Endocrine System: Hormones, Glands, and Regulation

Endocrine System Overview

Introduction to the Endocrine System

The endocrine system is a network of glands that secrete hormones directly into the bloodstream to regulate various bodily functions. Hormones are chemical messengers that influence growth, metabolism, reproduction, and homeostasis. The system includes major glands such as the pituitary, thyroid, parathyroid, adrenal, pineal, thymus, pancreas, ovaries, and testes.

• Hormones are classified as amino acid-based, steroids, or prostaglandins.

• Hormones act on target organs to regulate physiological processes.

• Most hormone release is controlled by negative feedback mechanisms.

Hypothalamus and Pituitary Gland

Hypothalamus-Pituitary Axis

The hypothalamus connects to the pituitary gland via the infundibulum. It regulates the release of hormones from both the anterior and posterior pituitary lobes.

• Posterior Pituitary stores and releases hormones produced by the hypothalamus.

• Anterior Pituitary synthesizes and secretes its own hormones under hypothalamic control.

Posterior Pituitary Hormones

• Oxytocin

  • Targets: Uterus and breasts

  • Stimulates uterine contractions during labor (triggered by cervical stretch)

  • Promotes milk ejection in response to suckling or breast pumping

• Antidiuretic Hormone (ADH)

  • Target: Kidneys

  • Stimulated by decreased blood volume or increased blood solute concentration (dehydration)

  • Promotes water reabsorption, reducing urine output and increasing hydration

Anterior Pituitary Hormones

• Growth Hormone (GH)

  • Stimulates growth in muscles, skeleton, and liver

  • Regulated by GH-releasing and GH-inhibiting hormones

• Prolactin (PRL)

  • Stimulates milk production in females

  • Increased by suckling; decreased by dopamine (prolactin-inhibiting hormone)

• Luteinizing Hormone (LH)

  • Females: Triggers ovulation, stimulates estrogen and progesterone release, and follicle maturation

  • Males: Stimulates testosterone production

  • Both sexes: Initiates puberty

• Follicle Stimulating Hormone (FSH)

  • Females: Stimulates egg production

  • Males: Stimulates sperm production

• Thyroid Stimulating Hormone (TSH)

  • Stimulates thyroid gland activity

  • Regulated by thyrotropin-releasing hormone and growth hormone-inhibiting hormone

• Adrenocorticotropic Hormone (ACTH)

  • Stimulates adrenal cortex to release corticosteroids

  • Regulated by corticotropin-releasing hormone (CRH)

Thyroid and Parathyroid Glands

Thyroid Gland

• Produces T3 (triiodothyronine) and T4 (thyroxine)

• Regulates metabolism, growth, and development

• Disorders:

  • Graves Disease: Hyperthyroidism (hot, energetic, underweight)

  • Hashimoto's Disease: Hypothyroidism (cold, overweight, hair loss, dry skin)

• Calcitonin: Lowers blood calcium by inhibiting osteoclast activity; antagonistic to parathyroid hormone

Parathyroid Gland

• Secretes Parathyroid Hormone (PTH) to regulate calcium homeostasis

• Actions:

  • Stimulates osteoclasts to break down bone and release calcium

  • Increases vitamin D3 and phosphate reabsorption in kidneys

  • Enhances calcium absorption in intestines

• Disorders:

  • Hyperparathyroidism: Excess bone breakdown, soft/deformed bones, kidney stones

  • Hypoparathyroidism: Low calcium, tetany, spasms, seizures, possible respiratory paralysis

Adrenal Glands

Adrenal Cortex

• Produces corticosteroids (e.g., cortisol, aldosterone)

• Regulates kidney function, energy metabolism, body hair, and libido

Adrenal Medulla

• Secretes epinephrine and norepinephrine

• Mimics sympathetic "fight or flight" response

Pineal and Thymus Glands

Pineal Gland

• Located near the roof of the third ventricle

• Secretes melatonin to regulate sleep-wake cycles and sexual maturity

• Assists with detoxification via antioxidants

Thymus Gland

• Located in the chest; larger in children, shrinks with age

• Produces thymulin, thymopoietin, and thymosins

• Essential for normal development of T lymphocytes (immune response)

