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Endocrine System Study Notes

Endocrine System Overview

  • The endocrine system consists of various glands that release hormones directly into the bloodstream or interstitial fluid.

  • Interstitial Fluid

    • Definition: The fluid found between cells in the body, also known as extracellular fluid but specifically within the cellular environment.

    • Function: Hormones secreted from endocrine glands enter interstitial fluid before reaching the bloodstream.

  • The endocrine system closely works with the nervous system in coordinating body functions.

Key Functions of the Endocrine System

  • Main role: Control and regulate the body's activities to maintain homeostasis.

  • Additional functions include:

    • Regulation of fertility and sexual function/maturity.

    • Control of mood and feelings.

    • Influence on growth and development.

Major Endocrine Glands

  • Pituitary Gland: Located in the brain.

  • Thyroid Gland: Butterfly-shaped gland in the throat.

  • Pancreas: Situated below and slightly inferior to the stomach.

  • Adrenal Glands: Positioned on top of the kidneys.

  • Testes and Ovaries: Related to biological sex; produce sex hormones.

Comparison Between Endocrine and Nervous Systems

  • Nervous System:

    • Communication via action potentials (electrical impulses).

    • Responses are fast but short-lived and target-specific.

  • Endocrine System:

    • Communication through hormones that travel in the blood.

    • Responses are slower but can last for a long time, influencing multiple areas of the body.

Hormones

  • Definition: Chemical messengers released by endocrine organs, affecting the function of other organs.

  • Hormones must attach to specific receptors on target cells to elicit responses. If a receptor is absent, the hormone cannot exert its effects.

Types of Glands

  • Exocrine Glands:

    • Have ducts to transport their products (e.g., sweat glands).

    • Release substances onto surfaces directly.

  • Endocrine Glands:

    • No ducts; secrete hormones into interstitial fluid.

    • Hormones diffuse into the bloodstream and spread throughout the body.

Hormone Types

  • Circulating Hormones: Travel through the bloodstream to distant target sites.

  • Local Hormones: Act on neighboring cells (paracrine) or on the same cell that secreted them (autocrine).

Hormonal Transportation in Blood

  • Plasma: The liquid component of blood that carries hormones.

  • Hormones can be:

    • Lipid-Soluble: Requires carriers to move through the plasma (nonpolar).

    • Mechanism: Hormones diffuse across the plasma membrane and bind to intracellular receptors, affecting gene transcription.

    • Water-Soluble: No carriers needed as they travel in the plasma (polar).

    • Mechanism: Hormones bind to receptors on the cell surface (e.g., GPCRs), initiating intracellular signaling cascades.

Mechanisms of Hormone Action

  • Lipid-Soluble Hormones:

    • Examples: Steroid hormones like testosterone and cortisol.

    • Activate transcription by binding to receptors inside cells and influencing gene expression.

  • Water-Soluble Hormones:

    • Examples: Insulin and glucagon.

    • Bind to cell surface receptors; activate secondary messengers (e.g., cAMP) leading to cellular responses without entering the cell directly.

Hormonal Effects on Target Cells

  • Changes in cell membrane permeability.

  • Altered metabolic rates.

  • Contraction/dilation of smooth and cardiac muscles (involuntary).

Feedback Mechanisms in Hormone Regulation

  • Negative Feedback:

    • Common mechanism to maintain homeostasis (prevents extreme fluctuations; inhibits hormone production).

    • Example: Insulin lowers blood glucose, stopping further insulin secretion when blood sugar is normalized.

  • Positive Feedback:

    • Amplifies the initiating signal (less common in the body).

    • Example: During childbirth, the pressure of the baby's head triggering the release of oxytocin results in more contractions until birth occurs.

Specific Hormones and Their Functions

  • Insulin:

    • Origin: Pancreas.

    • Stimulus: High blood glucose levels.

    • Function: Decreases blood glucose by facilitating glucose uptake into cells.

    • Target: All body cells, especially liver and muscle cells.

  • Oxytocin:

    • Origin: Hypothalamus (secreted through the posterior pituitary).

    • Function: Stimulates uterine contractions and milk ejection during breastfeeding.

  • ADH (Antidiuretic Hormone):

    • Origin: Hypothalamus (secreted through the posterior pituitary).

    • Function: Promotes water reabsorption in kidneys; regulates blood pressure.

The Pituitary Gland

  • Master Gland: Controls many other endocrine glands.

  • Anterior Pituitary:

    • Hormones: FSH, LH, TSH, ACTH, HGH, Prolactin, MSH.

