Endocrine System: Structure, Function, and Hormones

Endocrine System Overview

Introduction

The endocrine system is a network of glands that secrete hormones directly into the bloodstream to regulate various physiological processes. It plays a critical role in maintaining homeostasis, growth, metabolism, and reproduction. The following notes cover the anatomy, functions, and regulation of major endocrine glands and their hormones.

Hypothalamus and Pituitary Glands

Anatomy and Connection

• Hypothalamus: Located in the brain, links the nervous and endocrine systems. Connected to the pituitary gland via the infundibulum.

• Pituitary Gland (Hypophysis): Sits in the sella turcica of the sphenoid bone. Composed of two lobes:

  • Anterior lobe (Adenohypophysis): Endocrine tissue; divided into pars distalis, pars tuberalis, and pars intermedia.

  • Posterior lobe (Neurohypophysis): Nervous tissue; stores and releases hormones produced by the hypothalamus.

Relationship Between Hypothalamus and Pituitary

• Anterior Pituitary: Regulated by hypothalamic releasing (RH) and inhibiting (IH) hormones via the hypophyseal portal system.

• Posterior Pituitary: Stores and releases hypothalamic hormones in response to nerve impulses.

Anterior Pituitary Hormones

• Thyroid Stimulating Hormone (TSH): Stimulates thyroid hormone release.

• Adrenocorticotropic Hormone (ACTH): Stimulates adrenal cortex to secrete cortisols.

• Follicle Stimulating Hormone (FSH): Stimulates ovarian follicle development and sperm differentiation.

• Luteinizing Hormone (LH): Triggers ovulation and hormone secretion in ovaries; stimulates testosterone in testes.

• Prolactin (PRL): Stimulates milk production in mammary glands.

• Growth Hormone (GH): Regulates growth and metabolism.

• Melanocyte Stimulating Hormone (MSH): Stimulates melanin synthesis.

Growth Hormone Regulation and Disorders

• Regulation: GH secretion is controlled by GHRH and GHIH from the hypothalamus, responding to blood glucose levels.

• Disorders:

  • Dwarfism: Hyposecretion in childhood.

  • Giantism: Hypersecretion in childhood.

  • Acromegaly: Hypersecretion in adulthood.

  • Diabetogenic Effect: Hypersecretion leads to increased blood sugar, mimicking diabetes mellitus.

Posterior Pituitary Hormones

• Antidiuretic Hormone (ADH, Vasopressin): Promotes water reabsorption in kidneys, increases blood volume and pressure.

• Oxytocin (OT): Stimulates milk release and uterine contractions; regulated by positive feedback.

• Clinical Note: Diabetes insipidus results from ADH hyposecretion, causing excessive urination and dehydration.

Thyroid Gland

Anatomy and Hormones

• Location: Anterior to trachea, inferior to larynx; two lobes connected by isthmus.

• Follicles: Store hormones; rich blood supply.

• Hormones:

  • T3 (Triiodothyroxine): Tyrosine with 3 iodines.

  • T4 (Thyroxine): Tyrosine with 4 iodines.

  • Calcitonin (CT): Regulates blood calcium.

Functions and Regulation

• T3 and T4: Regulate metabolism, growth, development, cardiovascular and GI activity.

• Regulation: Controlled by TRH from hypothalamus and TSH from anterior pituitary.

Clinical Disorders

• Cretinism: Hyposecretion in infants; physical and mental retardation.

• Myxedema: Hyposecretion in adults; low metabolism, lethargy.

• Graves Disease: Hypersecretion; high metabolism, bulging eyes.

Calcitonin (CT)

• Produced by: Parafollicular (C) cells.

• Function: Lowers blood calcium by inhibiting osteoclasts and increasing renal excretion.

Parathyroid Glands

Anatomy and Hormone

• Location: Four glands on posterior thyroid surface.

• Hormone: Parathyroid Hormone (PTH) produced by chief cells.

Functions of PTH

• Stimulates osteoclasts, inhibits osteoblasts (increases blood calcium).

• Enhances renal reabsorption of calcium.

• Stimulates calcitriol formation in kidneys.

Adrenal Glands

Anatomy

• Location: Superior border of each kidney.

• Regions: Outer cortex (three zones) and inner medulla.

Adrenal Cortex

• Zona Glomerulosa: Mineralocorticoids (e.g., aldosterone); regulate sodium, water, potassium.

• Zona Fasciculata: Glucocorticoids (e.g., cortisol); increase blood glucose, protein/lipid breakdown, anti-inflammatory.

• Zona Reticularis: Androgens; main source in females.

Clinical Disorders

• Hyperaldosteronism: Causes hypertension.

• Addison's Disease: Hyposecretion; hypoglycemia, low BP, dehydration.

• Cushing's Disease: Hypersecretion; fat redistribution, moon face.

• Hirsutism: Excess androgens; excessive hair growth.

Adrenal Medulla

• Secretes epinephrine and norepinephrine.

