endocrine system basics

Endocrine System Basics

Overview

  • Endocrine glands produce and secrete hormones into the blood (process called "Secretion").

  • Hormones are chemical messengers that travel throughout the body, targeting sensitive parts known as "Target Cells".

  • The endocrine system acts as a communication system throughout the body, slower than the nervous system but capable of significant prolonged changes.

Hormonal Communication

  • Blood circulates throughout the body, mixing with hormones, but only specific cells with appropriate receptors respond to these hormones.

  • Paracrines: Send signals to neighboring cells (not in blood).

  • Autocrines: Target the same cells that produced them.

Major Endocrine Glands

  • Pineal Gland: Produces melatonin for sleep.

  • Hypothalamus: Controls the pituitary gland and produces important hormones.

  • Pituitary Gland (Hypophysis):

    • Anterior Pituitary: Produces at least 6 hormones.

    • Posterior Pituitary: Stores hormones produced by the hypothalamus (Oxytocin & ADH).

  • Thyroid Gland:

    • Largest pure endocrine gland, produces Thyroid hormones (T3 and T4) and Calcitonin.

  • Parathyroid Glands: Produce parathyroid hormone to regulate calcium levels in bones.

  • Thymus: Produces thymulin and thymopoietins, largest when a child.

  • Adrenal Glands (on kidneys):

    • Adrenal Medulla: Releases adrenaline and noradrenaline (fight or flight response).

    • Adrenal Cortex: Divided into three layers producing corticosteroids (mineralocorticoids, glucocorticoids, gonadocorticoids).

  • Pancreas: Functions as both an exocrine (digestive enzymes) and endocrine (insulin and glucagon) gland.

  • Gonads: Ovaries produce estrogens and progesterone; testes produce testosterone.

  • Placenta: During pregnancy, produces hormones like human chorionic gonadotropin (hCG).

Specific Hormones and Their Functions

  • Thyroid Hormones (T3/T4): Major hormones for metabolism; T4 (Thyroxine) is the most commonly released form.

  • Insulin: Lowers blood sugar by allowing glucose into body cells.

  • Glucagon: Raises blood sugar by stimulating the breakdown of glycogen.

  • ADH: Regulates water retention in the kidneys; excess can cause edema and headaches.

Regulation of Hormone Release

  • Hormone release is regulated through three types of stimuli:

    1. Humoral Stimuli: Respond to blood levels (e.g., insulin in response to high glucose).

    2. Neural Stimuli: Release due to nerve impulses (e.g., catecholamines during stress).

    3. Hormonal Stimuli: Hormone release triggered by other hormones (e.g., hypothalamic hormones stimulate the pituitary).

The Hypothalamus and Anterior Pituitary

  • Infundibulum: Connects hypothalamus to pituitary gland.

  • Hypothalamic hormones: Releasing and inhibiting hormones that regulate anterior pituitary hormones:

    • TSH (Thyroid-Stimulating Hormone): Stimulates thyroid hormone release.

    • ACTH (Adrenocorticotropic Hormone): Stimulates cortisol release from adrenal cortex.

    • FSH (Follicle-Stimulating Hormone) and LH (Luteinizing Hormone): Control gonadal function.

    • Prolactin: Stimulates breast milk production.

    • GH (Growth Hormone): Stimulates growth through IGFs (Insulin-like Growth Factors).

Negative Feedback Mechanism

  • High levels of thyroid hormone stop TRH release from hypothalamus, thus regulating hormone production.

Thyroid Gland Functions

  • Production of Thyroid Hormones: Requires iodine; TH is crucial for metabolism.

  • Conditions:

    • Hypothyroidism: Results in low TH levels (e.g., Myxedema, Goiter).

    • Hyperthyroidism: High TH levels (e.g., Graves' Disease, symptoms include high metabolic rate).

Adrenal Glands and Hormones

  • Adrenal Medulla: Releases catecholamines in response to sympathetic nervous activation.

  • Adrenal Cortex: Divided into three zones producing different corticosteroids:

    • Zona Glomerulosa: Produces aldosterone (mineralocorticoids).

    • Zona Fasciculata: Produces cortisol (glucocorticoids).

    • Zona Reticularis: Produces gonadocorticoids (androgens).

    • Medical Conditions:

      • Cushing's Syndrome: Excess cortisol. (buffalo hump/moon face)

      • Addison's Disease: Low cortisol levels. (too little aldosterone)

Pancreas Hormonal Function

  • Islets of Langerhans: Alpha cells produce glucagon; beta cells produce insulin.

  • Diabetes Mellitus: Results from insulin levels issues:

    • Type 1: No insulin production.

    • Type 2: Insulin resistance.

    • Symptoms: Polydipsia (excess thirst), Polyuria (excess urine), Polyphagia (excess hunger).

Hormonal Mechanism of Action

  • Nonsteroidal Hormones: Use G protein relay systems (e.g., cyclic AMP) as second messengers.

  • Steroid Hormones: Lipid-soluble, enter target cells directly and bind to receptors to influence gene expression.

Parathyroid Gland Functions

  • Regulates calcium levels in the blood through PTH (Parathyroid Hormone).

  • Medical Issues:

    • Hyperparathyroidism: High Ca2+ levels, risks of kidney stones.

    • Hypoparathyroidism: Can cause muscle twitching and possible respiratory failure.


Ensure to study each gland's specific functions, associated hormones, and related medical conditions for a comprehensive understanding.

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