocorticoids (Cortisol)

Cortisol maintains blood glucose and pressure, especially during stress. It promotes gluconeogenesis and increases blood glucose, fatty acids, and amino acids.

• Released in response to: ACTH, eating/activity patterns, and stress.

• Prime metabolic effect: Gluconeogenesis: $\text{Gluconeogenesis:} \quad \text{Glucose} \leftarrow \text{Fats and Proteins}$

Homeostatic Imbalances of Glucocorticoids

Imbalances can lead to Cushing’s syndrome (hypersecretion) or Addison’s disease (hyposecretion).

• Cushing’s syndrome: Depresses cartilage/bone formation, immune function, and causes fat redistribution.

• Addison’s disease: Deficits in glucocorticoids and mineralocorticoids, leading to low glucose/Na+, weight loss, dehydration, hypotension.

Gonadocorticoids (Sex Hormones)

Androgens are converted to testosterone or estrogens, contributing to puberty, secondary sex characteristics, and sex drive.

Adrenal Medulla Hormones

Chromaffin cells secrete epinephrine and norepinephrine, which mediate the body’s response to stress.

• Epinephrine: Stimulates metabolic activities, bronchial dilation, blood flow to muscles and heart.

• Norepinephrine: Influences vasoconstriction and blood pressure.

Pineal Gland

Melatonin

The pineal gland secretes melatonin, which regulates circadian rhythms, sexual maturation, and physiological processes with rhythmic variations.

Pancreas

Anatomy and Histology

The pancreas is a triangular gland with both exocrine (acinar cells) and endocrine (islets of Langerhans) functions. Alpha cells produce glucagon; beta cells produce insulin.

Glucagon

Glucagon is a hyperglycemic hormone targeting the liver, promoting glycogenolysis, gluconeogenesis, and glucose release into the blood.

• Glycogenolysis: Breakdown of glycogen to glucose.

• Gluconeogenesis: Synthesis of glucose from noncarbohydrates.

Insulin

Insulin lowers blood glucose by enhancing glucose uptake into cells, inhibiting glycogenolysis and gluconeogenesis, and promoting glycogen and fat formation.

• Activates tyrosine kinase receptor: Leads to increased glucose uptake and metabolism.

• Inhibits: Glycogenolysis and gluconeogenesis.

Homeostatic Imbalances of Insulin

Imbalances include diabetes mellitus (DM) and hyperinsulinism.

• DM: Due to hyposecretion or hypoactivity of insulin; characterized by polyuria, polydipsia, and polyphagia.

• Hyperinsulinism: Excessive insulin secretion causes hypoglycemia, disorientation, unconsciousness.

Gonads and Placenta

Ovaries and Placenta

Ovaries produce estrogens and progesterone, responsible for female reproductive organ maturation, secondary sexual characteristics, and uterine changes. The placenta secretes estrogens, progesterone, and hCG.

Testes

Testes produce testosterone, which initiates male reproductive organ maturation, secondary sexual characteristics, sex drive, and maintains reproductive function.

Other Hormone-Producing Structures

Several other organs produce hormones:

• Heart: ANP reduces blood pressure, volume, and Na+.

• GI tract: Gastrin, secretin, cholecystokinin regulate digestion.

• Kidneys: Erythropoietin stimulates RBC production; renin initiates renin-angiotensin mechanism.

• Skin: Cholecalciferol is a precursor of vitamin D.

• Adipose tissue: Leptin regulates appetite and energy expenditure.

• Skeleton: Osteocalcin stimulates insulin secretion and improves glucose handling.

• Thymus: Thymulin, thymopoietins, thymosins support T lymphocyte development.

Developmental Aspects of the Endocrine System

Hormone function can be disrupted by environmental pollutants. Sex hormones, thyroid hormone, and glucocorticoids are particularly vulnerable. Aging affects hormone levels and responsiveness, contributing to various physiological changes.

• Ovaries: Become unresponsive to gonadotropins with age; estrogen deficiency causes problems.

• Testosterone: Diminishes with age, effects seen in very old age.

• GH and TH: Decline with age, leading to muscle atrophy and lower metabolic rates.

• PTH: Remains constant, but estrogen deficiency increases bone-demineralizing effects.