Endocrine System Overview

Endocrine System Overview

• The endocrine system is a communication system in the body.
  • It releases hormones that travel through the bloodstream to target cells
• Hormones are chemical messengers.

Negative Feedback Systems

• Negative feedback systems maintain homeostasis.
  • A stimulus disrupts homeostasis causing a change
  • Receptors detect this change
  • The control center processes information & causes a response
  • Effectors cause a response
  • The feedback loop compares the response to the original stimulus and determines if further adjustments are needed, potentially returning to homeostasis.

Chemical Messengers in the Body

• Endocrine signaling: hormones released into the bloodstream, affecting distant cells.
• Paracrine signaling: signals released to nearby cells
• Autocrine signaling: signals acting on the same cell that released them.
• Synaptic signaling: signals released across synaptic clefts, used in the nervous system.
• Neuroendocrine signaling: neurons releasing hormones into the bloodstream.

Homeostatic Pathways

• Endocrine system (using hormones) and nervous system (using nerve impulses).
• The hypothalamus is central connecting the two systems.
  • Hormones: slow, enduring effects
  • Nerve impulses: fast, short-term effects

Endocrine System - General Mechanism

• Hormones are released into the blood or lymph.
  • Hormones travel via blood / lymph vessels.
  • Hormones bind to specific receptors on target cells.
  • This binding causes a response in the target cell.
• Different types of hormones exist, including water-soluble / lipid-soluble

Endocrine vs Nervous Pathways

• Endocrine signals travel everywhere via the bloodstream.
• Nervous signals travel along a specific path to specific locations.

Types of Glands

• Exocrine glands: secrete substances onto surfaces via ducts.
  • Examples include sweat, oil, salivary, mammary, and gastric glands.
• Endocrine glands: secrete hormones directly into surrounding tissue fluid.
  • Examples include pituitary, adrenal, gonads, and pancreatic islets.

Major Endocrine Glands & Tissues

• Several organs contain endocrine cells
  • Organs such as the thymus, heart, liver, stomach, kidneys, and small intestine

Functions of Hormones

• Hormones regulate reproductive processes, growth & development, metabolism, stimulate endocrine/exocrine glands, activate muscle or nervous tissue, and support immune function.

Hypothalamus & Pituitary Gland

• The hypothalamus and pituitary gland are closely linked, with the pituitary gland being divided into anterior and posterior lobes.
• The infundibulum connects both structures.

Posterior Pituitary Gland

• Releases ADH (antidiuretic hormone) and oxytocin.
  • ADH: targets kidney tubules, increases water reabsorption.
  • Oxytocin: targets mammary glands & uterine muscles, involved in milk release and childbirth.

Posterior Pituitary Gland - ADH

• ADH regulates water balance.
• It increases H₂O permeability and reabsorption in renal tubules, influencing urine volume & concentration and blood osmolarity.
• ADH hyposecretion (example: diabetes insipidus) results in disrupted water balance.

Posterior Pituitary Gland - Oxytocin

• Oxytocin is involved in uterine contractions and milk ejection.
• It also has positive feedback loops associated with breast feeding

Anterior Pituitary Gland

• The anterior pituitary gland releases hormones that regulate other endocrine glands. Examples include FSH, LH, TSH, ACTH, prolactin, and growth hormone.

Anterior Pituitary Gland - Hormonal Control

• Various hypothalamic hormones control the release of anterior pituitary hormones, including releasing and inhibiting hormones.

Anterior Pituitary Gland - Growth Hormone

• Growth hormone (GH) regulates growth and metabolism.
  • Growth promoting effects: on various tissues; skeletal cartilage and formation
  • Metabolic effects: on fat metabolism, and carbohydrate metabolism.

GH Related Disorders

• Hyposecretion during childhood leads to pituitary dwarfism
• Hyposecretion during adulthood leads to Simmonds disease
• Hypersecretion during childhood leads to Gigantism
• Hypersecretion during adulthood leads to Acromegaly

Thyroid Gland

• The thyroid gland produces hormones (T3, T4) that regulate metabolism.
• It's located in the neck, involving follicular and parafollicular cells

Thyroid Hormones

• Tyrosine in thyroid hormones increase metabolic rate and promote tissue & development.
• Thyroid hormones regulate metabolism, growth, and development and body temperature

Thyroid Hormone Disorders

• Hypersecretion: Graves' disease (autoimmune)

• Hyposecretion: Cretinism (severe hypothyroidism in childhood), Myxedema (hypothyroidism in adults) or goiter (enlarged thyroid due to iodine deficiency).

