endocrine system

Endocrine System Overview

13.1 General Characteristics of the Endocrine System

  • Endocrine System Functions:

    • Works alongside the nervous system to regulate bodily functions.

    • Maintains homeostasis (balance) in the body.

    • Coordinates communication among various body systems.

  • Unique Characteristics:

    • Organs are not anatomically connected unlike other systems.

  • Major Endocrine Glands:

    • Pituitary Gland

    • Thyroid Gland

    • Parathyroid Glands

    • Adrenal Glands

    • Pancreas

    • Pineal Gland

    • Thymus

    • Ovaries and Testes (reproductive glands)

13.2 Types of Glands and Messengers

  • Endocrine Glands:

    • Composed of cells, tissues, and organs that secrete hormones directly into body fluids.

    • Ductless, meaning hormones enter directly into the bloodstream.

    • The term "Endocrine" means "internal secretion."

    • Hormones only affect target cells that possess specific receptors for those hormones.

  • Exocrine Glands:

    • Secrete substances through ducts to body surfaces or cavities.

    • Deliver products directly to specific sites.

  • Local Hormones:

    • Paracrine Secretions: Affect nearby cells.

    • Autocrine Secretions: Affect only the cells that secrete them.

Comparison of Nervous and Endocrine Systems

  • Similarities:

    • Both systems facilitate communication within the body.

    • Both use chemical signals that bind to receptor molecules.

  • Differences:

    • Nervous System:

    • Releases neurotransmitters into synapses.

    • Responds rapidly (less than 1 second).

    • Effects are typically brief unless neuronal activity continues.

    • Endocrine System:

    • Secretes hormones into the bloodstream.

    • Responses may take seconds to hours.

    • Effects can last days, even if secretion ceases.

13.3 Hormone Names and Abbreviations

Source

Name

Abbreviation

Synonym

Hypothalamus

Corticotropin-releasing hormone

CRH

Hypothalamus

Gonadotropin-releasing hormone

GnRH

Hypothalamus

Luteinizing hormone-releasing hormone

LHRH

Hypothalamus

Somatostatin

SS

Growth hormone release-inhibiting hormone (GHRIH)

Hypothalamus

Growth hormone-releasing hormone

GHRH

Hypothalamus

Prolactin release-inhibiting hormone

PIH

Hypothalamus

Thyrotropin-releasing hormone

TRH

Anterior pituitary gland

Adrenocorticotropic hormone

ACTH

Corticotropin

Anterior pituitary gland

Follicle-stimulating hormone

FSH

Follitropin

13.4 Hormone Action

  • General Characteristics:

    • Hormones are released into extracellular fluid and diffuse into the bloodstream.

    • Their transport varies based on whether they are lipid-soluble or water-soluble.

    • They are active in very low concentrations.

  • Types of Hormones:

    • Steroid Hormones:

    • Derived from cholesterol.

    • Examples: testosterone, estrogens, cortisol, aldosterone.

    • Nonsteroid Hormones:

    • Amines: Derived from amino acids (e.g., epinephrine).

    • Proteins: Composed of long chains of amino acids (e.g., growth hormone).

    • Peptides: Short chains of amino acids (e.g., ADH).

    • Glycoproteins: Proteins with carbohydrates (e.g., TSH).

13.5 Actions of Hormones

  • Hormones exert effects by altering metabolic processes:

    • Can change enzyme activity.

    • Can change the rate of membrane transport.

    • Messages are delivered by binding to their receptors on/in target cell.

  • Receptor Dynamics:

    • Number of receptors on target cells can alter responsiveness.

    • Upregulation: Increase in receptor numbers due to decreased hormone levels.

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

13.6 Steroid and Thyroid Hormones

  • Characteristics:

    • Poor water-solubility, requiring transport via plasma proteins.

    • Can diffuse through cell membranes.

  • Mechanism of Action:

    • Bind to receptors inside the cell, usually in the nucleus, leading to gene transcription.

  • Hormonal Action Sequence:

    1. Hormone secretion from the endocrine gland.

    2. Hormone travels in blood.

    3. Diffusion into target cells.

    4. Hormone binds to intracellular receptors.

    5. Complex binds to DNA to promote messenger RNA synthesis.

    6. Protein synthesis yields specific hormonal effects.

13.7 Nonsteroid Hormones

  • Bind to target cell receptors on their membranes.

  • Hormonal cascade includes:

    • Hormone as first messenger.

    • Second messenger like cyclic adenosine monophosphate (cAMP).

    • The process is known as signal transduction.

13.8 Control of Hormonal Secretions

  • Hormone Secretion Regulation:

    • Primarily controlled by negative feedback mechanisms.

    • Can last from minutes to days; some are excreted urinary post-effects.

  • Negative Feedback Details:

    • Rising hormone levels signal a decrease in secretion.

    • Can be influenced by:

      • Hypothalamus and nervous system.

      • Changes in blood composition (ions, glucose).

  • Positive Feedback:

    • Increases hormone secretion in response to rising hormone levels, tied mostly to reproductive systems.

13.9 Pituitary Gland

  • Location and Structure:

    • Positioned at the brain's base in the sella turcica of the sphenoid bone.

    • Comprises anterior lobe (adenohypophysis) and posterior lobe (neurohypophysis).

  • Hormone Secretion:

    • Anterior lobe through hypothalamic releasing and inhibiting hormones.

    • Posterior lobe via direct nerve impulses from the hypothalamus.

13.10 Anterior Pituitary Hormones

  • Major Hormones:

    • Growth Hormone (GH): Stimulates growth and metabolism.

    • Prolactin (PRL): Stimulates milk production.

    • Thyroid-stimulating hormone (TSH): Regulates thyroid hormone release.

    • Adrenocorticotropic hormone (ACTH): Stimulates adrenal cortex hormone release.

    • Follicle-stimulating hormone (FSH): Regulates ovarian follicles/sperm production.

    • Luteinizing hormone (LH): Regulates sex hormone release and ovulation.

13.11 Thyroid Gland

  • Structure and Location:

    • Located below the larynx, consisting of two lobes connected by an isthmus.

    • Contains Follicular and Extrafollicular cells.

  • Hormones Produced:

    • Thyroxine (T4) and Triiodothyronine (T3) for metabolism.

    • Calcitonin: Manages calcium levels in blood.

13.12 Parathyroid Glands

  • Location: Found on the posterior surface of the thyroid gland.

  • Function: To secrete Parathyroid Hormone (PTH) that regulates calcium and phosphate levels in the blood.

13.13 Adrenal Glands

  • Structure: Comprised of an outer cortex and inner medulla.

  • Hormones: Cortex secretes steroid hormones (aldosterone, cortisol) while medulla secretes amine hormones (epinephrine, norepinephrine).

13.14 Pancreas

  • Structure: Elongated organ behind the stomach, having both endocrine (hormones) and exocrine (digestive juices) functions.

  • Key Hormones:

    • Insulin: Decreases blood glucose.

    • Glucagon: Increases blood glucose.

    • Somatostatin: Regulates other hormone levels.

13.15 Pineal and Thymus Glands

  • Pineal Gland: Regulates circadian rhythms by secreting melatonin.

  • Thymus Gland: Promotes T-lymphocyte development for immune response via thymosins.

13.16 Stress Response

  • Hypothalamic Control: Regulates reaction to stress through the General Adaptation Syndrome (GAS).

    • Stages: Alarm Stage (fight-or-flight) and Resistance Stage (long-term alteration responses).

    • Exhaustion Stage: Results from prolonged resistance leading to nutrient depletion and immune system suppression.