The Endocrine System

Endocrine

Ductless, secretes hormones directly into the bloodstream or interstitial fluid to hit distant target cells.

Exocrine

Produce nonhormonal substances (examples: sweat, saliva, digestive enzymes). Secretes via ducts onto the surface

Hormones

Chemical substances secreted by cells into the extracellular fluids that travel through the blood and regulate the metabolic function of other cells in the body

Amino-Based Hormones

Water-soluble. Cannot pass through the plasma membrane. Travels freely in blood. Bind to plasma membrane receptors which trigger second messenger systems

Humoral Stimulus

Hormone release caused by altered levels of certain ions or nutrients. (Example: Parathyroid hormone (PTH) is released in direct response to low blood calcium levels.)

Neural Stimulus

Hormone release caused by neural input. (Example: response to stress in which the sympathetic nervous system stimulates the adrenal medulla to release norepinephrine and epinephrine)

Hormonal Stimulus

Hormone release caused by another hormone. (Example: Luteinizing hormone (LH) from the anterior pituitary stimulates the testes to produce testosterone.)

Target cell activation depends on which factors

Blood levels of the hormone, Relative number of receptors for that hormone on or in the target cell, Affinity (strength) of the binding between the hormone and the receptor

Synergism

More than one hormone produces the same effects at the target cell, and their combined effects are amplified. (Example: let's say both hormone "A" and hormone "B" cause the liver to release glucose into the blood. When they act together, the amount of glucose released might be 150% of what is released if each hormone acts independently.)

Oxytocin

Produced by the hypothalamus. Stored and released in the posterior pituitary. stimulate uterine contractions during childbirth and promote milk ejection ("let-down") during breastfeeding, while also playing a key role in social bonding and emotional attachment.

Antidiuretic Hormone (ADH)

Produced by the hypothalamus. Stored and released in the posterior pituitary

Anterior Pituitary Hormones (Adenohypophysis)

• Growth Hormone (GH): Stimulates tissue growth, protein synthesis, and bone/muscle development; regulates metabolism.

• Thyroid-Stimulating Hormone (TSH): Stimulates the thyroid gland to produce and release thyroid hormones (T3 and T4)

• Adrenocorticotropic Hormone (ACTH): Promotes the release of glucocorticoids (like cortisol) and adrenal androgenic steroids from the adrenal cortex.

• Follicle-Stimulating Hormone (FSH): A gonadotropin that stimulates follicle production and estrogen secretion in females and sperm production in males.

• Luteinizing Hormone (LH): A gonadotropin that triggers ovulation and progesterone synthesis in females and testosterone production in males.

• Prolactin (PRL): Promotes milk production by the mammary glands.

Posterior Pituitary Hormones (Neurohypophysis)

• Oxytocin: Stimulates powerful uterine contractions during labor (via a positive feedback loop) and triggers milk ejection ("let-down") during breastfeeding.

• Antidiuretic Hormone (ADH / Vasopressin): Stimulates kidney tubule cells to reabsorb water back into the bloodstream, concentrating urine and preventing excess water loss.

Stores and releases neurohormones synthesized by the hypothalamus.

Tropins (tropic hormones)

Hormones secreted by one endocrine gland that target and regulate the secretory action of other endocrine glands.

Insulin

Lowers blood sugar

Glucagon

Raises blood sugar, triggers gluconeogenesis

Target Cells

Tissues with receptors for a specific hormone

Steroid-based hormones

Lipid-Soluble / Lipophilic. Synthesized from cholesterol with a long half life. Bind to intracellular receptors, which act on the DNA to increase transcription. (Example: Cortisol, Aldosterone, Estrogen, Progesterone, Testosterone, and Thyroid Hormones (T3 and T4).)

Type 1 Diabetes

The body makes little to no insulin.

Type 2 Diabetes

The body resists the insulin it makes.

The Endocrine System

regulates bodily functions (metabolism, growth, development, tissue function, fluid/electrolyte balance, reproduction) by releasing chemical messengers (hormones) directly into the bloodstream to act on target tissues.

Autocrine

Chemical messengers secreted by a cell that bind to receptors on that same cell.

Paracrine

Short-range signal to the immediate neighboring cell. Short-distance chemicals that act locally within the same tissue on neighboring cells.

Antagonism

When one hormone opposes the action of another hormone

How do Water-Soluble hormones trigger a response in target cells?

Mechanism: Second Messenger System (e.g., cAMP).

