Endocrine System: Hormones, Stress Responses, and Regulation

Key Takeaways

• Adrenal gland management of short- and long-term stress responses

• Pineal gland and melatonin

• Pancreas: insulin, glucagon, and diabetes

• Other hormones secreted by various organs

• Special feature on GLP-1 agonists

• Environmental endocrine disruptors

Adrenal Gland and Stress Response

General Adaptation Syndrome

The adrenal gland helps the body respond to stressors through a coordinated stress response, known as the general adaptation syndrome, regulated by the hypothalamus. The effects are time-dependent:

• Short-term stress (Fight or Flight):

  • Adrenal medulla releases catecholamines (epinephrine, norepinephrine)

  • Activates the autonomic nervous system (ANS)

• Long-term stress (Resistance Reaction):

  • Adrenal cortex releases corticosteroids (e.g., cortisol)

  • Helps survive prolonged stressors like starvation or drastic energy changes

Stages of Stress Response / Adaptation

Stage	Resistance to Stress	Symptoms/Characteristics
Alarm Reaction (Shock)	Below normal	Body acts as if injured; blood pressure and body temperature drop
Alarm Reaction (Countershock)	Above normal	Sympathetic nervous system activated; fight-flight-freeze response; adrenaline released
Resistance	Above normal	Cortisol released; nonessential functions shut down; individual appears normal
Exhaustion	Below normal	Resources depleted; immune system weakened; individual susceptible to illness

Short-Term vs. Long-Term Stress

Short-Term Stress

• Fight or flight response

• Useful for immediate emergencies (e.g., fire, sudden danger)

Long-Term Stress

• Resistance reaction

• Useful for surviving prolonged challenges (e.g., starvation)

Catecholamines

Role and Dysregulation

• Hyposecretion: Usually not an issue; other ANS components compensate

• Hypersecretion: Often due to adrenal medulla tumor (pheochromocytoma); leads to overactivation of sympathetic ANS

  • Symptoms: Increased metabolic rate, rapid heartbeat, palpitations, hypertension, nervousness, sweating

Pineal Gland and Melatonin

Pineal Gland

• Located at the roof of the third ventricle in the brain

• Contains pinealocytes that produce melatonin

Melatonin

• Regulates 24-hour (circadian) cycle

• Targets biological clock in hypothalamus, influencing sleep, body temperature, and appetite

• Has antioxidant properties

• May prevent early onset puberty (antigonadotropic effect)

Melatonin Regulation

• Production suppressed by light

• Increased production during winter (less daylight) may contribute to Seasonal Affective Disorder (SAD)

Other Hormone-Producing Tissues

• Pancreas

• Gonads (testes and ovaries)

• Placenta

• Adipose tissue

• Heart

• GI tract

• Kidneys

• Skeleton

• Skin

• Thymus

Pancreas: Endocrine Functions

Islets of Langerhans

• Alpha cells: produce glucagon

• Beta cells: produce insulin

Hormones of the Pancreas

• Glucagon: Increases blood glucose levels (hyperglycemic hormone)

  • Stimulates liver to perform glycogenolysis and gluconeogenesis

  • Release stimulated by low blood glucose, high amino acids, sympathetic stimulation

  • Release inhibited by insulin and high blood glucose

• Insulin: Decreases blood glucose levels (hypoglycemic hormone)

  • Promotes uptake of glucose by cells, inhibits glycogenolysis and gluconeogenesis

  • Promotes protein synthesis and fat storage

  • Release stimulated by high blood glucose, amino acids, parasympathetic stimulation

  • Release inhibited by sympathetic stimulation

Glucose Homeostasis

• Glucose is the primary energy source (1 glucose ≈ 30 ATP)

• Some cells (brain, kidney, red blood cells) rely almost exclusively on glucose

• Maintaining blood glucose is critical, especially for brain function

• Chronic high blood glucose damages tissues (kidneys, eyes, heart, neurons)

Glucose Dysregulation

Hyperinsulinism

• Hypersecretion of insulin leads to hypoglycemia

• Symptoms: Anxiety, tremors, weakness, confusion, convulsions, unconsciousness, death

• Treatment: Sugar ingestion

• Causes: Improper insulin injection, genetics, insulin resistance, cancer

Diabetes Mellitus

• Type 1: Hyposecretion of insulin (often autoimmune destruction of beta cells)

• Type 2: Hypoactivity of insulin (insulin resistance)

• Most cells can't take up glucose without insulin, leading to lipidemia and ketoacidosis

Symptoms of Diabetes Mellitus

• Ketoacidosis: Hyperpnea, arrhythmias, nausea, vomiting, abdominal pain, CNS depression, coma, death

• Hyperlipidemia: Plaque buildup, heart attack, stroke

• Hyperglycemia: Thirst, frequent urination, fatigue, blurred vision, weight loss, immune suppression, organ damage, death

