Endocrine System Overview & Hypothalamus–Pituitary Axis

These flashcards cover key concepts from endocrine system basics through hypothalamus–pituitary interactions, including hormone types, mechanisms, regulation, major glands, specific hormone functions, and related disorders.

What is a hormone?

A chemical messenger secreted by endocrine glands that travels in the blood to regulate functions of specific target cells.

Name three major functions hormones help maintain.

Fluid, electrolyte and acid–base homeostasis; promotion of growth; regulation of metabolic reactions.

How do the response times of the nervous and endocrine systems differ?

Nervous responses are almost immediate and short-lived; endocrine responses may take seconds to days and are generally longer-lasting.

Through what medium do endocrine cells deliver their chemical signals to target cells?

Bloodstream.

Define endocrine signaling.

Hormone is secreted into the blood and affects distant tissues.

Define paracrine signaling.

Chemicals are secreted into extracellular fluid to affect nearby but different cell types.

Define autocrine signaling.

A cell secretes chemicals that elicit effects on itself or the same cell type nearby.

What is a portal system?

A vascular arrangement in which capillaries drain into veins that lead to another set of capillaries (e.g., hypothalamic-hypophyseal portal system).

List six primary endocrine organs.

Anterior pituitary, thyroid gland, parathyroid glands, adrenal cortices, endocrine pancreas, thymus.

Give three examples of neuroendocrine organs.

Hypothalamus, pineal gland and adrenal medulla.

Where are receptors for hydrophilic hormones located?

Embedded in the target cell’s plasma membrane.

Where are receptors for hydrophobic hormones located?

Within the cytosol or nucleus of the target cell.

Most amino-acid hormones are (water or lipid soluble?).

Water-soluble (hydrophilic).

Steroid hormones are derived from .

Cholesterol and are lipid-soluble (hydrophobic).

Thyroid hormones (T₃, T₄) are chemically amino-acid based but are in terms of solubility.

Hydrophobic.

What is up-regulation?

A target cell increases the number of receptors in response to rising hormone levels.

What is down-regulation?

A target cell decreases the number of receptors after prolonged high hormone exposure.

Briefly describe the second-messenger mechanism of hydrophilic hormones.

Hormone binds membrane receptor → activates G-protein → activates enzyme → forms second messenger (e.g., cAMP) → alters cell activity.

How do hydrophobic hormones alter target cell activity?

Hormone-receptor complex enters nucleus or binds DNA and changes the rate of protein synthesis.

List the three stimuli that can initiate or inhibit hormone release.

Hormonal, humoral (ion/molecule concentration), and neural stimuli.

Outline the steps of a negative feedback loop regulating a hormone.

Stimulus → receptor detects change → control center alters hormone secretion → effector response moves variable toward normal → homeostatic range achieved; secretion decreases.

Define complementary hormone actions.

Different hormones act on different target cells to accomplish the same overall goal.

What is a permissive hormone interaction?

One hormone must be present for another to exert its effect (e.g., epinephrine requires normal thyroid hormone levels).

What is meant by synergist hormones?

Two or more hormones act on the same target to produce the same effect, which is amplified when they act together.

What are antagonist hormones?

Hormones that act on the same cells but produce opposite effects.

Where is the pituitary gland located?

In the sella turcica of the sphenoid bone.

What tissue type composes the anterior pituitary (adenohypophysis)?

Hormone-secreting glandular epithelium.

What tissue type composes the posterior pituitary (neurohypophysis)?

Nervous tissue.

What is the function of the hypothalamic-hypophyseal portal system?

Allows releasing and inhibiting hormones from the hypothalamus to reach the anterior pituitary rapidly and in high concentration.

Which two hormones are released from the posterior pituitary?

Antidiuretic hormone (ADH, vasopressin) and oxytocin.

Primary action of ADH?

Water retention by inserting aquaporins in kidney tubules; also causes vasoconstriction, raising blood pressure.

Name the disorder caused by hyposecretion or inactivity of ADH.

Diabetes insipidus.

Main reproductive functions of oxytocin?

Stimulates uterine contractions and milk ejection from mammary glands.

What type of feedback loop is the milk let-down reflex?

Positive feedback loop.

Differentiate tropic and trophic hormones.

Tropic hormones regulate secretion of other hormones; trophic hormones induce growth of target tissues (some hormones are both).

How many tiers are in the hypothalamic–anterior pituitary feedback system and what initiates tier one?

Three tiers; tier one begins with hypothalamic releasing or inhibiting hormones responding to a stimulus.

Function of thyroid-stimulating hormone (TSH)?

Stimulates development and hormone secretion of the thyroid gland; released in response to TRH.

Function of adrenocorticotropic hormone (ACTH)?

Stimulates adrenal cortex development and secretion of steroid hormones; released in response to CRH.

List two major actions of prolactin (PRL).

Growth of mammary tissue and initiation/maintenance of milk production.

LH roles in males vs. females?

Males—stimulates testosterone production; females—stimulates estrogen/progesterone production and triggers ovulation.

FSH roles in males vs. females?

Males—stimulates factors that bind/concentrate testosterone; females—stimulates estrogen production and maturation of ovarian follicles.

When is growth hormone secretion highest?

During sleep.

Give two short-term metabolic effects of growth hormone.

Increased lipolysis; increased gluconeogenesis and reduced glucose uptake (glucose-sparing).

What long-term mediator carries many of GH’s growth effects?

Insulin-like growth factor (IGF).

Name the two hypothalamic regulators of GH.

Growth hormone–releasing hormone (GHRH) stimulates; somatostatin inhibits.

Cause of gigantism.

Hypersecretion of GH before epiphyseal plates close.

Cause of acromegaly.

Hypersecretion of GH after epiphyseal plate closure.

Cause of pituitary dwarfism.

GH hyposecretion during childhood.

What is paraneoplastic syndrome?

Hormone-like secretion by cancer cells (commonly lung or GI) causing fluid, calcium, or electrolyte imbalances before other cancer symptoms appear.

What property chiefly determines a hormone’s mechanism of action?

Whether it is hydrophobic or hydrophilic.

Catecholamines such as epinephrine and norepinephrine belong to which hormone class and are they hydrophilic?

They are amine (amino-acid) hormones and are hydrophilic (water-soluble).

Why are oral hGH supplements ineffective?

GH is a protein that is destroyed by stomach acid and digestive enzymes; effective administration requires injection.

What is another name for ADH and how does it affect blood pressure?

Vasopressin; it raises blood pressure through vasoconstriction.

Contrast the adenohypophysis and neurohypophysis regarding hormone synthesis.

Adenohypophysis synthesizes and secretes its own hormones; neurohypophysis stores and releases hypothalamic hormones but does not synthesize them.