Endocrine System Review

Chapter 18 – Endocrine System

• General Overview

  • The endocrine system works alongside the nervous system to maintain homeostasis in the body.

  • It consists of organs and cells that release hormones, which are chemical messengers.

  • Organs specifically for hormone release:

  • Thyroid gland

  • Pituitary gland

  • Parathyroid gland

  • Adrenal gland

  • Pineal gland

  • Organs with mixed functions (release hormones and perform other functions):

  • Hypothalamus, thymus, pancreas, ovaries, testes, kidneys, stomach, liver, small intestines, skin, heart, adipose tissue, placenta

• Hormone Secretion and Regulation

  • Hormone release is managed by feedback mechanisms; primarily negative feedback systems.

  • Example factors regulating hormone release:

  • Blood levels of substances (e.g., low glucose inhibits insulin secretion)

  • Nervous system stimuli (e.g., release of epinephrine)

  • Hormones are released in bursts into interstitial fluid and may be circulating or local hormones.

  • Local hormones affect neighboring cells (paracrine) or the cell that released them (autocrine).

• Receptor Dynamics

  • Target cells require specific receptors for specific hormones.

  • Number of receptors changes according to hormone levels:

  • Down-regulation occurs with high hormone levels (decrease in receptors).

  • Up-regulation occurs with low hormone levels (increase in receptors).

• Classes of Hormones

  • Lipid Soluble Hormones:

  • Examples: Nitric oxide, thyroid hormones, and steroid hormones (e.g., cortisol, estrogen).

  • Require transport proteins (e.g., albumin) to travel in blood and act on intracellular receptors.

  • Bind to receptors, triggering gene expression and protein synthesis.

  • Water Soluble Hormones:

  • Examples: Amine hormones (e.g., epinephrine), peptide/protein hormones (e.g., insulin), eicosanoids (e.g., prostaglandins).

  • Travel dissolved in blood; bind to plasma membrane receptors and use second messengers to effect changes in existing proteins.

• Hormonal Interactions

  • Permissive Effect: One hormone's action enables another to work (e.g., thyroid hormone allows epinephrine action).

  • Synergistic Effect: Combined effect of hormones is greater than individual effects (e.g., glucagon and epinephrine).

  • Antagonistic Effect: One hormone opposes the action of another (e.g., insulin and glucagon).

• Hypothalamus and Pituitary Gland

  • The hypothalamus regulates the endocrine system and produces hormones affecting the pituitary gland.

  • Hypothalamic hormones:

  • Releasing hormones: GHRH, GHIH, PRH, TRH, CRH, GRH

  • Oxytocin and ADH stored in the posterior pituitary.

  • The pituitary gland (master gland) has:

  • Anterior pituitary (adenohypophysis): Produces GH, FSH, LH, ACTH, and TSH.

  • Posterior pituitary (neurohypophysis): Releases ADH and oxytocin.

• Growth Hormone (GH)

  • Promotes growth via IGF (Insulin-like Growth Factor); released mostly during sleep.

  • Excess GH before growth completion → gigantism; after completion → acromegaly.

• Thyroid Gland

  • Consists of follicles filled with colloid.

  • Produces T3 and T4 essential for metabolism; iodine is crucial for synthesis.

  • Functions include:

  • Increase metabolic rate

  • Promote nervous and skeletal system growth

  • Enhances catecholamine (epinephrine) effects

• Parathyroid Gland

  • Secretes PTH, which regulates calcium, magnesium, and phosphate.

  • PTH increases blood calcium levels by promoting bone resorption and calcium absorption in the gut.

• Adrenal Glands

  • Divided into cortex and medulla.

  • Cortex:

  • Zona glomerulosa: produces aldosterone (mineralocorticoid).

  • Zona fasciculata: produces cortisol (glucocorticoid).

  • Zona reticularis: produces androgens.

  • Medulla: releases catecholamines (epinephrine, norepinephrine) during stress.

• Pancreas

  • Functions as both endocrine (insulin/glucagon) and exocrine (digestive enzymes) gland.

  • Insulin decreases blood sugar; glucagon increases it.

  • An imbalance can lead to diabetes (Type I or II).

• Other Endocrine Organs

  • Pineal gland: produces melatonin for sleep regulation.

  • Thymus: produces thymosin for immune cell development.

  • Gonads: produce sex hormones (estrogen/progesterone in ovaries; testosterone in testes).

• Stress Response

  • Classified as eustress (positive) or distress (negative).

  • General Adaptation Syndrome (GAS):

  1. Fight or Flight: Immediate response driven by epinephrine and norepinephrine.

  2. Resistance Reaction: Long-term hormonal responses involving CRH, TRH, GHRH to sustain energy.

  3. Exhaustion Reaction: Depletion of resources leading to health issues.

• Homeostatic Imbalances

  • Growth Hormone Imbalances:

  • Giantism, acromegaly, achondroplasia.

  • Thyroid Hormone Imbalances:

  • Hypothyroidism: fatigue, hair loss; Cretinism in untreated congenital cases.

  • Hyperthyroidism (Graves Disease): fast metabolism, exophthalmos.

  • Insulin Issues:

  • Type I and II Diabetes Mellitus, characterized by symptoms like thirst, fatigue, and blurred vision.

  • Cortisol and Aldosterone Issues:

  • Cushing's syndrome from excess cortisol; Addison's disease from low levels.

  • ADH Deficiency: Leads to diabetes insipidus, characterized by excessive urination and thirst.