Hormones Knowledge-Based Study

Study notes

1. What Are Hormones?

   Hormones are chemical messengers produced by specialized ductless glands called endocrine glands. Unlike exocrine glands, which release their secretions through ducts onto epithelial surfaces or into body cavities (such as sweat or salivary glands), endocrine glands secrete hormones directly into extracellular fluid and the bloodstream.

   Once released into the circulatory system, hormones travel throughout the body. However, a hormone affects only specific cells known as target cells. These cells possess complementary protein structures called receptors that recognize and bind to the specific hormone molecule.

   • Endocrine secretion: Ductless; secretions diffuse directly into blood capillaries.
   • Exocrine secretion: Ducted; secretions travel through tubes to internal or external body surfaces.
   • Target specificity: Only cells expressing the matching receptor respond to a circulating hormone.

   Classification of Hormones

   Hormones fall into two main chemical classes based on their solubility, which dictates how they interact with target cells:

   1. Water-Soluble (Hydrophilic) Hormones

   These include peptide and protein hormones (such as insulin, glucagon, and growth hormone) as well as amine hormones derived from amino acids (such as epinephrine). Because they cannot cross the hydrophobic lipid bilayer of the cell membrane, they act via cell-surface mechanisms:

   • The hormone binds to an extracellular receptor on the plasma membrane.
   • This binding activates an intracellular signal transduction cascade, commonly involving a G protein and a second messenger such as cyclic AMP (cAMP).
   • The cascade leads to rapid enzymatic or metabolic changes in the cell.

   2. Lipid-Soluble (Lipophilic) Hormones

   These include steroid hormones synthesized from cholesterol (such as cortisol, aldosterone, estrogen, and testosterone) and thyroid hormones (T3 and T4). Because they are nonpolar, they easily diffuse across the plasma membrane:

   • The hormone enters the target cell and binds to an intracellular receptor in the cytoplasm or nucleus.
   • The hormone-receptor complex acts as a transcription factor, binding directly to specific DNA regulatory sites.
   • This alters gene transcription and protein synthesis, usually producing slower but longer-lasting physiological responses.

   Key Glands and Physiological Roles

   The human endocrine system consists of several interconnected glands that work collectively to regulate metabolism, growth, and fluid balance:

   • Hypothalamus and Pituitary Gland: The hypothalamus links the nervous and endocrine systems. It produces releasing and inhibiting hormones that control the anterior pituitary, which in turn secretes trophic hormones such as thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), and growth hormone (GH). The posterior pituitary stores and releases antidiuretic hormone (ADH) and oxytocin, both synthesized by hypothalamic neurons.
   • Thyroid Gland: Located in the anterior neck, it secretes thyroxine (T4) and triiodothyronine (T3) to regulate basal metabolic rate, as well as calcitonin to help lower blood calcium levels.
   • Parathyroid Glands: Produce parathyroid hormone (PTH), which increases blood calcium concentrations by stimulating bone resorption, kidney calcium reabsorption, and intestinal absorption via active vitamin D.
   • Pancreas (Islets of Langerhans): Functions as a heterocrine gland. Beta cells secrete insulin, which lowers blood glucose by promoting cellular glucose uptake and glycogen synthesis. Alpha cells secrete glucagon, which raises blood glucose by stimulating glycogen breakdown and gluconeogenesis in the liver.
   • Adrenal Glands: The adrenal cortex secretes steroid hormones including cortisol (glucocorticoid) and aldosterone (mineralocorticoid). The adrenal medulla produces catecholamines (epinephrine and norepinephrine) to drive the acute "fight-or-flight" sympathetic response.

   Maintaining Homeostasis

   Hormone secretion must be tightly controlled to prevent physiological imbalance. Most endocrine pathways are regulated by negative feedback loops, while a few specialized processes rely on positive feedback.

   Negative Feedback

   In a negative feedback loop, an increase in the physiological variable or the hormone itself inhibits further secretion, returning the parameter toward its normal set point:

   • Blood Glucose Regulation: Following a meal, elevated blood glucose stimulates pancreatic beta cells to release insulin. As cells absorb glucose and blood levels drop back to normal, insulin secretion is reduced. Conversely, fasting stimulates glucagon release, which raises blood glucose and subsequently turns off further glucagon release.
   • Hypothalamic-Pituitary-Thyroid Axis: Thyrotropin-releasing hormone (TRH) from the hypothalamus stimulates TSH release from the anterior pituitary, which stimulates thyroid hormone production. High circulating levels of T3 and T4 inhibit both TRH and TSH secretion.

   Positive Feedback

   In positive feedback, the output of a system enhances or amplifies the initial stimulus until an endpoint is reached. A classic example is the release of oxytocin during childbirth: pressure of the fetus against the cervix stimulates oxytocin release, which triggers stronger uterine contractions, causing even greater cervical stretch and further oxytocin release until delivery is complete.