Pancreas

Dual Function: Exocrine and Endocrine

• Exocrine: Secretes digestive enzymes via acinar cells

• Endocrine: Pancreatic islets contain:

  • Alpha cells: Secrete glucagon (raises blood sugar)

  • Beta cells: Secrete insulin (lowers blood sugar)

• Disorders:

  • Type 1 Diabetes: Insulin dependent (hypoinsulinism)

  • Type 2 Diabetes: Insulin resistant (hyperinsulinism)

Gonads and Placenta

Ovaries

• Produce estrogen and progesterone

• Regulate breast development, menstrual cycle, and secondary sex characteristics

• Estrogen matures reproductive organs

Placenta

• Temporary organ during pregnancy

• Secretes estrogen, progesterone, and human chorionic gonadotropin (HCG)

• HCG is detected by pregnancy tests

Testes

• Produce testosterone

• Responsible for male reproductive maturation, secondary sex characteristics, libido, and sperm production

Hormone Chemistry and Mechanisms

Classification of Hormones

• Amino acid-based hormones: Proteins, peptides, amines

• Steroid hormones: Derived from cholesterol

• Prostaglandins: Locally acting lipid-based hormones

Hormone Action

• Steroid hormones diffuse through plasma membranes to act on target cells

• Hormones can regulate the release of other hormones (hormonal control)

• Humoral control involves changes in body fluids (e.g., blood, bile) affecting hormone release

Anterior Pituitary Hormone Properties

• All are proteins

• Act through second messenger systems

• Regulated by hormonal stimuli and negative feedback

Table: Major Endocrine Glands and Their Hormones

Gland	Hormone(s)	Main Function(s)
Hypothalamus	Releasing & Inhibiting Hormones	Regulate pituitary hormone release
Posterior Pituitary	Oxytocin, ADH	Labor, milk ejection, water balance
Anterior Pituitary	GH, PRL, LH, FSH, TSH, ACTH	Growth, lactation, reproduction, thyroid & adrenal regulation
Thyroid	T3, T4, Calcitonin	Metabolism, calcium homeostasis
Parathyroid	PTH	Calcium homeostasis
Adrenal Cortex	Corticosteroids	Metabolism, stress response
Adrenal Medulla	Epinephrine, Norepinephrine	Fight or flight response
Pineal	Melatonin	Sleep-wake cycles
Thymus	Thymosins	T cell development
Pancreas	Insulin, Glucagon	Blood glucose regulation
Ovaries	Estrogen, Progesterone	Female reproduction
Testes	Testosterone	Male reproduction
Placenta	Estrogen, Progesterone, HCG	Pregnancy maintenance

Key Equations and Concepts

• Negative Feedback Example (Blood Glucose):

$\text{If blood glucose increases} \rightarrow \text{Insulin released} \rightarrow \text{Blood glucose decreases}$

$\text{If blood glucose decreases} \rightarrow \text{Glucagon released} \rightarrow \text{Blood glucose increases}$

• Calcium Homeostasis:

$\text{High blood calcium} \rightarrow \text{Calcitonin released} \rightarrow \text{Calcium deposited in bones}$

$\text{Low blood calcium} \rightarrow \text{PTH released} \rightarrow \text{Calcium released from bones}$

Summary

• The endocrine system is essential for regulating metabolism, growth, reproduction, and homeostasis through hormone secretion.

• Major glands include the pituitary, thyroid, parathyroid, adrenal, pineal, thymus, pancreas, ovaries, testes, and placenta.

• Hormones are classified by chemical structure and mechanism of action.

• Disorders can result from hypo- or hypersecretion of hormones, affecting multiple body systems.