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Chapter 18: The Endocrine System - Practice Flashcards

Intercellular Communication: Direct, Paracrine, and Autocrine

  • Direct communication: occurs between two cells in physical contact.

  • Paracrine communication: chemical messengers transfer information from a cell to a neighboring cell within the same tissue.

  • Autocrine communication: chemical messengers affect the same cells that secrete them.

Endocrine Communication and Hormones

  • Endocrine communication uses hormones, chemical messengers that travel in the blood to reach distant target cells.

  • Target cells possess receptors for the hormone.

Synaptic Communication

  • Neurons communicate with other cells by releasing neurotransmitters at a synapse.

  • This enables high-speed messages to specific destinations.

Endocrine vs Nervous System

  • Nervous regulation is faster but short-lived; endocrine regulation is slower but longer-lasting.

  • Both rely on chemical messengers that bind to specific receptors on target cells.

  • Chemicals can be hormones (into bloodstream) or neurotransmitters (across synapse).

  • Both systems are largely regulated by negative feedback and work together to regulate homeostasis.

Endocrine System: Overview

  • Endocrine cells, tissues, and organs produce hormones.

  • Endocrine cells release secretions into extracellular fluid (blood).

  • Exocrine cells release secretions onto epithelial surfaces via ducts.

  • Hormones regulate: growth and development, reproduction, cell metabolism and energy balance, body water content, electrolytes, and nutrients.

  • Note: Some content text appears jumbled in the transcript; the essential idea is that endocrine signaling coordinates physiological processes via blood-borne hormones.

General Properties of Hormones

  • Regulation of physiological processes.

  • Variety in chemical structure.

  • Released in very low quantities.

  • Movement through diffusion in plasma.

  • Bind to receptors on target cells.

  • Released in response to changes in homeostasis.

Types of Hormones: Chemical Messenger Classification

  • Steroids: Solubility — Lipophilic; Receptor location — Cytosol; Class — Endocrine; Example — Estrogen.

  • Thyroxine (T4) and Triiodothyronine (T3): Often listed with peptides in the transcript but are thyroid hormone derivatives; Lipophilic; Receptor location — Nucleus; Class — Endocrine; Example — Thyroxine (T4) / Triiodothyronine (T3).

  • Peptides: Solubility — Hydrophilic; Receptor location — Cell membrane; Class — Endocrine; Example — Adrenocorticotropic hormone (ACTH) and Growth hormone (GH).

  • Proteins: Solubility — Hydrophilic; Receptor location — Cell membrane; Class — Endocrine; Example — Growth hormone (GH).

  • Purinergic: Solubility — Hydrophilic; Receptor location — Cell membrane; Class — Paracrine; Example — Adenosine triphosphate (ATP).

  • Prostaglandins: Solubility — Lipophilic; Receptor location — Cell membrane; Class — Endocrine-like; Example — Prostaglandin E2 (PGE2).

  • Gases: Solubility — Lipophilic; Receptor location — Cytosol; Class — Paracrine; Example — Nitric oxide (NO).

  • Neurotransmitters: Solubility — Hydrophilic; Receptor location — Cell membrane; Class — Paracrine; Example — Acetylcholine (ACh).

Hormone Mechanisms: Lipid-Soluble Hormones and Genomic Signaling (Example Cascade)

  • A steroid hormone moves from the blood into the fluid bathing a target cell.

  • The hormone diffuses across the target cell's plasma membrane into the cytoplasm, then through the nuclear envelope.

  • Inside the nucleus, it binds a receptor molecule to form a hormone–receptor complex.

  • The hormone–receptor complex acts as a transcription factor to influence gene activity in DNA, altering protein synthesis and cellular activity.

  • In the cytoplasm, the hormone–receptor complex can also generate signaling cascades via secondary messengers when appropriate.

  • For a representative peptide/hormone signaling cascade (e.g., glucagon receptor):

    • Binding activates adenylyl cyclase.

    • Adenylyl cyclase catalyzes the formation of cyclic ext{cAMP} inside the target cell: ext{ATP}
      ightarrow ext{cAMP} + ext{PP}_i

    • cAMP activates protein kinase A (PKA).

