Endocrine System Regulation and Hormones

• The nervous system uses electrochemical impulses for rapid responses, whereas the endocrine system uses hormones for slower, longer-lasting metabolic effects.

Major Processes Controlled by Hormones

• Reproduction is regulated by gonadal hormones like testosterone, estrogen, and progesterone.
• Growth and development are stimulated by growth hormone (GH).
• Electrolyte, water, and nutrient balance in the blood are maintained by hormones like aldosterone, which regulates sodium (Na⁺) and potassium (K⁺) balance.
• Cellular metabolism and energy balance are regulated by thyroid hormones (T3 and T4).
• Body defenses are mobilized by hormones like cortisol, which aids in stress response and immune function.

Types of Glands

• Exocrine glands produce nonhormonal substances and use ducts to transport them to membrane surfaces
• Sweat and salivary glands are examples of exocrine glands.
• Endocrine glands release hormones directly into surrounding tissue fluids without using ducts.
• Endocrine glands are highly vascularized to ensure efficient hormone distribution.
• Endocrine organs include the pituitary, thyroid, parathyroid, adrenal, and pineal glands.
• The hypothalamus is a neuroendocrine organ.
• The pancreas, gonads (ovaries, testes), and placenta contain endocrine tissue.

Hormone Classification

• Hormones are classified into amino acid-based and steroid hormones.
• Amino acid-based hormones are water-soluble (except thyroid hormone) and cannot pass through the plasma membrane.
• Examples of amino acid-based hormones include biogenic amines (e.g., epinephrine, thyroxine), peptides, and proteins.
• Steroid hormones are derived from cholesterol, lipid-soluble, and can cross the plasma membrane to bind to intracellular receptors.
• The adrenal cortex (e.g., cortisol, aldosterone) and gonads (e.g., testosterone, estrogen, progesterone) produce steroid hormones.
• Eicosanoids mediate inflammation and allergic reactions (leukotrienes) and affect blood pressure regulation, uterine contractions, blood clotting, pain, and inflammation (prostaglandins).
• Prostaglandins increase blood clotting during injury

Types of Chemical Signals

• Autocrines affect the same cells that secrete them (short-distance).
• Prostaglandins in smooth muscle cells cause those cells to contract, as an example of autocrines.
• Paracrines affect neighboring cells (short-distance).
• Somatostatin inhibits insulin release by different pancreatic cells, as an example of paracrines.
• Juxtacrines require direct contact via gap junctions.
• Gap junctions between cardiac muscle cells allow them to contract in coordination, as an example of juxtacrines.
• Endocrine signals travel via blood or lymph (long-distance).
• Insulin secreted by pancreatic cells affects liver cells, as an example of endocrine signals.

Stimuli That Trigger Hormone Release

• Humoral stimuli are triggered by changing blood levels of ions or nutrients.
• Low blood calcium (Ca²⁺) levels cause the parathyroid glands to release PTH, increasing calcium levels.
• Neural stimuli involve nerve fibers stimulating hormone release.
• The sympathetic nervous system stimulates the adrenal medulla to release epinephrine and norepinephrine in response to stress.
• Hormonal stimuli involve hormones stimulating the release of other hormones.
• The hypothalamus releases hormones to stimulate the anterior pituitary to release its hormones.

Regulation of Hormone Activity

• Most hormone release is controlled by negative feedback loops.
• Rising hormone levels inhibit further release to maintain homeostasis.
• Activation of target cells depends on blood levels of the hormone, the number of receptors on/in target cells, and the binding affinity between the hormone and receptor.
• Up-regulation occurs when cells increase the number of receptors in response to low hormone levels.
• Oxytocin receptors increase in the uterus late in pregnancy
• Down-regulation occurs when cells reduce the number of receptors in response to high hormone levels
• Insulin resistance occurs due to persistent high insulin level

Hormone Transport and Half-Life

• Hormones circulate in the blood either free (water-soluble hormones) or bound to plasma proteins (lipid-soluble hormones).
• Lipid-soluble hormones have longer half-lives due to binding to carrier proteins.
• Water-soluble hormones have shorter half-lives because they are quickly removed by the kidneys.
• Epinephrine and norepinephrine have half-lives of a few seconds.
• Steroid hormones have half-lives of several hours to days.

Hormone Interactions

• Permissiveness requires one hormone's presence for another to be effective.
• Thyroid hormone is needed for normal reproductive system development.
• Synergism occurs when two hormones enhance each other’s effects.
• Glucagon & epinephrine together cause greater glucose release than alone.
• Antagonism occurs when one hormone opposes the action of another.
• Insulin lowers blood glucose, while glucagon raises it.

Hypothalamus and Pituitary Gland

• The pituitary gland sits in the sella turcica of the sphenoid bone, connected to the hypothalamus by the infundibulum.
• There are two major lobes (posterior and anterior) of the pituitary gland, and it secretes at least eight hormones.
• The posterior pituitary (neurohypophysis) is composed of neural tissue and stores neurohormones received from the hypothalamus, such as oxytocin and antidiuretic hormone (ADH).
• It does not produce hormones.
• Arterial blood is delivered to the pituitary gland via hypophyseal branches of the internal carotid arteries
• Veins from the pituitary drain into the dural sinuses.
• The anterior pituitary (adenohypophysis) is composed of glandular tissue.
• The posterior lobe is part of the brain, maintaining a neural connection to the hypothalamus via the hypothalamic-hypophyseal tract.
• The anterior pituitary originates from epithelial tissue and has a vascular connection to the hypothalamus via the hypophyseal portal system.
• The hypophyseal portal system carries releasing and inhibiting hormones from the hypothalamus to the anterior pituitary to regulate hormone secretion.
• All hypothalamic regulatory hormones are amino acid-based.

