Anatomy and Physiology of the Endocrine System

Introduction and Fundamentals of Endocrinology

Endocrinology is defined as the scientific study of hormones and endocrine organs. A hormone is a chemical messenger that regulates the physiological functions of the organism by acting on target cells, which are often located far from the site of hormonal secretion. Together, the nervous system (SN) and the endocrine system (SE) ensure communication and coordination between cells and the various systems of the organism to maintain homeostasis, which is the internal balance of the body. The endocrine system's functions are diverse and exhaustive, including the control of blood homeostasis regarding composition and volume. Specific hormones like aldosterone regulate natremia ($Na^+$) and kaliemia ($K^+$), while the parathyroid hormone (PTH) and calcitonin regulate calcemia ($Ca^{2+}$), and antidiuretic hormone (ADH) regulates water balance. The system also regulates digestive activity through hormones such as gastrin, secretin, and cholecystokinin, which influence the secretion and motility of the digestive tract. Furthermore, it regulates reproductive activities via GnRH, FSH, LH, estrogens, and testosterone, which govern the development of the genital apparatus and sexual characteristics. Oxytocin is responsible for childbirth and attachment, while prolactin governs lactation. Finally, the endocrine system regulates growth, development, and metabolism through Growth Hormone (GH), thyroid hormones ($T_3$ and $T_4$), insulin, glucagon, and cortisol, the latter of which is also involved in stress management.

Hormonal Regulation and Pathological Disruptions

Under normal conditions, hormones act as messengers to maintain homeostasis. They are produced by glands or endocrine tissues in response to specific signals, circulate through the blood, and produce specific effects on target cells through finely regulated secretion controlled by negative or positive feedback loops (retro-inhibition or retro-activation). In the context of tumors or endocrine diseases, hormonal production can become abnormal. This may involve a total lack of regulation, where feedback loops are ignored, or erratic and unpredictable secretions. In some cases, secretion is inappropriate, meaning it is produced by a tissue not intended for that hormone. For example, a lung cancer might secrete ACTH, which is normally only produced by the pituitary gland (hypophysis). This occurs because all cells in the body share the same DNA; a liver cell differs from a pancreatic cell only because certain genes are activated or silenced. In a tumor, mutations or loss of cellular specialization can activate normally silent genes, leading to the production of hormones by non-endocrine tissues. Hormonal imbalances can also manifest as hypersecretion, where excess hormone causes symptoms distant from the secretion site, or hyposecretion, where an organ decreases or ceases production entirely.

Anatomy and Classification of Glands

A gland is an organ specialized in the production and release of chemical substances. Glands are categorized into three types based on where their secretions are evacuated. Exocrine glands possess conduits (ducts) to transport non-hormonal secretions to the exterior of the body or onto a mucous membrane, such as sweat, sebaceous, mammary, and salivary glands. Endocrine glands are ductless; they produce hormones and release them into the interstitial fluid, where they enter the bloodstream to reach target cells. Examples include the thyroid, parathyroids, hypophysis, adrenals, and the pineal gland. Mixed glands, such as the pancreas, ovaries, and testicles, perform both endocrine and exocrine functions. Additionally, certain tissues that are not primarily glands also contain endocrine cells capable of secreting hormones into the blood while performing other primary functions. For instance, the heart acts as a pump, but a small group of cells in the right atrium can release Atrial Natriuretic Factor (FNA) to reduce high blood pressure. Other such tissues include the thymus, hypothalamus, stomach, small intestine, kidneys, skin, adipose tissue, and the placenta.

Chemistry and Action of Hormones

Hormones are classified chemically into two main groups. The first group consists of hormones derived from amino acids. These include amine hormones, which are small molecules similar to amino acids like tyrosine or tryptophan (e.g., adrenaline, noradrenaline, dopamine, serotonin, and melatonin). Peptide hormones are chains of amino acids; short polypeptides like ADH and oxytocin consist of nine amino acids, while small proteins like GH have $191$ amino acids and prolactin (PRL) has $198$ amino acids. Glycoproteins, such as TSH, LH, and FSH, contain over $200$ amino acids with carbohydrate side chains. The second group consists of lipid-derived hormones, which include steroids synthesized from cholesterol (such as sex hormones and adrenal cortex hormones) and eicosanoids (prostaglandins and leukotrienes). Thyroid hormones ($T_3$ and $T_4$) are a special case, derived from tyrosine and iodine, but they behave like lipid-soluble hormones. Hydrosoluble hormones (amino acid-based) cannot cross the plasma membrane; they bind to membrane receptors and use a second messenger system, such as $cAMP$, to trigger a cascade of reactions. Liposoluble hormones (steroids and thyroid hormones) cross the membrane, bind to intracellular receptors in the cytosol or nucleus, and directly activate or inhibit specific genes to alter protein synthesis. Hormonal activity is triggered in three ways: humoral stimulus (response to blood levels of ions or nutrients like $Ca^{2+}$), hormonal stimulus (one hormone triggers another), or nervous stimulus (direct stimulation by the nervous system, such as the sympathetic system stimulating the adrenal medulla).