    • PKA phosphorylates enzymes such as phosphorylase kinase, activating it.

    • Activated phosphorylase kinase activates glycogen phosphorylase, which breaks down glycogen to glucose-1-phosphate and ultimately glucose.

    • PKA also inhibits glycogen synthase, reducing glycogen synthesis.

  • This cascade exemplifies how a hormone can rapidly alter cellular metabolism via second messengers, separate from direct gene transcription.

Endocrine Glands and Key Hormones

  • Hypothalamus: Produces antidiuretic hormone (ADH), oxytocin (OXT), and regulatory hormones.

  • Pituitary Gland: Anterior lobe (adenohypophysis) and posterior lobe (neurohypophysis).

  • Pineal Gland: Melatonin.

  • Parathyroid Glands: Parathyroid hormone (PTH).

  • Heart: Atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP).

  • Thymus: Thymosins.

  • Adrenal Glands: Cortex (cortisol, corticosterone, cortisone, aldosterone, androgens) and Medulla (epinephrine E, norepinephrine NE).

  • Pancreas (Pancreatic Islets): Insulin and glucagon.

  • Adipose Tissue: Leptin.

  • Digestive Tract: Secretes numerous hormones involved in coordination of system functions, glucose metabolism, and appetite.

  • Kidneys: Erythropoietin (EPO); Calcitriol (active vitamin D).

  • See also: Gonads with sex hormones (testes and ovaries) and regulatory hormones.

The Pituitary Gland: Anatomy and Roles

  • Location: Lies within the sella turcica, inferior to the hypothalamus, connected by the infundibulum.

  • Structure: Anterior lobe and posterior lobe; regions of the anterior lobe include pars tuberalis, pars distalis, pars intermedia.

  • The venous portal system (hypophyseal portal system) connects hypothalamus to the anterior pituitary, delivering regulatory hormones.

  • Blood supply: Superior hypophyseal artery supplies an upper capillary network; portal vessels carry regulatory hormones to the anterior lobe; inferior hypophyseal artery supplies the posterior lobe; hypophyseal veins carry pituitary hormones to the cardiovascular system.

Hypothalamus and Portal System

  • The hypothalamus regulates both the anterior and posterior lobes of the pituitary.

  • It synthesizes hormones released from the posterior pituitary (ADH and OXT).

  • It secretes regulatory hormones that control the activity of the anterior pituitary via the hypophyseal portal system.

  • The hypophyseal portal system carries releasing and inhibiting hormones from the hypothalamus to the anterior pituitary.

Anatomy of the Hypothalamus and Portal System

  • Located inferior to the thalamus; part of the limbic system.

  • Contains nuclei that produce hypothalamic hormones.

  • Hypothalamus is sexually dimorphic.

  • The hypophyseal portal system links the hypothalamus to the anterior pituitary.

Anterior Pituitary Gland (Adenohypophysis)

  • Anterior lobe regulates other endocrine glands.

  • Regions include pars tuberalis, pars distalis (largest, most anterior), pars intermedia (narrow band bordering posterior lobe).

Hypophyseal Portal System: Pathway Details

  • Superior hypophyseal artery delivers blood to a capillary network in the upper infundibulum.

  • Portal vessels deliver blood with regulatory hormones to the anterior lobe capillary network.

  • Capillary network in the anterior lobe communicates with endocrine cells.

  • Inferior hypophyseal artery supplies the posterior lobe.

  • Hypophyseal veins carry pituitary hormones to the cardiovascular system for distribution to the rest of the body.

Hypothalamic Regulatory Hormones and Anterior Pituitary Hormones

  • Classes of hypothalamic regulatory hormones:

    • Releasing hormones (RH): stimulate secretion from the anterior lobe.

    • Inhibiting hormones (IH): prevent secretion from the anterior lobe.

    • Rate of secretion is controlled by negative feedback.

