Major Endocrine Glands and Their Locations

Overview

The endocrine system is composed of glands that secrete hormones directly into the bloodstream to regulate various physiological processes. The major endocrine glands are distributed throughout the body and play critical roles in growth, metabolism, stress response, and reproduction.

• Pineal gland: Located in the brain; regulates circadian rhythms.

• Hypothalamus: Brain region controlling the pituitary gland; integrates neural and endocrine functions.

• Pituitary gland: Below the hypothalamus; the "master gland" controlling other endocrine glands.

• Thyroid gland: In the neck; regulates metabolism.

• Parathyroid glands: On the posterior aspect of the thyroid; regulate blood calcium.

• Thymus: In the upper chest; involved in immune system development (more active in children).

• Adrenal glands: On top of each kidney; involved in stress response and metabolism.

• Pancreas: Behind the stomach; regulates blood glucose.

• Gonads: Ovaries (female) and testes (male); produce sex hormones.

• Kidneys: Produce hormones involved in blood pressure and red blood cell production.

• Heart: Produces hormones that regulate blood pressure and volume.

Hormones and Functions by Gland

Hypothalamus

• Hormones: TRH (thyrotropin-releasing hormone), CRH (corticotropin-releasing hormone), GnRH (gonadotropin-releasing hormone), GHRH (growth hormone-releasing hormone), and others.

• Function: Regulates anterior pituitary hormone release; integrates neural and endocrine signals.

• Stimulus: Neural input from the brain (stress, homeostatic changes) and hormone levels in the blood.

• If absent/dysfunctional: Pituitary gland fails to release its hormones, leading to breakdown of multiple systems (growth, thyroid, adrenal, reproductive).

Anterior Pituitary Gland

• Growth Hormone (GH): Stimulates growth of bones/muscles; increases protein synthesis.

• Prolactin (PRL): Promotes milk production.

• TSH (Thyroid-Stimulating Hormone): Stimulates thyroid to release T3/T4.

• ACTH (Adrenocorticotropic Hormone): Stimulates adrenal cortex to release cortisol.

• FSH (Follicle-Stimulating Hormone): Stimulates gamete (egg/sperm) production.

• LH (Luteinizing Hormone): Stimulates sex hormone secretion and ovulation.

• Stimulus: Hypothalamic releasing hormones.

• If absent/dysfunctional: Growth failure, infertility, low metabolism, poor stress response.

Posterior Pituitary Gland

• ADH (Antidiuretic Hormone): Increases kidney water reabsorption, conserves water.

• Oxytocin: Stimulates uterine contractions and milk ejection; bonding hormone.

• Stimulus: Neural signals (hypothalamic neurons firing).

• If absent/dysfunctional: Low ADH – Diabetes insipidus (excessive urination, dehydration). Low oxytocin – Impaired labor or milk release.

Thyroid Gland

• Hormones: T3, T4 (thyroid hormones), Calcitonin.

• Function: T3/T4 increase metabolic rate, growth, and energy use. Calcitonin lowers blood calcium.

• Stimulus: TSH from pituitary for T3/T4; high blood Ca2+ for calcitonin.

• If absent/dysfunctional: Low T3/T4 – hypothyroidism (weight gain, low energy). High T3/T4 – hyperthyroidism (weight loss, high energy).

Parathyroid Glands

• Hormone: Parathyroid hormone (PTH).

• Function: Raises blood calcium (stimulates osteoclasts, kidney Ca2+ reabsorption, activates vitamin D).

• Stimulus: Low blood Ca2+.

• If absent/dysfunctional: Low PTH – hypocalcemia (muscle spasms, seizures). High PTH – bone loss, kidney stones.

Adrenal Cortex

• Hormones: Aldosterone (mineralocorticoid), Cortisol (glucocorticoid), Androgens.

• Function: Aldosterone increases Na+ and water retention, raises BP. Cortisol regulates metabolism, stress adaptation, immune system.