Treatments for Diabetes

• Type 1: Insulin replacement therapy, blood glucose monitoring

• Type 2: Insulin therapy, glucose monitoring, lifestyle changes, medications:

  • Metformin: Decreases liver glucose production

  • GLP-1 receptor agonists: Increase insulin secretion

  • SGLT2 inhibitors: Increase glucose loss through urine

  • DPP-4 inhibitors: Prevent breakdown of GLP-1

  • Meglitinides: Increase insulin secretion

Glucagon Dysregulation

• High glucagon = high blood sugar (hyperglycemia)

• Not usually the cause of diabetes, but may play a permissive role

• High glucagon (hyperglucagonemia) present in diabetics, may facilitate insulin issues

Other Hormone-Producing Organs

Gonads

Testes

• Produce androgens (mainly testosterone) and inhibin

• Testosterone: Promotes male puberty, development of male sex characteristics, required for sperm production

• Inhibin: Inhibits FSH secretion

Ovaries

• Produce estrogens, progesterone, and inhibin

• Estrogen: Promotes female puberty, development of secondary sex characteristics

• Estrogen + progesterone: Regulate menstruation and fertility

• Inhibin: Inhibits FSH secretion

Placenta

• Temporary endocrine organ during pregnancy

• Connects mother and fetus via umbilical cord

• Functions as fetal kidneys, lungs, and liver until birth

Placental Hormones & Functions

• Estrogen: Stimulates uterine growth, contraction, mammary gland development

• Progesterone: Maintains uterine lining

• Human chorionic gonadotropin (hCG): Detected by pregnancy tests, maintains progesterone production

• Human placental lactogen: Mammary gland development, metabolism

• Placental growth hormone: Regulates fetal growth

• Relaxin: Increases ligament and muscle flexibility

Adipose Tissue

• Leptin: Increases satiety, stimulates energy use

• Resistin: Insulin antagonist, reduces cellular sensitivity to insulin

• Adiponectin: Insulin synergist, increases cellular sensitivity to insulin

Heart

• Atrial natriuretic peptide (ANP): Increases excretion of sodium, reduces blood pressure

• Brain natriuretic peptide (BNP): Similar function, less potent

Kidneys

• Erythropoietin: Stimulates red blood cell production in response to low oxygen

• Renin: Activates renin-angiotensin-aldosterone pathway in response to low blood pressure

Skeleton

• Osteocalcin: Produced by osteoblasts in response to insulin; stimulates pancreas to secrete more insulin, restricts fat storage, reduces body fat

Skin

• Produces cholecalciferol (inactive vitamin D), converted to calcitriol (active vitamin D) by kidneys and liver

• Increases intestinal absorption of calcium, decreases inflammation

Thymus

• Produces thymosins, thymulin, thymopoietins

• Promote T cell production (immune function)

Special Feature: GLP-1 Receptor Agonists

• GLP-1 receptor agonists increase insulin secretion by the pancreas

• Used in treatment of type 2 diabetes

Environmental Endocrine Disruptors

Definition and Examples

• Environmental chemicals that interfere with endocrine signaling, especially during pregnancy and development

• Atrazine: Herbicide, converts testosterone to estrogen

• Bisphenol A (BPA): Plastic additive, mimics estrogen

• PFAS: Nonstick chemicals, disrupt hormone signaling

• Phthalates: Found in cosmetics, toys; mimic or block sex hormones

Socioeconomic Health Determinants

• Higher exposure to endocrine disruptors in certain populations (e.g., Black women have higher body burdens of phthalates and parabens due to product marketing and usage patterns)

Key Equations

• ATP yield from glucose: $1 ext{ glucose} ightarrow ext{~30 ATP}$

• Glycogenolysis: $ext{Glycogen} ightarrow ext{Glucose}$

• Gluconeogenesis: $ext{Non-carbohydrate precursors} ightarrow ext{Glucose}$

Summary Table: Major Endocrine Organs and Hormones

Organ	Main Hormones	Primary Functions
Adrenal gland	Catecholamines, corticosteroids	Stress response, metabolism
Pineal gland	Melatonin	Regulates circadian rhythms
Pancreas	Insulin, glucagon	Glucose homeostasis
Gonads	Testosterone, estrogen, progesterone, inhibin	Reproduction, secondary sex characteristics
Placenta	Estrogen, progesterone, hCG, lactogen	Pregnancy support
Adipose tissue	Leptin, resistin, adiponectin	Energy balance, insulin sensitivity
Heart	ANP, BNP	Blood pressure regulation
Kidneys	Erythropoietin, renin	RBC production, blood pressure
Skeleton	Osteocalcin	Insulin regulation, fat storage
Skin	Cholecalciferol	Vitamin D synthesis
Thymus	Thymosins, thymulin	T cell production

Additional info: Academic context and expanded explanations have been added to ensure completeness and clarity for exam preparation.