Steps: Hormone (1st messenger) binds to a surface membrane receptor.

Activates G protein

G protein activates Adenylate Cyclase.

Adenylate cyclase converts ATP \rightarrow cAMP (2nd messenger).

cAMP activates Protein Kinases to alter cell activity.

How do Lipid-Soluble hormones trigger a response in target cells?

Mechanism: Direct Gene Activation.

Steps: Hormone diffuses across the plasma membrane.

Binds to an intracellular receptor in the cytoplasm or nucleus.

Receptor-hormone complex binds to specific DNA regions.

Initiates gene transcription \rightarrow mRNA \rightarrow Protein synthesis.

Permissiveness:

One hormone needs another present to exert its full effect (e.g., thyroid hormone permitted for reproductive development).

how does the Hypothalamus connect to the Anterior Pituitary?

Vascular connection via the Hypothalamo-Hypophyseal Portal System

how does the Hypothalamus connect to the Posterior Pituitary?

Neural connection via the Hypothalamo-Hypophyseal Tract.

The posterior pituitary gland synthesizes hormones

True

Which endocrine gland secretes melatonin

Pineal Gland

The hypothalamus-hypophysis tract is the neural pathway that connects the hypothalamus directly to which gland via nerve axons

Posterior Pituttary Gland

The hypophyseal portal system is the vascular pathway that connects the hypothalamus directly to which gland via blood vessles

Anterior Pitutary Gland

Which hormone is amino acid-based but lipid-soluble

Thyroid Hormone (T3 & T4)

Which of the following choices correctly outlines the sequential steps of the cAMP second-messenger system after a water-soluble hormone binds to its cell-surface receptor?

Zona Glomerulosa

Outermost layer of the adrenal cortex, Mineralocorticoids (ex: Alosterone)

Zona Fasciculata

Middle layer of the adrenal cortex, Glucocorticoids (e.g cortisol)

Zona Reticularis

Innermost layer of the adrenal cortex, gonadocorticoids (ex: androgens (DHEA))

Tropic Hormones

Hormones that cause other hormones to release hormones. (Ex TSH—thyroid-stimulating hormone stimulating the thyroid gland to release thyroid hormones)

Non-tropic Hormones

Acts directly on target cells. (ex; Prolactin acting directly on the mammary tissue to produce milk)

Renin-Angiotensin-Aldosterone System (RAAS)

a critical hormone cascade that regulates blood pressure, blood volume, and fluid balance in the body

What do alpha cells release

Glucagon

What do beta cells release

Insulin

PIP₂-Calcium Signaling Mechanism

An intracellular signaling pathway used by water-soluble hormones where a receptor activates Phospholipase C (PLC) to split membrane PIP2 into two second messengers (IP3 and DAG), ultimately triggering the release of calcium ions from the endoplasmic reticulum to alter cell behavior.

Eicosanoids

Biologically active lipids derived from arachidonic acid that act locally as paracrines or autocrines.

Prostaglandins

Mediates pain, fever, inflammation, and uterine contractions during labor and protects the stomach lining.

Thromboxanes

Produced primarily by blood platelets; coordinates blood clotting via platelet aggregation and blood vessel constriction.

Leukotrienes

Produced by white blood cells; triggers immune responses, vascular permeability, and severe smooth muscle bronchoconstriction in the lungs.

Cushing Disease

an endocrine disorder caused by a noncancerous pituitary tumor that triggers excess cortisol (glucocorticoid) production. ACTH releasing too much. (rapid weight gain and abnormal fat redistribution.)

Gigantism

A rare condition in children Hypersecretion of GH (growth hormone) is usually caused by anterior pituitary tumor

acromegaly

a rare, chronic hormonal disorder that occurs in adults when the pituitary gland produces too much growth hormone (GH) after the bone growth plates have closed

myxedema

severe, advanced hypothyroidism and the specific skin changes that occur when thyroid hormone levels are profoundly low

Graves Disease

an autoimmune disorder that serves as the most common cause of hyperthyroidism (an overactive thyroid). eyes protrude outward, neck goiter

Addison's disease

when cortisol levels are dangerously low because the adrenal glands are damaged. (wasting and color changes)

Conns Disease

The adrenal glands produce too much aldosterone, the hormone responsible for regulating the balance of salt (sodium), potassium, and water in your blood. (leading to sodium retention (hypernatremia), potassium excretion (hypokalemia), and hypertension.) Visible Muscle Twitches or Tremors