Endocrine System Overview

Endocrine System Overview

• The endocrine system is a network of glands that secrete hormones into the bloodstream.
• Hormones regulate various bodily functions, such as metabolism, growth, and reproduction.

Important Topics and Pages

• Growth Hormone (GH): Understand the mechanisms, effects, and glands that produce GH. Learn about disorders caused by homeostatic imbalance (pages 615-616).
• Thyroid Hormone (TH): Understand the mechanisms, effects, and glands that produce TH. Study disorders that result from homeostatic imbalance (pages 617-620).
• Insulin: Understand the mechanisms, effects, and glands that produce insulin. Study diabetes mellitus (pages 630-632).
• Student Research Presentations: Understand the pathophysiology of endocrine disorders covered.

Endocrine Glands and Specific Hormone Functions

• Specific examples of hormone pathways will be covered including:
  • Growth hormone
  • Thyroid hormone
  • Insulin

Hypothalamus and Pituitary Gland

• The hypothalamus is connected to the pituitary gland via the infundibulum.
• The pituitary gland secretes at least eight major hormones.
• It has two lobes:
  • Posterior pituitary: Composed of neural tissue; secretes neurohormones. Not technically an endocrine gland.
  • Anterior pituitary (adenohypophysis): Composed of glandular tissue.

Anterior Pituitary Hormones

• The anterior pituitary secretes 6 peptide hormones.
• Growth hormone (GH), also called somatotropin.
  • Produced by somatotropic cells.
  • Directly affects metabolism and indirectly promotes growth.
  • Anterior pituitary is stimulated to release GH when the hypothalamus secretes Growth Hormone Releasing Hormone (GHRH).

Growth Hormone (GH): Direct Actions on Metabolism

• Glucose-sparing actions decrease the rate of cellular glucose uptake and metabolism.
• Triggers the liver to break down glycogen to glucose.
• Increases blood levels of fatty acids.
• Encourages cellular protein synthesis via amino acid uptake.

Growth Hormone (GH): Indirect Actions on Growth

• GH triggers liver, skeletal muscle, and bone to produce insulin-like growth factors (IGFs).
• IGFs stimulate:
  • Uptake of nutrients for DNA and protein synthesis for cell division.
  • Formation of collagen and bone matrix deposition.
  • Growth and division of most cells, primarily bones and skeletal muscle.

Growth Hormone (GH): Daily Cycles and Imbalances

• GH levels cycle daily, highest during sleep.
• Levels are highest during adolescence, decreasing in adulthood.
• Important in metabolic regulation throughout life.
• Hypersecretion of GH in children causes gigantism, in adults acromegaly.
• Hyposecretion in children causes pituitary dwarfism.

Thyroid Gland

• Butterfly-shaped in the anterior neck.
• On the trachea, just inferior to the larynx.
• Consists of:
  • Lateral lobes (one on each side).
  • Isthmus (connecting the lobes).
  • Follicles with colloid (thyroglobulin plus iodine; precursor to thyroid hormone).
  • Parafollicular cells producing calcitonin.

Thyroid Hormone (TH): Structure

• Two forms:
  • T₄ (thyroxine): primary form secreted by thyroid follicles; two tyrosine molecules with four bound iodine atoms.
  • T₃ (triiodothyronine): has two tyrosines with three bound iodine atoms; more active form.
• Both are iodine-containing amine hormones, affecting virtually every cell.

Thyroid Hormone (TH): Synthesis

• Follicular cells synthesize thyroglobulin, bind it with iodine in the colloid.
• Tyrosines are linked to form T₃ and T₄, which are cleaved and released into blood, bound to transporter proteins.

Thyroid Hormone (TH): Effects

• Increases basal metabolic rate and heat production.
• Regulates tissue growth and development, particularly for skeletal and nervous systems, and reproductive capabilities.
• Maintains blood pressure and increases adrenergic receptors in blood vessels.

Thyroid Hormone (TH): Regulation

• Hypothalamus releases Thyrotropin Releasing Hormone (TRH).
• TRH stimulates TSH release from the anterior pituitary.
• TSH stimulates thyroid hormone release.
• Negative feedback loop: Thyroid hormones inhibit TRH and TSH release.

Thyroid Hormone (TH): Homeostatic Imbalances

• Hyposecretion (hypothyroidism): Myxedema (mucous swelling) in adults with symptoms like low metabolic rate, thick/dry skin, puffy eyes, feeling chilled, constipation, edema, mental sluggishness, lethargic feeling. Iodine deficiency can cause goiter.
• Hypersecretion (hyperthyroidism): Most commonly in Graves' disease, an autoimmune disease. Symptoms include high metabolic rate, sweating, rapid/irregular heartbeat, nervousness, weight loss and exophthalmos (eyes protrude).

Pancreas

• Triangular gland located behind the stomach.
• Has exocrine and endocrine cells.
• Acinar cells: produce digestive enzymes rich juice.
• Pancreatic islets (islets of Langerhans): Contain endocrine cells:
  • Alpha (α) cells produce glucagon (hyperglycemic hormone).
  • Beta (β) cells produce insulin (hypoglycemic hormone).

Insulin

• Secreted when blood glucose levels increase, lowering blood glucose.
• Synthesized as proinsulin, then modified.
• Binds to tyrosine kinase receptors on cell surfaces.
• Lowering blood glucose levels in three ways:
  • Enhances glucose transport into fat and muscle cells.
  • Inhibits glycogen breakdown to glucose.
  • Inhibits amino acid or fat conversion to glucose.
• Insults catalyzes glucose oxidation for ATP production, polymerizes glucose to glycogen, and converts glucose to fat (particularly in adipose tissue).

Factors Influencing Insulin Release

• Elevated blood glucose levels, a primary stimulus.
• Rising amino acid and fatty acid blood levels.
• Release of acetylcholine by parasympathetic nerve fibers.
• Hormones: glucagon, epinephrine, growth hormone, thyroxine, glucocorticoids.
• Somatostatin and sympathetic nervous system inhibit insulin release.

Insulin and Blood Sugar Homeostasis

• Insulin helps maintain blood sugar homeostasis along with glucagon.
• High blood glucose levels stimulate insulin release; cells take up glucose, glycogen formation in the liver, and glucose conversion to fat.
• Low blood glucose stimulates glucagon release; liver breaks down glycogen to glucose.

Diabetes Mellitus: Insulin Homeostatic Imbalance

• Hyposecretion or ineffective insulin leads to Diabetes mellitus.
• Blood glucose remains high and not properly absorbed.
• Cardinal signs: Polyuria (large urine output), Polydipsia (excessive thirst), Polyphagia (excessive hunger).
• Severe cases: Causes fats to be used instead of sugar (lipidemia); ketone bodies build up, acidosis could occur, and eventually lead to death.