11. Endocrine System

Comprehensive vocabulary flashcards covering the organs, hormones, functions, feedback mechanisms, and associated diseases of the endocrine system.

Endocrine System

A body system comprising organs and structures that release hormones directly into the bloodstream to regulate bodily functions and maintain homeostasis.

Exocrine System

A body system comprising organs or structures that secrete substances such as enzymes, organic matter, and ions into an organ cavity or onto an organ surface rather than directly into the bloodstream.

Negative Feedback

A homeostatic regulatory mechanism in which an initial action leads to a secondary action that suppresses or inhibits the initial action (e.g., elevated blood glucose stimulating insulin release, which then lowers blood glucose levels).

Positive Feedback

A physiological mechanism in which an initial action triggers a secondary action that reinforces and continues the initial action (e.g., the formation of a blood clot inducing thromboxane release to stimulate further clot formation).

Hypothalamus

An endocrine organ located ventral to the thalamus and adjacent to the pituitary glands; synthesizes oxytocin and antidiuretic hormone (ADH), and secretes corticotropic-releasing hormone (CRH).

Posterior Pituitary Gland

The caudal lobe of the pituitary gland located ventral to the hypothalamus that stores and secretes oxytocin and antidiuretic hormone (ADH).

Anterior Pituitary Gland

The rostral portion of the pituitary gland located ventral to the hypothalamus that synthesizes and secretes hormones including growth hormone, prolactin, TSH, ACTH, FSH, and LH.

Oxytocin

A hormone synthesized by the hypothalamus and stored/released by the posterior pituitary gland that induces uterine contractions during parturition.

Antidiuretic Hormone (ADH)

A hormone synthesized by the hypothalamus and released by the posterior pituitary that signals the kidneys to retain water, stimulates thirst via the nervous system, and acts as a vasoconstrictor to maintain blood pressure.

Corticotropic-Releasing Hormone (CRH)

A regulatory neurohormone released by the hypothalamus that signals the anterior pituitary gland to secrete adrenocorticotropic hormone (ACTH).

Growth Hormone (GH)

An anterior pituitary hormone that stimulates young animal growth and encourages protein anabolism.

Prolactin

An anterior pituitary hormone responsible for stimulating and sustaining lactation.

Thyroid-Stimulating Hormone (TSH)

An anterior pituitary hormone that promotes thyroid gland growth and triggers the synthesis and release of thyroid hormones ($T_3$ and $T_4$).

Adrenocorticotropic Hormone (ACTH)

An anterior pituitary hormone that stimulates the zona fasciculata of the adrenal cortex to secrete cortisol.

Follicle-Stimulating Hormone (FSH)

An anterior pituitary hormone that stimulates the growth and development of ovarian follicles.

Luteinizing Hormone (LH)

An anterior pituitary hormone responsible for triggering ovulation.

Thyroid Gland

A paired endocrine gland located on the lateral surfaces of the trachea near the larynx that secretes thyroid hormones ($T_3$ and $T_4$) and calcitonin.

Thyroid Hormone ($T_4$ & $T_3$)

Metabolic hormones produced by the thyroid gland; $T_4$ acts as an inactive prohormone converted into active $T_3$ within the liver, kidneys, and skeletal muscle to drive cellular metabolism.

Calcitonin

A thyroid hormone that lowers blood calcium levels by encouraging calcium deposition into bone tissue.

Parathyroid Hormone (PTH)

A hormone produced by the parathyroid glands that elevates blood calcium levels by stimulating renal calcium retention, intestinal calcium absorption, and bone calcium resorption.

Adrenal Gland Regions

The two primary anatomical layers of the adrenal gland from superficial to deep: the outer adrenal cortex and the inner adrenal medulla.

Zona Glomerulosa

The outermost zone of the adrenal cortex that produces mineralocorticoids, primarily aldosterone, which stimulates renal sodium retention to preserve blood pressure.

Zona Fasciculata

The middle layer of the adrenal cortex that secretes glucocorticoids, primarily cortisol (the body's chief natural steroid).

Zona Reticularis

The innermost layer of the adrenal cortex that synthesizes and secretes androgens (estrogen and testosterone).

Adrenal Medulla

The central core of the adrenal gland that produces and secretes catecholamines, namely epinephrine and norepinephrine.

Islets of Langerhans

Microscopic endocrine cell clusters in the pancreas containing alpha cells (which produce glucagon) and beta cells (which produce insulin).

Insulin

A pancreatic hormone secreted by beta cells in the islets of Langerhans that decreases blood glucose by stimulating peripheral cells to absorb glucose and synthesize glycogen.

Glucagon

A pancreatic hormone secreted by alpha cells in the islets of Langerhans that raises blood glucose by stimulating the liver to convert stored glycogen into glucose.

Erythropoietin

A renal hormone that stimulates bone marrow hematopoietic tissue to produce erythrocytes (red blood cells).

Renin

An enzyme/hormone released by the kidneys during hypotension, hyponatremia, or reduced renal perfusion that catalyzes the conversion of angiotensinogen to angiotensin I.

Renin-Angiotensin-Aldosterone System (RAAS)

A multi-organ hormonal pathway involving the kidneys, liver, lungs, and adrenal cortex that regulates systemic vascular resistance, fluid balance, and salt homeostasis.

Gastrin

A hormone produced by the stomach mucosa that triggers gastric wall contractions and stimulates hydrochloric acid ($HCl$) release.

Secretin

A hormone secreted by the small intestine that prompts the release of sodium bicarbonate ($NaHCO_3$) into the duodenum to neutralize incoming gastric acid.

Diabetes Insipidus

A disorder resulting from inadequate synthesis of antidiuretic hormone (ADH) by the hypothalamus or inadequate release by the posterior pituitary, causing extreme polyuria and dehydration.

Hypoadrenocorticism (Addison’s Disease)

A disease caused by a deficiency of ACTH or CRH, or primary adrenal atrophy, leading to inadequate production of cortisol and aldosterone.

Hyperadrenocorticism (Cushing’s Disease)

A pathological state characterized by excessive circulating cortisol levels, resulting in clinical signs such as bilateral symmetrical alopecia and a pot-bellied appearance.

Diabetes Mellitus

A metabolic disorder characterized by persistent hyperglycemia caused by either absolute beta-cell insufficiency (Type I) or peripheral tissue resistance to insulin (Type II).

Hyperthyroidism

An endocrine disorder common in feline patients resulting from excessive secretion of thyroid hormones, causing marked hypermetabolism, polydipsia, polyuria, and polyphagia.

Hypothyroidism

An endocrine disease most commonly diagnosed in dogs characterized by insufficient circulating thyroid hormone levels, manifesting as lethargy, ataxia, and rapid weight gain.