The Pancreas and Glucose Regulation

The pancreas is a mixed gland. Its exocrine part consists of acinar cells that secrete pancreatic juices for digestion. Its endocrine part consists of the Islets of Langerhans, containing alpha cells ($α$) which secrete glucagon and beta cells ($β$) which secrete insulin. Insulin is a hypoglycemic agent that lowers blood glucose levels by promoting glucose transport into cells (especially myocytes and adipocytes) via GLUT transporters. It stimulates glycogenesis (storage of glucose as glycogen in the liver and muscles), lipogenesis (conversion of glucose to lipids), and protein synthesis. Normal glycemia ranges between $3.5$ and $6.0\,mmol/L$. Glucagon is a powerful hyperglycemic agent that raises blood glucose levels by stimulating glycogenolysis (breakdown of glycogen) and gluconeogenesis (synthesis of glucose from non-carbohydrates like lactic acid or glycerol) in the liver. Diabetes mellitus occurs when this balance is lost. Type 1 diabetes is an autoimmune disease characterized by the sudden destruction of beta cells and an absence of insulin, requiring insulin injections ($10\%$ of cases). Type 2 diabetes involves insulin resistance or insufficient production, often linked to obesity and sedentary lifestyle, appearing gradually and affecting $90\%$ of cases.

The Hypothalamus-Hypophysis Axis

The hypothalamus is the junction between the nervous and endocrine systems, receiving signals from the cerebral cortex, thalamus, and limbic system. It controls the hypophysis (pituitary gland) through releasing hormones (e.g., GHRH) and inhibiting hormones (e.g., GHIH). The hypophysis has two parts. The neurohypophysis (posterior) does not synthesize hormones but stores and releases ADH and oxytocin produced by the hypothalamus via nervous stimulus. ADH (antidiuretic hormone) targets the kidneys to reduce urine production and increase blood pressure; its deficiency causes diabetes insipidus, characterized by intense thirst and polyuria ($4$ to $20\,L/day$). The adenohypophysis (anterior) synthesizes six major hormones: GH, PRL, TSH, ACTH, FSH, and LH. These are triggered by hormonal stimuli. Growth Hormone (GH) stimulates cell division and growth; its levels peak during adolescence (about $700\,μg/day$ compared to $400\,μg/day$ in adults) and during early sleep stages. GH imbalances include gigantism (childhood hypersecretion), acromegaly (adult hypersecretion causing bone thickening in hands, feet, and face), and pituitary dwarfism (childhood hyposecretion leading to a maximum height of approximately $1.4\,m$).

The Thyroid and Parathyroid Glands

The thyroid gland produces $T_3$, $T_4$, and calcitonin. $T_3$ and $T_4$ (thyroxine) contain iodine and are essential for regulating the basal metabolic rate, heat production, and the development of the nervous system. A deficiency in iodine can lead to a goiter (hypertrophy of the thyroid) or cretinism in children. Hyperthyroidism increases metabolism and heart rate, while hypothyroidism decreases them. The four to eight parathyroid glands secrete parathyroid hormone (PTH) in response to low blood calcium ($Ca^{2+}$). PTH is vital because it maintains stable calcemia ($2.2$ to $2.5\,mmol/L$), which is necessary for nerve communication, muscle contraction, and blood coagulation. PTH stimulates osteoclasts to resorbe bone and release $Ca^{2+}$ into the blood, and it stimulates the kidneys to produce calcitriol (activated Vitamin D) to increase intestinal calcium absorption. In contrast, calcitonin (from the thyroid) inhibits osteoclasts to lower blood calcium, though its effects are negligible in healthy adults. Bone remodeling is a constant balance between destruction by osteoclasts (stimulated by PTH and cortisol) and reconstruction by osteoblasts (stimulated by GH, testosterone, and estrogens).

The Adrenal Glands and Stress Management

The adrenal glands sit atop the kidneys and consist of the cortex and the medulla. The adrenal cortex produces over $30$ corticosteroids in three layers: mineralocorticoids (mainly aldosterone), glucocorticoids (mainly cortisol), and androgens. Aldosterone maintains electrolyte balance by stimulating the kidneys to retain $Na^+$ and water while excreting $K^+$. Cortisol is the primary stress hormone that increases blood glucose (anti-insulin effect), breaks down fats and proteins for energy, and provides an anti-inflammatory and immunosuppressive effect. In excess, cortisol inhibits the immune system and bone formation. The adrenal medulla secretes catecholamines—adrenaline ($80\%$) and noradrenaline—in response to immediate nervous stimulus. Stress management occurs in two phases. Short-term (acute) stress involves the sympathetic nervous system and the adrenal medulla, causing increased heart rate, bronchodilation, and glucose release for immediate "fight or flight." Long-term (chronic) stress involves the endocrine system, specifically the release of cortisol and aldosterone to maintain high energy levels and blood pressure, though this may eventually damage health by depressing the immune system.

Eco-citizenship and Endocrine Disruptors

Endocrine disruptors are foreign substances that interfere with the synthesis, secretion, transport, action, or elimination of natural hormones. These substances, which can be natural or synthetic, penetrate the body and send harmful signals, leading to metabolic disorders, reduced sperm quality, and cancers of the reproductive system. They act by mimicking hormones, blocking receptors, or altering the number of cellular receptors. Common disruptors include BPA, phthalates, parabens, and perfluorinated substances (PFAS), found in plastics, pesticides, hygiene products, and processed foods. To limit exposure, it is recommended to avoid heating food in plastic containers (using glass, ceramic, or stainless steel instead), prefer fresh and non-transformed foods, buy local or organic to reduce pesticide contact, and use natural cleaning products like white vinegar or baking soda. Reading labels to avoid ingredients like phenoxyethanol or triclosan is also essential for personal health and eco-citizenship.