The Endocrine System

Overview of the Endocrine System

The endocrine system is a major regulatory system in the human body, responsible for maintaining homeostasis through the release of hormones. It coordinates long-term processes such as growth, development, and reproduction by using chemical messengers to relay information between cells.

• Homeostasis is preserved through intercellular communication.

• Regulates long-term processes:

  • Growth

  • Development

  • Reproduction

• Uses hormones as chemical messengers.

Endocrine Communication

• Endocrine cells release hormones into the bloodstream.

• Hormones affect the metabolic activity of many tissues and organs simultaneously.

• Allows for broad and long-lasting regulation of physiological processes.

Target Cells and Hormones

Hormones only affect specific cells that possess receptors needed to bind and read the hormonal message. These are called target cells.

• Hormones can:

  • Stimulate synthesis of enzymes or structural proteins

  • Increase or decrease the rate of synthesis

  • Turn existing enzyme or membrane channel 'on' or 'off'

The Structure of Hormones

Hormone Types

Hormones can be divided into three groups based on their chemical structure:

• Amino acid derivatives

• Peptide hormones

• Lipid derivatives

Hormones circulate freely or bound to transport proteins in the blood.

Mechanisms of Hormone Action

Hormone Receptor Interaction

• Hormones must bind to a specific receptor molecule to affect a target cell.

• Receptors may be located on the plasma membrane or inside the cell.

• The location of the receptor determines the mechanism of hormone action.

Hormones and Plasma Membrane Receptors

• Catecholamines and peptide hormones are not lipid soluble.

• They cannot penetrate the plasma membrane and must bind to receptors on the outer surface of the plasma membrane.

Second Messengers

• Second messengers are molecules that relay signals received at receptors on the cell surface to target molecules inside the cell.

• Common second messengers include:

  • Cyclic-AMP (cAMP)

  • Cyclic-GMP (cGMP)

  • Calcium ions (Ca2+)

Hormones and Intracellular Receptors

• Steroid and thyroid hormones are lipid soluble and can cross the plasma membrane to bind to intracellular receptors.

• They alter DNA transcription in the nucleus, changing patterns of protein synthesis and directly affecting metabolic activity and structure of target cells.

The Control of Endocrine Activity

Endocrine Reflexes

• Endocrine reflexes are controlled by different stimuli:

  • Humoral stimuli (changes in composition of extracellular fluid)

  • Hormonal stimuli (arrival or removal of a specific hormone)

  • Neural stimuli (arrival of neurotransmitters at neuroglandular junctions)

• Most endocrine reflexes are regulated by negative feedback mechanisms.

The Pituitary Gland

Structure and Function

• Also called the hypophysis, located within the sella turcica of the sphenoid bone.

• Connected to the hypothalamus by the infundibulum.

• Divided into:

  • Anterior lobe (adenohypophysis): produces and releases hormones.

  • Posterior lobe (neurohypophysis): releases hormones produced by the hypothalamus.

Hormones of the Anterior Pituitary

• Thyroid-stimulating hormone (TSH)

• Adrenocorticotropic hormone (ACTH)

• Follicle-stimulating hormone (FSH)

• Luteinizing hormone (LH)

• Prolactin (PRL)

• Growth hormone (GH)

• Melanocyte-stimulating hormone (MSH)

Hormones of the Posterior Pituitary

• Antidiuretic hormone (ADH): regulates water balance in the body.

• Oxytocin (OXT): stimulates labor contractions and milk ejection.

The Thyroid Gland

Structure and Hormones

• Located on the anterior surface of the trachea, inferior to the thyroid cartilage.

• Contains thyroid follicles that produce:

  • Thyroxine (T4)

  • Triiodothyronine (T3)

• Parafollicular (C) cells produce calcitonin (CT).

Thyroid-Stimulating Hormone (TSH)

• Produced by the anterior pituitary gland.

• Stimulates the thyroid gland to release thyroid hormones.

• Regulated by negative feedback.

Functions of Thyroid Hormones

• Increase energy utilization, oxygen consumption, growth, and development.

• Essential for normal development of skeletal, muscular, and nervous systems.

Calcitonin

• Produced by C (clear) cells of the thyroid gland.

• Helps regulate concentrations of Ca2+ in body fluids by inhibiting osteoclast activity and stimulating calcium excretion by the kidneys.

Parathyroid Glands

Parathyroid Hormone (PTH)

• Produced by parathyroid (chief) cells.

• Increases Ca2+ concentrations in body fluids by:

  • Stimulating osteoclasts

  • Enhancing reabsorption of Ca2+ by the kidneys

  • Stimulating formation and secretion of calcitriol by the kidneys

Adrenal (Suprarenal) Glands

Structure and Hormones

• Located on top of each kidney.

• Divided into:

  • Adrenal cortex: produces corticosteroids (mineralocorticoids, glucocorticoids, and androgens).

  • Adrenal medulla: produces catecholamines (epinephrine and norepinephrine).

Adrenal Cortex Hormones

Adrenal Medulla Hormones

• Produces epinephrine (adrenaline) and norepinephrine (noradrenaline).

• Responsible for the 'fight or flight' response.

Pineal Gland

• Located in the brain, secretes melatonin.

• Regulates circadian rhythms and reproductive timing.

Pancreas

Endocrine Function

• Contains clusters of endocrine cells called islets of Langerhans.

• Alpha cells produce glucagon (raises blood glucose).

• Beta cells produce insulin (lowers blood glucose).

• Hormone release is regulated by negative feedback.

Blood Glucose Regulation

Endocrine Tissues of Other Systems

• Kidneys: produce erythropoietin (EPO), calcitriol, and renin.

• Heart: produces natriuretic peptides (ANP and BNP) to reduce blood pressure.

• Thymus: produces thymosins for immune function.

• Gonads (ovaries and testes): produce sex hormones (estrogens, progesterone, and androgens).

• Adipose tissue: produces leptin (regulates appetite) and other hormones.

Hormone Interactions

• Antagonistic effects: hormones oppose each other's actions.

• Synergistic effects: hormones enhance each other's actions.

• Permissive effects: one hormone is necessary for another to produce its effect.

• Integrative effects: hormones produce different but complementary results.

Hormones and Growth

• Growth hormone (GH): stimulates growth and metabolism in most tissues.

• Thyroid hormones: essential for normal development, especially of the nervous system.

• Parathyroid hormone (PTH) and calcitriol: regulate calcium ion concentrations for bone growth.

• Reproductive hormones: androgens and estrogens stimulate cell growth and differentiation.

Hormones and Stress

General Adaptation Syndrome (GAS)

• Describes the body's response to stress-causing factors.

• Divided into three phases:

  • Alarm phase

  • Resistance phase

  • Exhaustion phase

Key Terms

• Endocrine system

• Hormone

• Target cell

• Second messenger

• Negative feedback

• Hypothalamus

• Pituitary gland

• Thyroid gland

• Adrenal gland

• Pancreas

• Gonads

• Melatonin

• Insulin

• Glucagon

• Calcitonin

• Parathyroid hormone