Endocrine System Overview

Endocrine System Characteristics

• Composed of endocrine glands and specialized endocrine cells
• Secrete hormones, which are chemical messengers that diffuse into the bloodstream
• Hormones travel to their target tissues or effectors

Nervous System vs. Endocrine System

• Nervous System: Uses neurotransmitters for short-duration responses, released directly onto target cells
• Endocrine System: Uses hormones for long-duration responses, circulate in the blood, affect receptors through G proteins

Hormone Communication & Distribution

• Hormones are able to communicate with their target cells
• Stability of hormones is defined by their half-life (exponential decrease)
• Hormones are distributed throughout the body via the bloodstream

Chemical Composition of Hormones

• Lipid-Soluble Hormones: Nonpolar, including steroid hormones, thyroid hormones, and fatty acid derivative hormones
  • Travel bound to proteins in the bloodstream
  • Removed from circulation through conjugation (enzymes attach water-soluble molecules)
• Water-Soluble Hormones: Polar molecules, including protein hormones, peptide hormones, and most amino acid derivative hormones
  • Travel as free hormones in the bloodstream
  • Have shorter half-lives due to proteases (hydrolytic enzymes)
  • Some modifications occur to protect them from destruction (glycoproteins, modified terminal end, binding proteins)

Hormone Secretion Patterns

• Chronic: Relatively constant blood levels over long periods (e.g., thyroid hormones)
• Acute: Concentrations change suddenly and irregularly (e.g., epinephrine)
• Episodic: Secreted at predictable intervals and concentrations (e.g., reproductive hormones)

Regulation of Hormone Secretion

• Humoral Stimuli: Stimulation by metabolites and molecules in the bloodstream (e.g., calcium, sodium, glucose)
• Neural Stimuli: Action potentials release neurotransmitters into synapses of hormone-producing cells (e.g., releasing hormones from the hypothalamus)
• Hormonal Stimuli: Hormones stimulate the secretion of other hormones (e.g., tropic hormones from the anterior pituitary)

Hormone Receptors & Mechanisms of Action

• Hormones exert their actions by binding to receptors on target cells
• Receptor Site: Specific portion of each receptor molecule where a hormone binds
• Specificity: Each type of hormone tends to bind to one type of receptor

Hypothalamus

• Rostral/Preoptic Area: Involved in thermoregulation, fever, electrolyte balance, sleep-wake cycles, and sexual behavior
• Tuberal Hypothalamus: Involved in feeding, sexual behavior, aggression, and autonomic/endocrine responses
• Posterior Area: Regulates wakefulness and the stress response

Pituitary Gland/Hypophysis

• Pea-sized gland located in the sella turcica of the sphenoid bone
• Posterior Pituitary (Neurohypophysis): Continuous with the hypothalamus, stores and releases neuropeptides
• Anterior Pituitary (Adenohypophysis): Develops from the embryonic oral cavity, produces traditional hormones

Pituitary Gland & Hypothalamus Interaction

• Anterior Pituitary Regulation: Hypothalamohypophyseal portal system (specialized blood vessels)
  • Releasing hormones: Stimulate hormone secretion
  • Inhibiting hormones: Decrease hormone secretion
• Posterior Pituitary Regulation: Hypothalamohypophyseal tract (neural pathway)
  • Posterior pituitary stores and releases hormones produced in the hypothalamus

Growth Hormone (GH)

• Stimulates growth in most tissues & regulates metabolism
  • Promotes protein synthesis, fat breakdown, and glucose synthesis
• GH secretion increases in response to low blood glucose, stress, and increased amino acids
• Regulated by hypothalamic hormones: GHRH (releasing) and GHIH (inhibiting)

Thyroid Gland

• Largest endocrine gland located in the anterior neck
• Composed of follicles that produce thyroglobulin
• Parafollicular cells produce calcitonin

Thyroid Hormones

• Triiodothyronine (T3): Two tyrosines with three bound iodine atoms (90%)
• Tetraiodothyronine (T4): Two tyrosines with four bound iodine atoms (10%)

Effects of Thyroid Hormones

• Increase metabolic rate (glucose, fat, and protein metabolism)
• Increase body temperature
• Regulate blood pressure, tissue growth, skeletal/nervous system development, maturation, and reproductive capabilities

Regulation of Thyroid Hormone Secretion

• Regulated by TRH (hypothalamus) and TSH (anterior pituitary)
  • TRH increases TSH secretion
  • TSH increases T3 and T4 secretion
  • T3 and T4 negatively feedback to inhibit TSH and TRH secretion

Calcitonin

• Produced by parafollicular cells of the thyroid gland
• Directly regulated by blood Ca2+ levels
  • Low Ca2+ = Low Calcitonin
  • High Ca2+ = High Calcitonin
• Targets skeleton to inhibit osteoclast activity and stimulate calcium uptake