• Stimulus: Aldosterone – low Na+, low BP, Angiotensin II. Cortisol – ACTH from anterior pituitary.

• If absent/dysfunctional: Low aldosterone/cortisol – Addison's disease (fatigue, low BP). Excess cortisol – Cushing's syndrome (obesity, high BP).

Adrenal Medulla

• Hormones: Epinephrine (adrenaline), Norepinephrine.

• Function: Increases heart rate, BP, blood glucose – "fight or flight" response.

• Stimulus: Neural (sympathetic nervous system).

• If absent/dysfunctional: Slower stress response, low BP, poor glucose mobilization.

Pancreas

• Hormones: Glucagon (α-cells), Insulin (β-cells).

• Function: Glucagon raises blood glucose. Insulin lowers blood glucose.

• Stimulus: Humoral – blood glucose levels.

• If absent/dysfunctional: Low insulin – Diabetes Mellitus. Low glucagon – hypoglycemia.

Ovaries & Placenta

• Hormones: Estrogen, Progesterone, Relaxin, hCG (placenta only).

• Function: Estrogen – female sexual traits, menstrual regulation. Progesterone – maintains pregnancy. Relaxin – softens pelvis. hCG – maintains corpus luteum early in pregnancy.

• Stimulus: Hormonal (FSH, LH, or fertilization/pregnancy).

Testes

• Hormone: Testosterone.

• Function: Male sexual traits, sperm production.

• Stimulus: Hormonal (LH from pituitary).

Thymus

• Hormone: Thymosin.

• Function: Matures T-lymphocytes for immune defense.

• Stimulus: Developmental and immune status.

• If absent/dysfunctional: Weakened immune function.

Kidneys

• Hormones: Erythropoietin (EPO), Renin, Calcitriol (active vitamin D).

• Function: EPO stimulates RBC production. Renin triggers RAAS to increase BP. Calcitriol promotes calcium absorption in intestines.

• Stimulus: EPO – low oxygen. Renin – low BP/Na+. Calcitriol – low Ca2+ or PTH stimulation.

• If absent/dysfunctional: Anemia, low BP, poor calcium absorption.

Heart

• Hormone: Atrial Natriuretic Peptide (ANP).

• Function: Decreases blood pressure by promoting sodium and water excretion.

• Stimulus: Stretching of atrial walls (high BP).

• If absent/dysfunctional: Persistent high BP, fluid retention.

Pineal Gland

• Hormone: Melatonin.

• Function: Regulates circadian rhythm and sleep cycle.

• Stimulus: Darkness (low light levels).

• If absent/dysfunctional: Sleep disturbances, Seasonal Affective Disorder (SAD) due to excess melatonin in winter.

Similarities and Differences: Endocrine vs. Nervous Systems

Similarities

• Both control and coordinate body functions (growth, metabolism, stress, reproduction).

• Both use chemical messengers (hormones for endocrine, neurotransmitters for nervous).

• Both systems can be interlinked (e.g., hypothalamus connects nervous and endocrine systems).

Differences

Hormone Transport and Elimination

Where Hormones Are Found in Whole Blood

• Hormones circulate in the plasma (liquid component of blood).

• Water-soluble hormones (e.g., insulin, epinephrine, most peptides): travel freely dissolved in plasma.

• Lipid-soluble hormones (e.g., steroid hormones, thyroid hormones): must bind to plasma transport proteins for solubility.

Elimination of Hormones from Bloodstream

• Hormones are inactivated and removed by:

  • Enzymatic degradation: Broken down by enzymes in the liver, kidneys, or target tissues.

  • Excretion: Inactive metabolites are filtered out by the kidneys (urine) or excreted in bile.

Summary

The endocrine system is essential for long-term regulation of physiological processes. It works in concert with the nervous system to maintain homeostasis, using hormones as chemical messengers to coordinate the activities of various organs and tissues throughout the body.