Chapter 7_Endocrine SystemsMHDG (1)

7.1 Principles of Endocrinology

• Definition: Endocrinology is the study of hormones, their physiological functions, and their evolution.

• Role of Endocrine System:

  • Regulates and coordinates distant organs through hormone secretion.

  • Hormones act as signal molecules and reach target cells via circulatory systems.

  • Functions over longer durations rather than quick responses.

When Endocrine Homeostasis Fails

• Examples of disorders caused by endocrine dysfunction:

  • Acromegaly: Excess growth hormone leading to enlarged bones.

  • Cretinism: Severe thyroid deficiency, leading to stunted growth and development.

  • Gigantism: Excess growth hormone during childhood results in abnormal height.

  • Goiter: Enlargement of the thyroid due to iodine deficiency.

  • Rickets: Vitamin D deficiency affecting bone development.

  • Graves Disease: Autoimmune disorder leading to overproduction of thyroid hormones.

Chemical Bioregulation

• Bioregulators: Hormones (endocrine), neurocrines (nervous system), and cytocrines (immune system).

• All three systems work together for overall homeostasis.

Overview of the Mammalian Endocrine System

• Key Glands:

  • Pineal, hypothalamus, pituitary, testes, parathyroid, thyroid, thymus, heart, stomach, adrenal gland, pancreas, duodenum, kidney, skin, ovaries, placenta.

• Functions:

  • Vary from solely endocrine, mixed functions, to uncertain roles.

Chemical Classification of Hormones

• Types of Hormones:

  • Peptide and Protein Hormones: Hydrophilic and consist of amino acid chains.

  • Amines: Include catecholamines and thyroid hormones.

  • Steroids: Lipophilic, derived from cholesterol.

• Hormones Differentiation:

  • Variance in receptor localization and permeability.

Hormone Synthesis and Secretion

• Peptide Hormones:

  • Synthesized in rough endoplasmic reticulum and secreted via exocytosis.

• Steroid Hormones:

  • Derived from cholesterol and synthesized in mitochondria and smooth ER.

  • Diffuse directly after synthesis.

Hormone Mechanism of Action

• Steroid Hormones:

  • Lipophilic hormones diffuse through the plasma membrane.

  • Bind to intracellular receptors and activate gene expression via mRNA.

  • Produce new proteins for cellular responses.

Endocrine System Functional Conceptualization

• Input Factors:

  • Both endogenous (internal) and exogenous (external) factors influence hormone levels.

• Regulatory Response:

  • Hormone secretion can be affected by blood chemistry levels and environmental factors.

Regulation of Plasma Hormone Concentration

• Negative Feedback Control:

  • Increased hormone levels inhibit further secretion.

• Neuroendocrine Reflexes:

  • Sudden increases in hormone release following specific stimuli.

• Biological Rhythms:

  • Hormone levels fluctuate according to biological clocks, like cortisol secretion peaking in the morning.

Central Endocrine Glands

• Pineal Gland:

  • Secretes melatonin to maintain circadian rhythms; influenced by light.

• Pituitary Gland:

  • Anterior and posterior lobes; regulates several hormone secretions including TSH, ACTH, FSH, LH, and GH.

Hormones of the Pituitary Gland

• Posterior Pituitary:

  • Secretes oxytocin (for social bonding and childbirth) and vasopressin (for water retention).

• Anterior Pituitary:

  • Releases GH, TSH, ACTH, FSH, LH, and prolactin, which regulate numerous bodily functions.

Thyroid Gland—Structure and Function

• Anatomy:

  • Located below the larynx, composed of follicular cells surrounding colloid.

• Hormone Synthesis:

  • Thyroglobulin produced by follicular cells is iodinated to form T3 and T4.

• Release Mechanism:

  • Colloid uptake followed by lysosomal processing results in active thyroid hormones entering circulation.

Effects of Thyroid Hormones

• Physiological Effects:

  • Raise metabolic rate, influence growth, increase heart rate, affect CNS development, and support growth hormone's actions.

Regulation of Thyroid Hormone Secretion

• Feedback Loops:

  • TSH stimulates synthesis and release; elevated T3 and T4 levels suppress TSH.

• Environmental Factors:

  • Stress (inhibitory) and cold (stimulatory) influence TSH secretion.