Parathyroid Gland

• Tiny glands embedded in the posterior aspect of the thyroid
• Secrete parathyroid hormone (PTH)
• Plays a crucial role in regulating blood calcium balance (more important than calcitonin)

Parathyroid Hormone (PTH)

• Increases release of Ca2+ from bones into the blood (increased osteoclast activity)
• Promotes calcium reabsorption in kidneys and active vitamin D formation
• Active vitamin D increases calcium absorption in the intestine
• Secreted in response to decreased blood Ca2+ levels

Adrenal Glands

• Paired, pyramid-shaped organs located on top of the kidneys
• Adrenal Medulla (Inner Area): Arises from sympathetic nervous system cells, secretes epinephrine and norepinephrine
• Adrenal Cortex (Outer Area): Glandular tissue derived from mesoderm, composed of three layers:
  • Zona glomerulosa
  • Zona fasciculata
  • Zona reticularis

Adrenal Medulla Hormones

• Epinephrine (Adrenaline): 80% of hormones released
• Norepinephrine: 20% of hormones released
• Prepare the body for physical activity by:
  • Increasing blood glucose levels
  • Increasing glycogen and glucose use in muscle
  • Increasing heart rate and force of contraction
  • Causing vasoconstriction in skin and viscera
  • Causing vasodilation in skeletal and cardiac muscle
• Released in response to emotions, injury, and stress by the sympathetic nervous system

Adrenal Cortex Hormones

• Corticosteroids: Steroid hormones
• Zona Glomerulosa: Mineralocorticoids (Aldosterone)
  • Regulates electrolytes in extracellular fluids
  • Maintains Na+ balance and decreases K+ and H+ levels in the blood
  • Secreted in response to high K+, low Na+, or decreased blood volume/pressure
• Zona Fasciculata: Glucocorticoids (Cortisol)
  • Help the body resist stress by maintaining blood sugar, blood volume, and preventing water shifts
  • Provoke gluconeogenesis, increase blood glucose, fatty acids, and amino acids
  • Excessive levels can suppress immune function and cause other adverse effects
• Zona Reticularis: Gonadocorticoids (Androgens)
  • Primarily testosterone, contributes to puberty, secondary sex characteristics, and sex drive in females

Pancreas

• Triangular gland located behind the stomach with exocrine and endocrine functions
• Acinar Cells: Produce digestive enzymes (exocrine)
• Pancreatic Islets (Islets of Langerhans): Produce hormones (endocrine)
  • Alpha (α) Cells: Produce glucagon
  • Beta (β) Cells: Produce insulin

Insulin

• Target tissues: Liver, adipose tissue, muscle, satiety center in the hypothalamus
• Increases glucose and amino acid uptake by cells
  • Glucose used for energy, stored as glycogen, or converted into fat
  • Amino acids used for protein synthesis
• Low insulin levels promote ketone body formation by the liver

Glucagon

• Target tissue primarily the liver
• Promotes glycogen breakdown to glucose
• Stimulates glucose synthesis from amino acids
• Liver releases glucose into the blood

Hormonal Regulation of Nutrients

• After a meal:
  • High glucose stimulates insulin and inhibits glucagon, cortisol, GH, and epinephrine
  • Insulin increases glucose, amino acid, and fat uptake
  • Blood glucose drops: Insulin decreases, other hormones increase, glucose released
• During Exercise:
  • Sympathetic activity increases epinephrine and glucagon secretion, leading to glucose release
  • Low blood sugar stimulates epinephrine, glucagon, GH, and cortisol secretion, increasing fatty acids and ketones for energy

Testes & Ovaries

• Testes: Secrete testosterone
  • Initiates male reproductive organ maturation
  • Causes secondary sex characteristics and sex drive
  • Necessary for sperm production
• Ovaries: Secrete estrogen and progesterone
  • Maturation of female reproductive organs
  • Appearance of secondary sex characteristics
  • Breast development and uterine cycle

Pineal Body

• Small, pine cone-shaped structure located superior and posterior to the thalamus
• Secretes melatonin
• May inhibit reproductive maturation and regulate sleep-wake cycles

Thymus

• Lobulated gland located deep to the sternum
• Produces thymopoietins and thymosins
• Essential for T-lymphocyte development in the immune system

Other Endocrine Glands

• Gastrointestinal Tract: Produces gastrin, secretin, and cholecystokinin (regulate digestion)
• Kidneys: Produce erythropoietin (stimulates red blood cell production)
• Placenta: Secretes human chorionic gonadotropin (maintains pregnancy)

Hormone-Like Substances

• Autocrine Agents: Local effects on cells of the same type (e.g., prostaglandins, thromboxanes)
• Paracrine Agents: Local effects on cells of a different type (e.g., growth factors, clotting factors)
• Key differences from hormones: Not secreted from discrete glands, local effects, functions not always fully understood

Developmental Aspects of the Endocrine System

• Endocrine glands derive from all three germ layers: ectoderm, mesoderm, and endoderm