Chapter 16: The Endocrine System

Comprehensive vocabulary flashcards covering the anatomy, physiology, and pathology of the endocrine system based on Chapter 16 of Marieb's Human Anatomy & Physiology.

Endocrinology

The study of hormones and endocrine organs.

Hormones

Long-distance chemical messengers utilized by the endocrine system that are transported in the blood to influence metabolic activities.

Autocrines

Local chemical messengers that exert their effects on the same cells that secrete them; not considered part of the endocrine system.

Paracrines

Locally acting chemicals that affect neighboring cells; not considered part of the endocrine system.

Exocrine glands

Glands that produce nonhormonal substances, such as sweat or saliva, and utilize ducts to carry secretions to a membrane surface.

Endocrine glands

Ductless glands that produce hormones secreted directly into the surrounding extracellular fluid; examples include the pituitary, thyroid, parathyroid, adrenal, and pineal glands.

Amino acid–based hormones

A class of hormones including derivatives, peptides, and proteins that are usually water soluble (except thyroxine) and cannot cross the plasma membrane.

Steroids

Lipid-soluble hormones synthesized from cholesterol that can cross the plasma membrane; includes gonadal and adrenocortical hormones.

Target cells

Specific tissue cells that possess the receptors required to be affected by a particular hormone.

Cyclic AMP (cAMP) signaling mechanism

A second-messenger system where a hormone binds to a receptor, activates a G protein, which then activates adenylate cyclase to convert ATP to cAMP.

Humoral stimuli

A type of endocrine gland stimulation where changing blood levels of certain ions and nutrients directly stimulate the release of hormones.

Neural stimuli

A type of endocrine gland stimulation where nerve fibers stimulate hormone release, such as sympathetic fibers triggering the adrenal medulla to secrete catecholamines.

Hormonal stimuli

A control mechanism where hormones produced by one organ stimulate other endocrine organs to release their own hormones, often following a hypothalamic–pituitary–target organ feedback loop.

Up-regulation

The process by which target cells add receptors in response to persistently low levels of a hormone to increase sensitivity.

Down-regulation

The process by which target cells remove receptors in response to persistently high hormone levels to desensitize the cells.

Half-life

The time required for the level of a hormone in the blood to decrease by half.

Permissiveness

A hormone interaction where one hormone cannot exert its full effects without the presence of another hormone, such as reproductive hormones requiring thyroid hormone.

Synergism

An interaction where more than one hormone produces the same effects at a target cell, and their combined effects are amplified.

Antagonism

An interaction where one or more hormones oppose the action of another hormone, such as the relationship between insulin and glucagon.

Neurohypophysis

The region consisting of the posterior pituitary lobe and the infundibulum; it is composed of neural tissue and secretes neurohormones.

Adenohypophysis

The anterior pituitary gland, which consists of glandular tissue that manufactures and secretes six major hormones.

Hypothalamic-hypophyseal tract

A bundle of axons that maintains the neural connection between the hypothalamus (paraventricular and supraoptic nuclei) and the posterior pituitary.

Hypophyseal portal system

A vascular connection between the hypothalamus and the anterior pituitary consisting of a primary capillary plexus, hypophyseal portal veins, and a secondary capillary plexus.

Oxytocin

A peptide hormone produced by the paraventricular nuclei that stimulates uterine contractions during labor and triggers milk ejection during breastfeeding.

Antidiuretic hormone (ADH)

Also called vasopressin, this peptide is produced by the supraoptic nuclei and signals kidney tubules to reabsorb more water back into the blood.

Diabetes insipidus

A homeostatic imbalance caused by ADH deficiency, resulting in intense thirst and large urine output.

Tropic hormones (tropins)

Hormones that regulate the secretory action of other endocrine glands; these include TSH, ACTH, FSH, and LH.

Growth hormone (GH)

Also called somatotropin, it has direct metabolic actions like glucose-sparing and lipolysis, and indirect growth-promoting actions mediated by insulin-like growth factors (IGFs).

Acromegaly

A condition in adults caused by hypersecretion of GH after the growth plates have closed, resulting in excessive thickening of bones in the hands, feet, and face.

Thyroid hormone (TH)

The body’s major metabolic hormone, found in two forms: $T_4$ (thyroxine) and $T_3$ (triiodothyronine).

Myxedema

The full-blown hypothyroid syndrome in adults, characterized by low metabolic rate, thick/dry skin, and mental sluggishness.

Graves’ disease

The most common hyperthyroid condition, an autoimmune disease where antibodies mimic TSH and stimulate continuous TH secretion.

Calcitonin

A hormone produced by the parafollicular (C) cells of the thyroid in response to high blood $Ca^{2+}$ levels; it is an antagonist to parathyroid hormone.

Parathyroid hormone (PTH)

The single most important hormone in $Ca^{2+}$ homeostasis, it stimulates osteoclasts, enhances kidney reabsorption of calcium, and promotes the activation of vitamin D.

Corticosteroids

A general term for the over 24 different steroid hormones produced by the adrenal cortex.

Mineralocorticoids

Hormones produced by the zona glomerulosa of the adrenal cortex that regulate electrolyte concentrations, primarily $Na^+$ and $K^+$.

Aldosterone

The most potent mineralocorticoid, it stimulates $Na^+$ reabsorption and $K^+$ secretion by the kidneys.

Glucocorticoids

Hormones produced by the zona fasciculata, primarily cortisol, that help the body resist stressors by regulating nutrient metabolism and maintaining blood pressure.

Cushing’s syndrome

A condition caused by an excess of glucocorticoids, resulting in high blood glucose, loss of muscle/bone protein, and water retention.

Addison’s disease

A condition involving hyposecretion of corticosteroids, including glucocorticoids and mineralocorticoids, often marked by a characteristic bronzing of the skin.

Catecholamines

Amine hormones produced by medullary chromaffin cells, consisting of epinephrine (80%) and norepinephrine (20%).

Melatonin

An amine hormone secreted by the pineal gland that reaches peak levels at night and influences the biological clock and sleep-wake cycles.

Glucagon

A hyperglycemic hormone produced by pancreatic alpha cells that stimulates glycogenolysis and gluconeogenesis in the liver.

Insulin

A hypoglycemic hormone produced by pancreatic beta cells that enhances glucose uptake into cells and inhibits the breakdown of glycogen.

Diabetes mellitus (DM)

A metabolic disorder resulting from hyposecretion (Type 1) or hypoactivity (Type 2) of insulin, characterized by polyuria, polydipsia, and polyphagia.

Erythropoietin

A glycoprotein hormone secreted by the kidneys that stimulates the production of red blood cells.

Leptin

A hormone secreted by adipose tissue that serves to suppress appetite and increase energy expenditure.

Osteocalcin

A peptide hormone secreted by osteoblasts that prods the pancreas to secrete more insulin and restricts fat storage.