Endocrine System Overview Quiz

Endocrine System Overview

• The endocrine system is the body's second major control system, influencing cellular metabolism via hormones.
• Endocrine system effects are generally slower and more prolonged than nervous system effects.
• Endocrine organs are small, unimpressive, and scattered throughout the body.
• Hormone-producing cells are arranged in branching patterns maximizing capillary contact for hormone release.
• "Within" = endo, "to secrete" = crine.
• The pancreas and gonads produce both hormones and exocrine products.
• The hypothalamus regulates neural functions and hormone release.
• Other tissues (intestine, stomach, kidneys, heart, brain) also produce hormones.

Hormones

• Hormones are biological chemicals secreted by endocrine glands, carried in the bloodstream to target cells.
• "Hormone" means "to excite."
• Hormones initiate and regulate numerous bodily functions, coordinating via complex interactions.
• Hormone potency varies with concentration.
• A single hormone can have multiple functions depending on its level.
• Hormone production must be tightly regulated due to their potency even in trace amounts.
• Hormone measurements are in nanograms (ng) and picograms (pg).
• Hormones regulate metabolic activities and responses from seconds to weeks.
• Hormone effects can be long-lasting.
• Hormones are either amino acid-based or steroids.
• Steroids are biologically active lipids, including local hormone-like activity.

Types of Hormones

• Amino acid-based hormone types include amines, thyroxine, peptides, and proteins.
• Steroid hormones are mostly gonadal and adrenocortical.
• Eicosanoids (leukotrienes, prostaglandins, thromboxane) are another type.

Hormone Action

• Hormones alter cell activity via two mechanisms:
  • Second messengers (amino acid-based hormones).
  • Direct activation (steroid hormones).
• The precise response depends on the target cell.

Mechanisms of Hormone Action

• Cellular changes due to hormones may include altering plasma membrane permeability, stimulating protein synthesis, activating/deactivating enzymes, inducing secretory activity, or stimulating mitosis.

Amino Acid-Based Hormone Action: cAMP Second Messenger

• Hormone binding initiates a cascade involving G proteins, adenylate cyclase, cAMP activation of protein kinases, and cellular effects.

Amino Acid-Based Hormone Action: PIP2-Calcium

• Hormone binding activates phospholipase, splitting PIP2 into DAG and IP3, which act as second messengers.
• DAG activates protein kinases; IP3 releases calcium ions (a messenger); resulting in cellular effects.

Steroid Hormones

• Steroid hormones diffuse into the target cell and activate an intracellular receptor.
• The hormone-receptor complex binds to DNA, initiating transcription, mRNA production, protein synthesis, and cellular effects.

Hormone-Target Cell Specificity

• Hormones circulate to all tissues but affect only target cells that possess their specific receptors.
• Receptors are located on the plasma membrane or intracellularly.
• Target cell activation depends on hormone concentration, receptor number, and receptor affinity.

Target Cell Activation

• Up-regulation increases receptor numbers and sensitivity while down-regulation reduces them.
• Multiple hormones can act on a single target, or synergistically amplify each other’s response .
• Some hormones require others to exert their complete effect.
• Antagonism occurs when one hormone opposes another's action.

Hormone Concentrations in the Blood

• Blood hormone levels reflect release rates, inactivation/removal speeds, and various removal mechanisms.

Control of Hormone Synthesis and Release

• Hormone levels are controlled by negative feedback mechanisms.
• Hormones are released in response to various stimuli including humeral, neural, and hormonal stimuli.

Humoral Stimuli

• Humoral stimuli are direct responses to changing blood levels of certain substances (e.g., Ca²⁺, PTH).

Neural Stimuli

• Neural stimuli are nerve fiber stimulations, e.g., sympathetic fibers stimulate adrenal medulla catecholamine release.

Hormonal Stimuli

• Hormonal stimuli involve release in response to other hormones (e.g., hypothalamus/pituitary hormones triggering other endocrine targets).

Nervous System Modulation

• The nervous system modifies endocrine gland stimulation, providing adjustments for dynamic needs and overriding normal endocrine controls (e.g., glucose control during stress).

Major Endocrine Glands

• Include pineal, hypothalamus, pituitary, thyroid, parathyroid, thymus, adrenal, pancreas, gonads (testes and ovaries).

Pituitary (Hypophysis)

• Two-lobed organ secreting nine major hormones.
• Posterior lobe (neurohypophysis) receives, stores, and releases hypothalamic hormones (ADH, oxytocin).
• Anterior lobe (adenohypophysis) synthesizes and secretes hormones.
• The hypothalamus and pituitary have important relationships via neural and vascular connections.

Pituitary-Hypothalamic Relationships

• Posterior lobe has direct neural connections (hypothalamic-hypophyseal tract).
• Anterior lobe has vascular connections (hypophyseal portal system).

Adenohypophyseal Hormones

• Six adenohypophysis hormones (GH, TSH, ACTH, FSH, LH, PRL) regulate other endocrine glands.
• POMC is a prohormone precursor for some anterior pituitary hormones.

Activity of the Adenohypophysis

• Hypothalamic releasing and inhibiting hormones regulate anterior pituitary hormone synthesis and release.

Growth Hormone (GH)

• Stimulates most cells, targets bone and muscle.
• Promotes protein synthesis and fat use for fuel.
• Effects are mediated by insulin-like growth factors (IGFs).
• Antagonistic hypothalamic hormones control GH.