• Activated by sympathetic nerves; mediates "fight or flight" response.

• Effects: Increased heart rate, BP, breathing, blood sugar, metabolism, muscle contraction.

Pancreas

Anatomy and Function

• Location: Inferior and posterior to stomach; consists of head, body, tail.

• Exocrine: 99% of tissue; produces digestive enzymes.

• Endocrine: 1% of tissue; Islets of Langerhans produce hormones.

Pancreatic Islet Cells and Hormones

Cell Type	Hormone	Function
Alpha cells	Glucagon	Raises blood glucose by stimulating liver to release glucose
Beta cells	Insulin	Lowers blood glucose by promoting uptake and glycogen formation
Delta cells	Somatostatin (GHIH-like)	Regulates insulin and glucagon secretion

Diabetes Mellitus

• Definition: High blood glucose overwhelms kidney reabsorption.

• Symptoms: Hyperglycemia, glucosuria, polyuria, poor circulation.

• Type I: Childhood onset; decreased beta cells, requires insulin shots.

• Type II: Adult onset; fewer insulin receptors, managed by diet, exercise, sometimes insulin.

• Hyperinsulinism: Insulin overdose; can cause coma and death.

Pineal Gland

Anatomy and Function

• Location: Posterior to thalamus in brain.

• Cells: Neuroglial and secretory cells.

• Hormone: Melatonin; regulated by light, affects gonadotropin secretion and circadian rhythms.

Ovaries and Testes

Functions and Regulation

• Primary function: Gamete formation (eggs and sperm).

• Secondary function: Hormone production.

• Ovaries: Estrogen, progesterone, relaxin, inhibin; support follicle maturation, female characteristics, pregnancy.

• Testes: Testosterone, inhibin; support sperm maturation and male characteristics.

• Regulation: Hypothalamus (GnRH) → anterior pituitary (FSH, LH) → gonads.

Thymus Gland

Anatomy and Function

• Location: Mediastinum, superior to heart; large in infants, atrophies in adults.

• Primary function: Maturation of T lymphocytes.

• Secondary function: Secretes thymosin to aid T cell maturation.

Other Endocrine Glands

Hormones and Functions

• Stomach: Gastrin; regulates gastric juice secretion.

• Small Intestine: Secretin, cholecystokinin; stimulate pancreas and gall bladder, inhibit stomach.

• Placenta: Progesterone, estrogen, HCG; support pregnancy.

• Kidneys: Erythropoietin; stimulates RBC production in bone marrow.

Key Terms and Concepts

• Tropic Hormones: Hormones that stimulate other endocrine glands (e.g., TSH, ACTH, FSH, LH).

• Positive Feedback: A regulatory mechanism where the response amplifies the stimulus (e.g., oxytocin during labor).

• Negative Feedback: A regulatory mechanism where the response reduces the stimulus (e.g., most hormone regulation).

Sample Equations

• Blood Glucose Regulation:

  • Insulin lowers blood glucose: $\text{Glucose}_{\text{blood}} \xrightarrow{\text{Insulin}} \text{Glucose}_{\text{cells}}$

  • Glucagon raises blood glucose: $\text{Glycogen}_{\text{liver}} \xrightarrow{\text{Glucagon}} \text{Glucose}_{\text{blood}}$

• Calcium Regulation:

  • Calcitonin: $\text{Ca}^{2+}_{\text{blood}} \xrightarrow{\text{Calcitonin}} \text{Ca}^{2+}_{\text{bone}}$

  • PTH: $\text{Ca}^{2+}_{\text{bone}} \xrightarrow{\text{PTH}} \text{Ca}^{2+}_{\text{blood}}$

Summary Table: Major Endocrine Glands and Hormones

Gland	Hormone(s)	Main Function
Hypothalamus	RH, IH	Regulate pituitary hormones
Pituitary (Anterior)	TSH, ACTH, FSH, LH, PRL, GH, MSH	Stimulate target glands, growth, lactation, pigmentation
Pituitary (Posterior)	ADH, OT	Water balance, uterine/mammary contraction
Thyroid	T3, T4, Calcitonin	Metabolism, calcium regulation
Parathyroid	PTH	Increase blood calcium
Adrenal Cortex	Aldosterone, Cortisol, Androgens	Electrolyte balance, stress response, sex characteristics
Adrenal Medulla	Epinephrine, Norepinephrine	Fight or flight response
Pancreas	Insulin, Glucagon, Somatostatin	Blood glucose regulation
Pineal	Melatonin	Circadian rhythm
Ovaries	Estrogen, Progesterone, Inhibin, Relaxin	Female reproduction
Testes	Testosterone, Inhibin	Male reproduction
Thymus	Thymosin	T cell maturation
Other	Gastrin, Secretin, Erythropoietin, HCG	Digestion, RBC production, pregnancy

Additional info: Some explanations and clinical details were expanded for clarity and completeness.