  • Hypothalamic releasing hormones (examples):

    • TRH (thyrotropin-releasing hormone)

    • PRH (prolactin-releasing hormone)

    • CRH (corticotropin-releasing hormone)

    • GnRH (gonadotropin-releasing hormone)

    • GHRH (growth hormone-releasing hormone)

    • SS (somatostatin) acts as an inhibitory hormone

    • PIH (prolactin-inhibiting hormone) inhibits prolactin

  • Anterior pituitary hormones and their targets:

    • ACTH (adrenocorticotropic hormone): targets adrenal cortex; stimulates glucocorticoid release.

    • TSH (thyroid-stimulating hormone): targets thyroid; stimulates thyroid hormone release.

    • GH (growth hormone): targets bone, muscle, adipose tissue; promotes growth and metabolism.

    • PRL (prolactin): targets mammary glands; promotes milk production; regulated by PRH and PIH.

    • FSH (follicle-stimulating hormone): targets ovaries and testes; promotes gametogenesis and sex hormone production.

    • LH (luteinizing hormone): targets ovaries and testes; promotes ovulation and sex hormone production.

    • MSH (melanocyte-stimulating hormone): targets melanocytes; role in pigmentation; secretion influenced by pars intermedia and inhibited by dopamine.

The Posterior Pituitary Gland (Neurohypophysis)

  • Consists of axons from hypothalamic neurons.

  • Neurons release hormones into capillaries of the posterior lobe.

  • Hormones released: Oxytocin (OT) and Antidiuretic hormone (ADH, also called vasopressin).

Oxytocin and Vasopressin (ADH)

  • Oxytocin (OXT): stimulates uterine contractions during labor and milk ejection from mammary glands.

  • Antidiuretic hormone (ADH, vasopressin): promotes water reabsorption by kidneys, increasing blood volume and pressure.

  • Alcohol inhibits ADH release, leading to dehydration.

  • Diabetes insipidus: disease characterized by excessive thirst and urination due to low ADH levels.

The Thyroid Gland

  • Location: lies inferior to the thyroid cartilage of the larynx; consists of two lobes connected by an isthmus.

  • Structure: contains thyroid follicles with a colloid-filled follicle cavity.

  • Hormones produced:

    • Thyroid hormones: T3 (triiodothyronine) and T4 (thyroxine)

    • Calcitonin (CT) produced by parafollicular (C) cells

  • Regulation: TSH from the anterior pituitary stimulates thyroid hormone release.

Thyroid Hormones: T3 and T4

  • T3 and T4 increase the rate of oxygen consumption and ATP production by mitochondria.

  • They increase the metabolic rate of cells by activating genes involved in energy use and glycolysis.

  • They increase blood pressure, heart rate, and contraction force.

Calcitonin

  • Produced by parafollicular cells (C cells).

  • Promotes calcium excretion by kidneys and decreases calcium absorption in the digestive tract.

  • Thyroid follicular cells produce T3/T4; parafollicular cells produce calcitonin.

Parathyroid Glands

  • Four small glands embedded on the posterior surface of the thyroid gland.

  • Principal cells secrete parathyroid hormone (PTH) to increase blood calcium.

Adrenal Glands: Structure and Function

  • Located superior to each kidney; composed of cortex (outer) and medulla (inner).

  • Blood supply and internal organization: cortex has three zones; medulla secretes catecholamines.

  • Cortex zones and products:

    • Zona glomerulosa: mineralocorticoids (e.g., aldosterone) — regulate Na+ and water balance.

    • Zona fasciculata: glucocorticoids (e.g., cortisol) — regulate metabolism and stress response.

    • Zona reticularis: androgens — contribute to secondary sex characteristics.

  • Medulla secretes catecholamines: epinephrine (E) and norepinephrine (NE).

Mineralocorticoids and Glucocorticoids

  • Aldosterone: main mineralocorticoid; increases Na+ reabsorption in kidneys.

  • Cortisol (and other glucocorticoids like corticosterone and cortisone): regulate metabolism and respond to stress; production stimulated by ACTH from the anterior pituitary.

  • Androgens: contribute to male traits and have other tissue-specific effects.

Adrenal Medulla: Catecholamines

  • Epi and NE released in response to sympathetic stimulation.