Metabolic Action of Growth Hormone

• GH stimulates multiple tissues to produce insulin-like growth factors (IGFs)
• GH has direct effects promoting lipolysis and inhibiting glucose uptake.

Thyroid Stimulating Hormone (Thyrotropin)

• Stimulates thyroid gland development and secretion, triggered by TRH (Thyroid-releasing Hormone).
• Negative feedback from thyroid hormones controls TSH release.

Adrenocorticotropic Hormone (Corticotropin)

• Stimulates adrenal cortex corticosteroid release triggered by CRH from the hypothalamus.
• Internal/external factors (stress, fever) can trigger corticotropin release.

Gonadotropins (FSH, LH)

• FSH stimulates gamete production.
• LH regulates gonadal hormone production.
• In females LH works with FSH to facilitate ovulation.
• In males LH stimulates testosterone production.

Prolactin (PRL)

• Stimulates milk production in females and is triggered by PRH.
• PRL function is inhibited by PIH.
• PRL levels increase during pregnancy and with suckling.

Posterior Pituitary and Hypothalamic Hormones

• Posterior pituitary stores and releases ADH and oxytocin, synthesized in the hypothalamus.
• ADH influences water balance (antidiuretic), and oxytocin has uterine and breast stimulation functions.

Oxytocin

• A powerful uterine stimulant involved in labor and milk "letdown."
• Oxytocin is regulated by positive feedback.

Antidiuretic Hormone (ADH)

• ADH conserves water by reducing urine output based on blood solute concentration.
• Alcohol inhibits ADH, causing copious urination

Thyroid Gland

• Largest endocrine gland, located in the anterior neck.
• Composed of follicles producing thyroglobulin (colloid).
• Parafollicular cells secrete calcitonin.

Thyroid Hormone (TH)

• Main metabolic hormone, consisting of T4 (thyroxine) and T3 (triiodothyronine).
• TH affects glucose oxidation, increasing metabolic rate, and heat production.

Transport and Regulation of TH

• T4 and T3 bind to transport proteins (TBGs).
• T3 is more active than T4.
• Regulation is by negative feedback from TH.

Calcitonin

• Hormone that lowers blood calcium levels, by inhibiting osteoclasts.
• Calcitonin is regulated by negative feedback.

Parathyroid Glands

• Tiny glands embedded in the posterior thyroid.
• Chief cells secrete parathyroid hormone (PTH).

Parathyroid Hormone (PTH)

• Regulates calcium balance via
  • Stimulating osteoclast activity.
  • Increasing Ca²⁺ reabsorption in kidneys.
  • Increasing Ca²⁺ absorption in intestines.
• Ca²⁺ rise inhibits PTH.

Adrenal (Suprarenal) Glands

• Paired organs atop the kidneys, composed of medulla and cortex.

Adrenal Cortex

• Synthesizes corticosteroids (mineralocorticoids, glucocorticoids, gonadocorticoids).
• Zones within the cortex produce different steroids.

Mineralocorticoids (Mostly Aldosterone)

• Regulate electrolyte balance via Na+ retention and K+ excretion by kidneys.
• Aldosterone release is triggered by decreased blood volume/pressure.
• Several mechanisms trigger aldosterone release, including the renin-angiotensin mechanism and blood Na+/K+ levels.

Glucocorticoids (Cortisol and others)

• Maintain blood glucose levels during stress, promote gluconeogenesis, and prevent tissue water shift.
• Excessive glucocorticoids can cause numerous adverse effects.

Gonadocorticoids (Sex Hormones)

• Primarily androgens, like testosterone, producing effects such as puberty, secondary sexual characteristics, and libido.

Adrenal Medulla

• Composed of chromaffin cells, secreting epinephrine and norepinephrine.
• Increased activity (e.g., heart rate, blood pressure) occurs with hormone release.
• Norepinephrine more strongly affects vasoconstriction.

Pancreas

• Dual-function gland with exocrine (acinar cells) and endocrine (pancreatic islets) functions.

Pancreatic Islets (Islets of Langerhans)

• Contain alpha, beta, and delta cells producing glucagon, insulin, and somatostatin.

Glucagon

• Hyperglycemic hormone promoting glycogenolysis and gluconeogenesis.

Insulin

• Hyoglycemic hormone that lowers blood glucose.
• Increases glucose transport, storage, and conversion into fat.

Gonads: Female

• Ovaries produce estrogens and progesterone, impacting development, secondary sex characteristics, uterine changes.
• Estrogen has many functions like producing female characteristics, bone health, and brain function.

Gonads: Male

• Testes produce testosterone, initiating/maintaining male characteristics, and affecting sperm production and sex drive.

Pineal Gland

• Small brain gland releasing melatonin, involved in circadian rhythms.

Thymus

• Thoracic gland producing hormones for T lymphocyte development.

Other Hormone-Producing Structures

• Heart: atrial natriuretic peptide (ANP).
• Gastrointestinal tract: digestive hormones.
• Placenta: hormones influencing pregnancy.
• Kidney: erythropoietin.
• Adipose tissue: leptin (involved in satiety).
• Stomach/pancreas: Ghrelin.

Developmental Aspects

• Endocrine glands arise from all three germ layers.
• Endocrine functions remain consistent, despite structural changes.
• Hormone levels decline with age (e.g., GH, TH), impacting processes like muscle maintenance and bone health. Hormonal changes can affect aging and menopause.

Test your knowledge of the endocrine system and its functions with this overview quiz. Explore the roles of hormones, endocrine organs, and their effects on cellular metabolism. Ideal for students learning about human biology and the endocrine system.