THE ENDOCRINE SYSTEM

Endocrine system

The second great control system of the body, along with the nervous system, that coordinates and directs the activity of the body's cells using chemical messengers called hormones.

Hormones

Chemical substances secreted by endocrine cells into the extracellular fluids that regulate the metabolic activity of other cells in the body. They can be classified as amino acid-based molecules, steroids, or prostaglandins.

Target cells/organs

Tissue cells or organs that are specifically affected by a hormone. They have specific protein receptors to which the hormone can attach.

Hormone action

The way hormones influence the workings of a cell by altering cellular activity, such as changing plasma membrane permeability, activating or inactivating enzymes, stimulating or inhibiting cell division, promoting or inhibiting secretion, or turning on or off transcription of certain genes.

Direct gene activation

The mechanism by which steroid hormones can trigger changes in cells by diffusing through the plasma membranes of their target cells, entering the nucleus, binding to specific hormone receptors, and activating certain genes to transcribe messenger RNA (mRNA) for protein synthesis.

Second-messenger system

The mechanism by which protein and peptide hormones influence cell activity by binding to hormone receptors on the target cell's plasma membrane and using a second-messenger system, in which the hormone activates an enzyme that produces second-messenger molecules to promote the typical response of the target cell to the hormone.

Negative feedback

The chief means of regulating blood levels of nearly all hormones, in which a rising level of a hormone inhibits further hormone release, maintaining blood levels within a narrow range.

Hormonal stimuli

The most common stimulus for hormone release, in which endocrine organs are prodded into action by other hormones. For example, hormones of the hypothalamus stimulate the anterior pituitary gland to secrete its hormones.

Humoral stimuli

Changing blood levels of certain ions and nutrients that stimulate hormone release. For example, a decreasing blood calcium ion level prompts the release of parathyroid hormone.

Neural stimuli

Nerve fibers that stimulate hormone release. For example, sympathetic nervous system stimulation of the adrenal medulla to release catecholamines during periods of stress.

Pituitary gland

The "master endocrine gland" that controls the activity of many other endocrine glands. It has two functional lobes - the anterior pituitary (glandular tissue) and the posterior pituitary (nervous tissue).

Pituitary-hypothalamus relationships

The interaction between the hypothalamus and the anterior pituitary gland, where releasing hormones and inhibiting hormones produced by the hypothalamus control the release of hormones by the anterior pituitary.

Hypothalamus

A region of the brain that produces hormones and regulates the release of hormones from the pituitary gland.

Oxytocin

A hormone produced by the hypothalamus and released by the posterior pituitary gland, involved in childbirth, breastfeeding, and milk ejection.

Antidiuretic hormone (ADH)

A hormone produced by the hypothalamus and released by the posterior pituitary gland, involved in water reabsorption by the kidneys and regulation of blood pressure.

Posterior pituitary

A storage area for hormones produced by the hypothalamus, located at the back of the pituitary gland.

Let-down reflex

The release of milk from the breasts in response to oxytocin stimulation during breastfeeding.

Tropic hormones

Hormones released by the anterior pituitary gland that stimulate other endocrine glands to release their own hormones.

Growth hormone (GH)

A hormone produced by the anterior pituitary gland that promotes growth and development of muscles and bones.

Prolactin (PRL)

A hormone produced by the anterior pituitary gland that stimulates milk production in breastfeeding women.

Gonadotropic hormones

Hormones released by the anterior pituitary gland that regulate the activity of the gonads (ovaries and testes).

Thyrotropic hormone (TH) or thyroid-stimulating hormone (TSH)

A hormone released by the anterior pituitary gland that stimulates the growth and activity of the thyroid gland.

Adrenocorticotropic hormone (ACTH)

A hormone released by the anterior pituitary gland that regulates the activity of the adrenal cortex.

Pineal gland

A small gland in the brain that secretes the hormone melatonin, involved in regulating sleep-wake cycles and reproductive function.

Melatonin

A hormone secreted by the pineal gland that regulates sleep-wake cycles and inhibits the reproductive system.

Thyroid gland

A gland located in the neck that produces thyroid hormones and calcitonin.

Thyroid hormone

A hormone produced by the thyroid gland that regulates metabolism and tissue growth and development.

Calcitonin

A hormone produced by the thyroid gland that decreases blood calcium levels by promoting calcium deposition in bones.

Parathyroid glands

Small glands located on the posterior surface of the thyroid gland that secrete parathyroid hormone (PTH).

Parathyroid hormone (PTH)

A hormone released by the parathyroid glands that regulates blood calcium levels by stimulating bone breakdown and increasing calcium absorption in the kidneys and intestines.

Thymus

An organ located in the chest that plays a role in the development and maturation of immune cells.