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.