Chapter 9: The Endocrine System – Essentials of Human Anatomy & Physiology

The Endocrine System

Overview and Functions

The endocrine system is the body's second major controlling system, complementing the rapid responses of the nervous system. It utilizes chemical messengers called hormones that are released into the bloodstream to regulate various physiological processes.

• Hormones control key processes such as:

  • Reproduction

  • Growth and development

  • Mobilization of body defenses

  • Maintenance of homeostasis

  • Regulation of metabolism

• Hormones are produced by specialized cells and secreted into extracellular fluids.

• Blood transports hormones to target sites, where they regulate the activity of other cells.

• Endocrinology is the scientific study of hormones and endocrine organs.

The Chemistry of Hormones

Classification of Hormones

Hormones are classified based on their chemical structure, which determines their mechanism of action and solubility.

• Amino acid-based hormones:

  • Proteins

  • Peptides

  • Amines

• Steroid hormones: Derived from cholesterol.

• Prostaglandins: Made from highly active lipids that act as local hormones.

Hormone Action

Target Cells and Specificity

Hormones affect only specific tissues or organs known as target cells or target organs. Target cells must possess specific protein receptors for the hormone to bind and exert its effect.

• Hormone binding alters cellular activity.

• Specificity of hormone-receptor interaction is analogous to the enzyme-substrate relationship.

Cellular Effects of Hormones

Hormones can arouse cells or alter their activity in several ways:

1. Change plasma membrane permeability or membrane potential by opening or closing ion channels.

2. Activate or inactivate enzymes.

3. Stimulate or inhibit cell division.

4. Promote or inhibit secretion of a product.

5. Turn on or off transcription of certain genes.

Mechanisms of Hormone Action

Hormones act via two primary mechanisms:

1. Direct gene activation (used by steroid and thyroid hormones)

2. Second-messenger system (used by protein and peptide hormones)

Direct Gene Activation

• Steroid hormone diffuses through the plasma membrane of target cells.

• Once inside, the hormone enters the nucleus and binds to a specific protein.

• The hormone-receptor complex binds to specific sites on the cell's DNA.

• Certain genes are activated to transcribe messenger RNA (mRNA).

• New proteins are synthesized, leading to the cellular response.

Second-Messenger System

• Hormone (first messenger) binds to a receptor protein on the plasma membrane.

• The activated receptor initiates a cascade of reactions that activates an enzyme.

• The enzyme catalyzes the formation of a second-messenger molecule (e.g., cyclic AMP, cAMP).

• The second messenger oversees additional intracellular changes to promote a specific response in the target cell.

Example equation:

• Formation of cAMP: $\text{ATP} \xrightarrow{\text{Adenylate cyclase}} \text{cAMP} + \text{PP}_i$

Stimuli for Control of Hormone Release

Feedback Mechanisms

Hormone levels in the blood are primarily regulated by negative feedback mechanisms:

• A stimulus or low hormone levels trigger the release of more hormone.

• Hormone release stops once an appropriate level is reached.

Categories of Stimuli

Endocrine glands are activated by three major types of stimuli:

1. Hormonal stimuli: Endocrine organs are activated by other hormones. Example: Hormones of the hypothalamus stimulate the anterior pituitary to secrete its hormones.

2. Humoral stimuli: Changing blood levels of certain ions and nutrients stimulate hormone release. Examples: Parathyroid hormone and calcitonin are produced in response to changing levels of blood calcium; insulin is produced in response to changing levels of blood glucose.

3. Neural stimuli: Nerve fibers stimulate hormone release, often under the control of the sympathetic nervous system. Example: Sympathetic stimulation of the adrenal medulla to release epinephrine and norepinephrine.

The Major Endocrine Organs

List of Major Endocrine Organs

• Hypothalamus

• Pituitary gland

• Pineal gland

• Thyroid gland

• Parathyroid glands

• Thymus

• Adrenal glands

• Pancreas

• Gonads (testes and ovaries)

Endocrine vs. Mixed Glands

• Some glands have purely endocrine functions (anterior pituitary, thyroid, adrenals, parathyroids).

• Others are mixed glands with both endocrine and exocrine functions (pancreas, gonads).

• Endocrine glands are ductless; hormones are released directly into blood or lymph.

Pituitary Gland and Hypothalamus

Pituitary Gland

• Pea-sized gland hanging by a stalk from the hypothalamus, protected by the sella turcica of the sphenoid bone.

• Two functional lobes:

  • Anterior pituitary – glandular tissue

  • Posterior pituitary – nervous tissue

• Called the "master endocrine gland" due to its regulatory role over other endocrine glands.

Hypothalamus

• Produces releasing and inhibiting hormones that control the anterior pituitary via portal circulation.

• Makes two hormones: oxytocin and antidiuretic hormone (ADH), which are stored in the posterior pituitary and released in response to nerve impulses.

Posterior Pituitary

• Does not synthesize hormones; stores and releases oxytocin and ADH made by the hypothalamus.

• Oxytocin:

  • Released during childbirth and nursing.

  • Stimulates uterine contractions and milk ejection (let-down reflex).

• Antidiuretic hormone (ADH):

  • Inhibits urine production by promoting water reabsorption in the kidneys.

  • Decreases urine volume, increases blood volume and pressure.

  • In large amounts, causes vasoconstriction (hence also called vasopressin).

  • Alcohol inhibits ADH secretion.

Additional info:

• Anterior pituitary hormones include growth hormone, prolactin, follicle-stimulating hormone, luteinizing hormone, thyroid-stimulating hormone, and adrenocorticotropic hormone.