Introduction to the Endocrine System

Practice flashcards covering the introductory concepts of endocrinology, including signaling types, major glands, hormone classifications, and the hypothalamic-pituitary axis.

Autocrine signaling

A signaling process where a cell releases a signal for which it has its own receptor and responds to that signal itself.

Paracrine signaling

Signaling that affects neighboring cells nearby, which is important in the blood, circulation, and the immune system.

Neurocrine signaling

Also known as synaptic transmission, this involves the release of a signal that travels a short distance across a synapse to a neighboring postsynaptic cell.

Endocrine signaling

A signaling method where an endocrine cell or gland secretes a hormone into the bloodstream to reach cells throughout the body.

Hormone

A signaling molecule meant to set things in motion, which travels through the blood and only affects cells with specific receptors.

Fenestrated capillaries

Porous capillary beds with little windows or pores that allow hormones to easily enter the bloodstream.

Exocrine gland

A gland that maintains a duct to the surface of the epithelium, such as sweat glands or mucus glands.

Endocrine gland

A ductless gland that releases hormones directly into an associated capillary bed.

Pineal gland

Located in the epithalamus, this gland releases melatonin and is involved in sleep-wake cycle regulation.

Hypothalamus

The integration center for both the autonomic nervous system and the endocrine system; it releases neurohormones.

Pituitary gland

Commonly called the master gland, it sits in the sella turcica and releases various hormones under the control of the hypothalamus.

Humoral stimulus

A stimulus for hormone secretion triggered by changes in blood levels of certain chemicals high or low concentrations, such as calcium.

Neural stimulus

A stimulus where a neuron directly triggers the release of a hormone, such as the preganglionic neuron triggering epinephrine release from the adrenal medulla.

Hormonal stimulus

A stimulus where one hormone triggers the release of another hormone, such as hypothalamic hormones affecting the anterior pituitary.

Eicosanoids

A group of hormones derived from arachidonic acid, a 20-carbon unsaturated fatty acid found in membrane phospholipids.

Half-life

The time it takes for the concentration of a hormone in the plasma to reach one-half of its original released concentration.

Thyroxine ($T_4$)

A hydrophobic thyroid hormone consisting of two tyrosines and four iodines with a half-life of approximately $6$ days.

Triiodothyronine ($T_3$)

The most active form of thyroid hormone, which is five times more potent than $T_4$ and has a half-life of $1$ day.

Infundibulum

The stalk, meaning funnel, that connects the hypothalamus to the pituitary gland.

Magnocellular neuroendocrine cells

Large cells located in the supraoptic and paraventricular nuclei of the hypothalamus that release $ADH$ and oxytocin.

Hypophyseal portal system

A system of two capillary beds (median eminence and anterior pituitary) connected by veins that allow tropic hormones to travel directly from the hypothalamus to the pituitary.

Antidiuretic hormone ($ADH$)

Also called vasopressin, it promotes water reabsorption in the kidney collecting ducts and causes vasoconstriction.

Aquaporin

A transmembrane protein channel inserted into the luminal membrane of kidney cells via a $G$ protein and cyclic $AMP$ pathway to allow water passage.

Oxytocin

A peptide hormone that triggers smooth muscle contraction for childbirth and milk ejection, often involved in positive feedback loops.

Tropic hormone

A hormone that stimulates or inhibits the secretion of another hormone.

Growth Hormone ($GH$)

A protein hormone that promotes bone and muscle growth and increases blood glucose levels; its largest peak occurs during deep sleep.

Somatostatin

An inhibitory tropic hormone released by the hypothalamus that inhibits the secretion of growth hormone.

Acromegaly

A condition caused by excessive growth hormone in adults, characteristically affecting the hands, feet, and face.

Follicular cells

Specific cells in the thyroid gland that synthesize thyroglobulin and form the follicles.

Colloid

The material stored inside thyroid follicles consisting of thyroglobulin, which serves as a precursor to thyroid hormones.

Parafollicular cells ($C$ cells)

Cells in the thyroid gland that produce calcitonin to decrease blood calcium levels.

Parathyroid hormone ($PTH$)

A peptide hormone released in response to low blood calcium that stimulates osteoclasts and promotes calcium reabsorption in the kidneys.

Calcitriol

The active form of Vitamin $D$ that promotes the absorption of calcium from food in the digestive system.

Anabolism

The metabolic process of building things up, such as protein synthesis and muscle mass increase facilitated by growth hormone.