Posterior Pituitary Hormones

• The posterior pituitary consists of axon terminals from hypothalamic neurons.
• Hypothalamic paraventricular neurons mainly produce oxytocin.
• Hypothalamic supraoptic neurons mainly produce antidiuretic hormone (ADH), also called vasopressin.
• Oxytocin & ADH are peptide hormones.

Oxytocin

• Oxytocin causes stronger contractions, by mobilizing calcium ions, is a strong stimulant of uterine contractions during childbirth.
• Also, it plays a rile in social bonding, affectionate behavior, and trust (cuddle hormone)
• Stretching of the cervix and uterus during birth dispatches afferent impulses to the hypothalamus, which then synthesizes oxytocin.
• Oxytocin is released from the posterior pituitary and acts via the PIP₂–Ca²⁺ second-messenger system.
• Oxytocin triggers milk ejection (let-down reflex) in lactating women by targeting myoepithelial cells.
• Synthetic oxytocin is used to induce or hasten labor and stop postpartum bleeding.

Antidiuretic Hormone (ADH, Vasopressin)

• ADH prevents wide swings in water balance.
• Hypothalamic osmoreceptors monitor blood solute concentration, triggering ADH release when solute concentration rises too high.
• ADH targets kidney tubules, causing them to reabsorb more water, reducing urine output, & returning water to the bloodstream.
• Pain, drugs, nicotine, morphine, and barbiturates enhance ADH release.
• Alcohol inhibits ADH secretion, leading to increased urine output.
• Reduced hydration inhibits ADH release

ADH Homeostatic Imbalances

• Diabetes insipidus is caused by ADH deficiency, leading to intense thirst and huge urine output, often due to damage to the hypothalamus or posterior pituitary; patients must stay well-hydrated, and severe cases may require synthetic ADH.
• Syndrome of Inappropriate ADH Secretion (SIADH) leads to fluid retention, brain edema, weight gain, and decreased blood solute concentration, often occurring in children with meningitis, adults with brain trauma, or cancer; treated with fluid restriction and monitoring of blood sodium levels.

Anterior Pituitary Hormones

• Production and release are controlled by the hypothalamus.
• Six major hormones produced/released: Growth Hormone (GH), Thyroid-Stimulating Hormone (TSH), Adrenocorticotropic Hormone (ACTH), Follicle-Stimulating Hormone (FSH), Luteinizing Hormone (LH), and Prolactin (PRL).
• Some hypothalamic hormones inhibit pituitary hormone release

Growth Hormone (GH)

• Also called somatotropin, is produced by somatotropic cells of the anterior pituitary.
• Has metabolic and growth-promoting effects.
• Direct actions include mobilizing fats from fat depots, decreasing glucose uptake and metabolism, encouraging glycogen breakdown in the liver, and increasing amino acid uptake into cells.
• Indirect actions are exerted through insulin-like growth factors (IGFs), stimulating protein synthesis, cell growth, and cartilage formation.
• GH release is stimulated by growth hormone-releasing hormone (GHRH) and inhibited by growth hormone-inhibiting hormone (GHIH) (somatostatin).
• GH secretion follows a diurnal cycle, peaking during adolescence and sleeping hours.
• Release is regulated by negative feedback from GH and IGFs.

Homeostatic Imbalances of GH

• Hyposecretion of GH in children leads to pituitary dwarfism, characterized by short stature but normal body proportions; can be treated with GH replacement if diagnosed before puberty.
• Hypersecretion of GH before epiphyseal plates close leads to gigantism, abnormally tall stature with normal body proportions.
• Hypersecretion of GH after epiphyseal plates close causes acromegaly, resulting in overgrowth of bones in the hands, feet, and face, often caused by a tumor on the anterior pituitary gland; treated with surgical removal of the tumor.

Other Anterior Pituitary Hormones

• Thyroid-Stimulating Hormone (TSH), also called thyrotropin, stimulates normal thyroid gland development and function; release is controlled by thyrotropin-releasing hormone (TRH) and inhibited by rising thyroid hormone levels
• Adrenocorticotropic Hormone (ACTH), also called corticotropin, stimulates the adrenal cortex to release corticosteroids, mainly glucocorticoids (cortisol); release is controlled by corticotropin-releasing hormone (CRH) and inhibited by glucocorticoids
• Gonadotropins (FSH and LH) regulate gonadal function; FSH stimulates egg and sperm production; LH promotes gonadal hormone production (testosterone, estrogen, progesterone); gonadal hormones inhibit further FSH and LH release.
• Prolactin (PRL) stimulates milk production in females; release is controlled by prolactin-inhibiting hormone (PIH) (dopamine); infant suckling stimulates PRL release

Homeostatic Imbalances of Prolactin

• Hyperprolactinemia (excess PRL) is the most common anterior pituitary tumor disorder, causing inappropriate lactation, lack of menses (females), infertility, and impotence (males).
• Hyposecretion of PRL leads to poor milk production.

Thyroid Gland

• The thyroid is located in the anterior neck, inferior to the larynx, with two lobes connected by the isthmus.
• Has a high blood supply.
• The gland is composed of follicles lined by follicular cells, which produce thyroglobulin (a precursor to thyroid hormone).
• Colloid (thyroglobulin + iodine) fills the follicles.
• Thyroid hormone (TH) is derived from iodinated thyroglobulin.

Explore the endocrine system's role in regulating body functions, hormone types (steroid vs. amino acid-based), and key organs involved. Differentiate between endocrine and exocrine glands. Learn about hormone secretion and processes affected by pituitary gland tumors.