Human Anatomy and Physiology: The Endocrine System

Human Anatomy and Physiology

Overview of the Endocrine System

• Introduction

  • The endocrine system functions together with the nervous system to coordinate and integrate the activities of body cells.

  • General Function: Slow communication and control throughout the body.

  • It influences the metabolic activities of cells via hormones transported in the blood.

  • Responses are slower and have longer-lasting effects compared to nervous system responses.

  • Endocrinology is the study of hormones and endocrine organs.

Endocrine Glands

Major Endocrine Organs

• Key Endocrine Glands:

  • Pituitary Gland

  • Thyroid Gland

  • Parathyroid Glands

  • Adrenal Glands

  • Pineal Gland

  • Pancreas

  • Gonads (Testes and Ovaries)

• Hypothalamus:

  • Considered a neuroendocrine organ and is the master regulator of the endocrine system.

Functions of Endocrine Glands

• Endocrine glands secrete hormones directly into the bloodstream, contrasting with exocrine glands that secrete substances via ducts.

• Examples of glands that have both functions:

  • Pancreas: Functions as both an exocrine (digestion) and endocrine (hormonal control) organ.

  • Gonads:

  • Testes (male hormone production)

  • Ovaries (female hormone production)

  • Thymus: Another organ that produces hormones.

Steps in Endocrine System Functioning

1. Endocrine Gland Receives STIMULUS

2. Endocrine Gland Secretes HORMONE into BLOODSTREAM

3. Blood Transports Hormone to TARGET

4. Hormone Binds to RECEPTOR at Target

5. CHANGE IN TARGET

• Examples of Changes in Target Cells:

  • Alters plasma membrane permeability and/or membrane potential by opening or closing ion channels.

  • Stimulates synthesis of enzymes or other proteins.

  • Activates or deactivates enzymes.

  • Induces secretory activity.

  • Stimulates mitosis.

Types of Stimuli for Hormone Release

• Three Types of Stimuli:

  • Neural: Initiated by the nervous system.

  • Hormonal: Release triggered by other hormones.

  • Humoral: Triggered by blood levels of ions and nutrients.

Hormone Types and Their Structure

Water Soluble Hormones

• Characteristics:

  • They dissolve in plasma (the liquid part of blood), which is primarily composed of water.

  • Most hormones are protein derivatives and water-soluble.

Lipid (Fat) Soluble Hormones

• Characteristics:

  • They dissolve in fats and cannot dissolve in water; hence cannot dissolve in plasma.

• To be transported in plasma, lipid-soluble hormones must bind to plasma proteins produced by the liver.

Mechanisms of Hormone Action

Binding to Receptors

• Hormones specifically influence cells that have receptors for them.

• Receptors: Cellular proteins that bind to specific substances like hormones.

• Receptors for Hormones:

  • Lipid soluble hormone receptors are located intracellularly (inside cells).

  • Water soluble hormone receptors are located within plasma membranes of target cells.

Direct Gene Activation (Lipid Soluble Hormones)

• Upon reaching the target cell, a lipid-soluble steroid hormone will:

  1. Diffuse through the plasma membrane into the cell.

  2. The receptor-hormone complex then enters the nucleus and binds to specific DNA regions.

  3. This binding activates genes and initiates protein synthesis, thereby altering gene expression.

Second Messenger Systems (Water Soluble Hormones)

• Water soluble hormones cannot pass through plasma membranes.

• They bind with receptors in the plasma membrane, initiating a cascade of reactions known as a second messenger system.

• Example: Cyclic AMP (cAMP) is a common second messenger that carries the signal into the cell to cause a change at the target.

Regulation of Hormone Levels

Blood Levels of Hormones

• The amount of hormone in the blood can influence the number of receptors for that hormone.

• Up-regulation: Target cell increases receptor numbers in response to low hormone levels.

• Down-regulation: Target cell decreases receptor numbers in response to high hormone levels.