    • Functions: Stimulates growth, metabolic processes, and other endocrine glands.

  • Posterior Pituitary: Secretory site for oxytocin and ADH (does not produce its own hormones).

Importance of Hormone Timing

  • Hormonal secretion often fluctuates and is tightly regulated based on physiological needs and circadian rhythms (e.g., sleep increases HGH production).

  • Hormones Produced by the Anterior Pituitary Gland

    The anterior pituitary gland produces seven hormones, playing crucial roles in growth, metabolism, and the function of other endocrine glands:

    1. Follicle-Stimulating Hormone (FSH)

      • Origin: Anterior Pituitary Gland

      • Function: Stimulates ovarian follicle development in females and sperm production in males.

      • Stimulus: Gonadotropin-releasing hormone (GnRH) from the hypothalamus.

      • Target: Ovaries (follicle cells), Testes (Sertoli cells).

    2. Luteinizing Hormone (LH)

      • Origin: Anterior Pituitary Gland

      • Function: Triggers ovulation and corpus luteum formation in females; stimulates testosterone production in males.

      • Stimulus: Gonadotropin-releasing hormone (GnRH) from the hypothalamus.

      • Target: Ovaries (theca cells, corpus luteum), Testes (Leydig cells).

    3. Thyroid-Stimulating Hormone (TSH)

      • Origin: Anterior Pituitary Gland

      • Function: Stimulates the thyroid gland to produce and release thyroid hormones (T3 and T4).

      • Stimulus: Thyrotropin-releasing hormone (TRH) from the hypothalamus.

      • Target: Thyroid Gland.

    4. Adrenocorticotropic Hormone (ACTH)

      • Origin: Anterior Pituitary Gland

      • Function: Stimulates the adrenal cortex to produce and release glucocorticoids (e.g., cortisol).

      • Stimulus: Corticotropin-releasing hormone (CRH) from the hypothalamus, stress.

      • Target: Adrenal Cortex.

    5. Human Growth Hormone (HGH)

      • Origin: Anterior Pituitary Gland

      • Function: Promotes growth of body tissues (bones, muscles) and influences metabolic processes (e.g., protein synthesis, fat breakdown).

      • Stimulus: Growth hormone-releasing hormone (GHRH) from the hypothalamus; sleep, exercise, low blood glucose.

      • Target: Liver (which produces IGF-1), bones, muscles, and other tissues.

    6. Prolactin (PRL)

      • Origin: Anterior Pituitary Gland

      • Function: Stimulates milk production in mammary glands after childbirth.

      • Stimulus: Prolactin-releasing hormone (PRH) from the hypothalamus; suckling during breastfeeding.

      • Target: Mammary Glands.

    7. Melanocyte-Stimulating Hormone (MSH)

      • Origin: Anterior Pituitary Gland

      • Function: Influences skin pigmentation (less impactful in humans than in other vertebrates).

      • Stimulus: Also derived from POMC, can be influenced by CRH.

      • Target: Melanocytes in the skin.

    Hormones Secreted by the Posterior Pituitary Gland

    The posterior pituitary gland stores and secretes two hormones that are produced by the hypothalamus:

    1. Oxytocin

      • Origin: Hypothalamus (neurons in paraventricular and supraoptic nuclei)

      • Function: Stimulates uterine contractions during childbirth and milk ejection (let-down reflex) during breastfeeding.

      • Stimulus: Pressure on the cervix (during labor), suckling, touch, sight, or sound of baby.

      • Target: Uterus (smooth muscle cells), Mammary Glands (myoepithelial cells).

    2. Antidiuretic Hormone (ADH) / Vasopressin

      • Origin: Hypothalamus (neurons in paraventricular and supraoptic nuclei)

      • Function: Promotes water reabsorption in the kidneys to conserve body water; also causes vasoconstriction to increase blood pressure.

      • Stimulus: Increased blood osmolarity (dehydration), decreased blood volume/pressure.

      • Target: Kidneys (renal tubules and collecting ducts), arterioles (smooth muscle cells).

    Importance of Hormone Timing

    • Hormonal secretion often fluctuates and is tightly regulated based on physiological needs and circadian rhythms (e.g., sleep increases HGH production).

    Summary

    • Hormones play vital roles in regulating body systems from metabolism to growth and stress response.

    • Understanding the functions, interactions, and regulatory mechanisms of hormones is essential for grasping endocrine physiology, which influences all bodily functions and homeostasis.