  • Effects include increased heart rate, increased blood glucose, and mobilization of energy stores.

Pineal Gland

  • Located in the epithalamus; pinealocytes synthesize melatonin using serotonin.

  • Melatonin influences circadian rhythms.

Pancreas: Endocrine Pancreas and Blood Glucose Regulation

  • Anatomy: pancreas lies in the loop between stomach and small intestine; retroperitoneal; exocrine and endocrine functions.

  • Islets of Langerhans: endocrine clusters containing alpha (glucagon-producing) and beta (insulin-producing) cells, among others.

  • Primary pancreatic hormones:

    • Insulin: released by beta cells in response to high blood glucose; lowers blood glucose by promoting glucose uptake into cells.

    • Glucagon: released by alpha cells in response to low blood glucose; raises blood glucose by promoting glucose release from liver stores.

  • Mechanism of insulin action involves cellular uptake of glucose and metabolic effects that reduce hyperglycemia; glucagon promotes glycogenolysis and gluconeogenesis.

  • Practical note: insulin and glucagon coordinate to maintain glucose homeostasis.

Diabetes Mellitus: Hyperglycemia and Its Consequences

  • Characterized by hyperglycemia, glycosuria, and polyuria.

  • Hyperglycemia: blood glucose is abnormally high.

  • Glycosuria: glucose appears in urine.

  • Polyuria: excessive urine volume.

  • Causes: inadequate insulin production or faulty insulin receptors.

  • Type 1 diabetes: inadequate insulin production by pancreatic beta cells; requires daily injections or infusion; often develops in children and young adults.

  • Type 2 diabetes: most common; normal insulin but tissues have insulin resistance; commonly associated with obesity; treatment focuses on weight loss and medications.

Additional Notes: Hormone Regulation and System Integration

  • The endocrine system operates with feedback mechanisms to maintain homeostasis.

  • Hormones can have multiple targets and can interact with other signaling systems (e.g., the nervous system) to coordinate responses.

  • Some hormones exert rapid effects via second messenger systems (e.g., cAMP) while others exert slow effects via gene transcription changes.

Quick Reference: Key Hormones and Functions (Selected)

  • ADH (vasopressin): promotes water retention in kidneys; diuretic effect of alcohol occurs via ADH inhibition.

  • Oxytocin: stimulates uterine contractions and milk ejection.

  • TSH: stimulates thyroid hormone release; regulated by TRH.

  • ACTH: stimulates glucocorticoid release; regulated by CRH.

  • GH: promotes growth and tissue growth, especially muscle and bone; regulated by GHRH and GHIH (somatostatin).

  • PRL: promotes mammary gland development and milk production; regulated by PRH and PIH.

  • FSH/LH: regulate gametogenesis and sex hormone production; regulated by GnRH.

  • MSH: influences melanin production; regulated by PIF (dopamine acts as an inhibitor).

  • Calcitonin: lowers blood calcium levels; from C cells in the thyroid.

  • PTH: raises blood calcium levels; from parathyroid glands.

  • Insulin: lowers blood glucose by promoting uptake in cells.

  • Glucagon: raises blood glucose via liver glucose release.

  • Melatonin: regulates circadian rhythms; secreted by the pineal gland.

Summary of Key Relationships and Pathways

  • The hypothalamus links the nervous and endocrine systems via the pituitary gland.

  • The hypothalamus secretes regulatory hormones into the hypophyseal portal system to control the anterior pituitary.

  • The posterior pituitary releases hormones produced in the hypothalamus (ADH and oxytocin) directly into the bloodstream.

  • The thyroid, adrenal glands, pancreas, and gonads are major endocrine organs with diverse regulatory hormones.

  • Hormone action ranges from rapid second-messenger cascades (e.g., cAMP) to slower genomic effects via transcriptional regulation.

Note: The content follows the structure and points presented in the provided transcript. Some formatting and ordering reflect the pedagogical flow of the material to support study and review. For clarity, biological terms are kept consistent with standard endocrinology terminology where possible; minor typographical inconsistencies in the source transcript have been reconciled in the notes where they do not alter the core concepts.