The Hypothalamus and Pituitary Gland

Hypothalamus Structure and Control

• The hypothalamus connects to the pituitary gland (hypophysis) via the infundibulum.

• The pituitary has two major lobes:

  • Posterior Pituitary (Neurohypophysis): Contains neural tissue and stores hormones produced by the hypothalamus (e.g., oxytocin, ADH).

  • Anterior Pituitary (Adenohypophysis): Composed of glandular tissue and regulated by hypothalamic hormones through a portal system.

Functions of Posterior Pituitary

• Oxytocin:

  • In females, stimulates uterine contractions during labor and childbirth and is also involved in milk ejection from mammary glands (positive feedback mechanism).

• Antidiuretic Hormone (ADH):

  • Related to blood osmotic pressure; secreted when solute concentration divides from normal levels.

  • ADH targets kidney tubules promoting water reabsorption to increase blood volume.

Relationship Between Hypothalamus and Anterior Pituitary

• Hormones from hypothalamus (releasing and inhibiting hormones) are released into the hypophyseal portal system to control anterior pituitary hormone secretion.

• Releasing Hormones Include:

  • Thyrotropin-releasing hormone (TRH)

  • Corticotropin-releasing hormone (CRH)

  • Gonadotropin-releasing hormone (GnRH)

  • Growth hormone-releasing hormone (GHRH)

• Inhibiting Hormones Include:

  • Prolactin-inhibiting hormone (PIH)

  • Growth hormone-inhibiting hormone (GHIH)

Anterior Pituitary Hormones

• All six hormones produced are peptide hormones (protein derivatives), with the following being tropic:

  • Thyroid-stimulating hormone (TSH)

  • Adrenocorticotropic hormone (ACTH)

  • Follicle-stimulating hormone (FSH)

  • Luteinizing hormone (LH)

• Other important hormones:

  • Prolactin (PRL)

  • Growth Hormone (GH)

Thyroid Gland

• Thyroid Hormones:

  • Thyroxine (T4) and Triiodothyronine (T3): Major metabolic hormones that target body cells to increase metabolism.

  • Both T4 and T3 are iodine-containing and lipid-soluble.

  • Their secretion is regulated by negative feedback; low blood levels stimulate TSH release while high levels inhibit TSH.

Calcium Homeostasis

Blood Calcium Ion Levels

• Normal levels maintained between 9 - 11 mg/100 ml due to:

  • Essential for muscle contraction, neuron impulse conduction, and blood coagulation.

• Calcitonin: Secreted in response to high Ca2+ levels, functioning as an antagonist to parathyroid hormone (PTH).

Parathyroid Glands

• Four to eight glands located on the posterior aspect of the thyroid, containing cells that secrete PTH.

• PTH is critical in maintaining Ca2+ homeostasis and is released in response to low blood calcium levels.

Adrenal Glands

• Structure: Paired pyramid-shaped organs positioned atop the kidneys, consisting of two main components:

  • Adrenal Cortex: Produces corticosteroids (over 24 types), categorized into three layers:

  • Zona glomerulosa (mineralocorticoids)

  • Zona fasciculata (glucocorticoids)

  • Zona reticularis (gonadocorticoids)

  • Adrenal Medulla: Part of the sympathetic nervous system, producing catecholamines (epinephrine and norepinephrine) that trigger fight-or-flight responses.

Pineal Gland

• A small gland hanging from the roof of the third ventricle that secretes melatonin.

• Secretion of melatonin increases during the evening, influencing sleep cycles and rhythmic physiological processes such as body temperature and appetite.

Pancreas

• A triangular gland located partially behind the stomach with both exocrine (digestive enzymes) and endocrine (hormonal) functions.

• Islets of Langerhans:

  • Alpha (α) Cells: Produce glucagon, which increases blood glucose levels.

  • Beta (β) Cells: Produce insulin, which decreases blood glucose levels.

Thymus

• A large organ in infants and children that shrinks with age, which produces thymulin, thymopoietins, and thymosins critical for T-lymphocyte (immune response) development.