Calcitonin

• Calcitonin regulates calcium levels in blood for calcium homeostasis
  • Inhibits bone resorption by osteoclasts.
  • Promotes calcium uptake into bones.

Calcium Homeostasis and Thyroid Gland

• The thyroid gland plays a role in calcium homeostasis

Parathyroid Glands

• Parathyroid glands are found behind the thyroid gland and produce parathyroid hormone (PTH)
• PTH plays a key role in calcium regulation, including bone resorption, increasing blood calcium levels.

Calcium Homeostasis and Parathyroid Gland

• The parathyroid gland plays a key roles in calcium homeostasis (including high and low blood calcium).

Parathyroid Hormone (PTH)

• Regulates calcium homeostasis, raising blood calcium levels in numerous ways.

Parathyroid Gland Disorders

• Hyperparathyroidism, often rare, reduces bone density
• Hypoparathyroidism causing muscle twitching and convulsions

Pancreatic Endocrine Tissue

• The pancreas has endocrine tissue (pancreatic islets) that regulate blood glucose levels.
  • Alpha (α) cells release glucagon.
  • Beta (β) cells release Insulin

Pancreas- Main Functions

• Digestion (exocrine).
• Metabolism (endocrine).
• Secretion of pancreatic juice (digestion).
• Glucose-regulating hormones (metabolism - insulin and glucagon)

Blood Glucose Homeostasis and Hormonal Actions

• Insulin lowers blood glucose.
• Glucagon increases blood glucose.

Blood Glucose Homeostasis - Diabetes Mellitus

• Type 1 Diabetes: autoimmune destruction of beta cells;
• Type 2 Diabetes: insulin resistance or deficiency

Blood Glucose Homeostasis Hormones

• Insulin promotes glucose uptake by cells, glycogen synthesis
• Glucagon promotes glycogenolysis & gluconeogenesis

Adrenal Glands

• Adrenal glands have an outer cortex and an inner medulla.
  • Cortex: produces mineralocorticoids, glucocorticoids, gonadocorticoids.
  • Medulla: produces catecholamines.

Adrenal Hormones

• Aldosterone: mineralocorticoid, regulates sodium and water balance.
  • Cortisol: glucocorticoid, regulates carbohydrate/protein metabolism, stress response
• Gonadocorticoids: sex hormones, primarily androgens.
• Epinephrine and norepinephrine: catecholamines, involved in the fight-or-flight response

Regulation of Adrenal Hormones

• Various factors regulate secretion of the adrenal hormones including: stress, blood pressure, blood volume, electrolytes, feedback mechanisms.

Role of Adrenal Hormones

• Regulate numerous metabolic processes in the body, including carbohydrate/protein metabolism, blood volume, blood pressure, electrolytes

Adrenal Disorders

• Hyposecretion of mineralocorticoids and glucocorticoids leads to Addison's disease.
• Hypersecretion of glucocorticoids leads to Cushing's syndrome.

Organs with Secondary Endocrine Functions

• Many organs have secondary endocrine function, in addition to their primary function.
• Examples include gonads, digestive organs, placenta, thymus, kidneys, heart, and adipose tissue.

Gonadal Hormones

• Gonads: produce sex hormones (estrogens & androgens) that regulate sexual development and function.
  • Estrogens in females
  • Androgens in males

Placenta Endocrine Function

• The placenta produces hormones critical for pregnancy, maintaining receptiveness of the uterus, and supporting pregnancy.
  • Human chorionic gonadotropin (hCG)
  • Estrogens & progesterones

Digestive Organs Endocrine Function

• Digestive organs produce gastrointestinal hormones that regulate digestion and absorption.
  • Gastrin helps regulate gastric juice.
  • Secretin promotes pancreatic & bile secretion.
  • CCK helps promote pancreatic and bile secretion.

Thymus Endocrine Function

• The thymus promotes T lymphocyte development, critical for immune responses.
  • Thymosins, thymulin, and thymopoietins

Pineal Gland Endocrine Function

• Melatonin is produced in the pineal gland to regulate circadian rhythms and protect the central nervous system.

Prostaglandins (Local Signaling)

• Prostaglandins are lipid-based paracrine signaling molecules.
• They regulate numerous bodily processes, such as inflammation, digestion, blood pressure.