BIO 254 - Week 8 - Endocrine System (2)

General Overview of the Endocrine System

• The endocrine system, in conjunction with the nervous system, regulates body functions to maintain homeostasis and coordinates communication across the body.

• Unique features:

  • Organs of the endocrine system are not anatomically connected.

  • Major endocrine glands include:

    • Pituitary gland

    • Thyroid gland

    • Parathyroid gland

    • Adrenal glands

    • Pancreas

    • Pineal gland

    • Thymus

    • Reproductive glands (ovaries and testes)

Types of Glands

• Endocrine glands:

  • Makeup of endocrine system, consist of cells, tissues, and organs.

  • Ductless, secrete hormones directly into bodily fluids (blood).

  • Hormones act only on target cells with specific receptors.

• Exocrine glands:

  • Glands that secrete into ducts or tubes that lead to body surfaces, deliver products externally.

  • Secrete local hormones (not true hormones):

    • Paracrine secretions: affect nearby cells.

    • Autocrine secretions: affect the cells that secrete them.

Comparison of Nervous and Endocrine Systems

• Both systems communicate through chemical signals but differ in mechanisms and response time:

  • Nervous System:

    • Releases neurotransmitters into synapses.

    • Responds quickly (milliseconds).

    • Effects are brief unless neuronal activity continues.

  • Endocrine System:

    • Secretes hormones into the bloodstream.

    • Responses can be slower (seconds to hours) and effects can be longer-lasting.

Hormones: Names and Functions

Hormone Abbreviations

• Hypothalamus:

  • Corticotropin-releasing hormone (CRH)

  • Gonadotropin-releasing hormone (GnRH)

  • Growth hormone-releasing hormone (GHRH)

  • Thyrotropin-releasing hormone (TRH)

• Anterior Pituitary:

  • Adrenocorticotropic hormone (ACTH)

  • Follicle-stimulating hormone (FSH)

  • Luteinizing hormone (LH)

  • Thyroid-stimulating hormone (TSH)

  • Prolactin (PRL)

• Posterior Pituitary:

  • Antidiuretic hormone (ADH or vasopressin)

  • Oxytocin (OT)

Hormone Actions

• Hormones released into extracellular fluid diffuse into blood, affecting target cells.

• Transport method depends on lipid or water solubility.

• Hormones function effectively in low concentrations; receptor number determines response strength:

  • Upregulation: Increase in receptor numbers in response to lower hormone levels.

  • Downregulation: Decrease in receptor numbers due to higher hormone levels.

Steroid vs Nonsteroid Hormones

Steroid Hormones

• Lipid-based, derived from cholesterol.

• Examples: Testosterone, estrogens, cortisol, aldosterone.

• Bind to receptors inside cell, triggering gene transcription and protein synthesis.

Nonsteroid Hormones

• Derived from amino acids; unable to penetrate cell membranes.

• Bind to cell surface receptors, initiating a signaling cascade often involving second messengers like cyclic AMP (cAMP).

  • Process from hormone binding to cellular changes is called signal transduction.

Hormonal Regulation

• Hormone secretion is controlled primarily by negative feedback mechanisms, ensuring homeostasis:

  • Increased hormone levels lead to decreased secretion.

  • Mechanisms include hypothalamic control and changes in blood composition.

• Positive feedback: Rare; increases secretion, primarily in reproductive scenarios.

Pituitary Gland

• Master gland situated at the base of the brain, controlled by the hypothalamus through:

  • Hypophyseal Portal Veins (anterior regulation).

  • Nerve impulses (posterior regulation).

• Anterior Lobe Hormones:

  • Produce hormones affecting growth, metabolism, and reproductive processes.

• Posterior Lobe Hormones:

  • Includes ADH (reduces urine output) and Oxytocin (stimulates contractions and milk ejection).

Thyroid Gland

• Positioned below the larynx and involved in metabolism.

• Produces T4 (Thyroxine), T3 (Triiodothyronine), and Calcitonin.

• Regulation via Thyroid-Stimulating Hormone (TSH) from the anterior pituitary.

Disorders of Thyroid and Parathyroid Glands

Hyperthyroidism

• Increased metabolic rate, restlessness, weight loss; often caused by Graves' disease.

Hypothyroidism

• Low metabolic rate, sluggishness, stunted growth in infants (Hashimoto's disease leading cause).

• Hypoparathyroidism: Resulting in low calcium levels and muscle spasms.

Pancreas and Diabetes Mellitus

• Functions as both endocrine (hormones) and exocrine (digestive enzymes) organ.

• Endocrine hormones:

  • Glucagon: Raises blood glucose levels.

  • Insulin: Lowers blood glucose levels.

  • Somatostatin: Regulates glucose and insulin secretion.

• Diabetes Mellitus:

  • Type 1: Autoimmune destruction of insulin-producing cells.

  • Type 2: Insulin insensitivity in body cells. Treatment varies based on type.

Additional Endocrine Glands

• Pineal Gland: Produces melatonin, regulates circadian rhythms.

• Thymus Gland: Releases thymosins for immune function.

• Other organs (heart, kidneys) produce hormones influencing blood pressure, red blood cell formation, and digestion.

Life Span Changes

• Endocrine glands generally decrease in size and hormone levels with age, affecting metabolism and overall health risk factors (e.g., osteoporosis).