Chapter 17

17.1 Overview of the Endocrine System

  • Endocrine system:

    • Composed of ductless endocrine glands that synthesize and secrete hormones.

    • Hormones are released into the blood and transported throughout the body.

    • Target cells have specific receptors for a hormone, binding and responding to it.

    • Hormone transport:

    • Released into interstitial fluid, enter blood, randomly leave blood to bind to target cells’ receptors.

17.1b Comparison of the Two Control Systems

  • Endocrine vs. Nervous System:

    • Both release ligands (chemical messengers) and bind to cellular receptors.

    • Endocrine System:

    • Transmits hormones via blood.

    • Targets any body cells with correct receptors.

    • Longer reaction times, widespread effects, and lasts longer (minutes to weeks).

17.1c General Functions of the Endocrine System

  • Regulation of:

    • Development, growth, metabolism, blood composition, digestive processes, and reproductive activities.

17.2 Location of Major Endocrine Glands

  • Major Glands:

    • Pituitary, pineal, thyroid, parathyroid, adrenal glands.

    • Endocrine cells also found in hypothalamus, skin, thymus, heart, liver, stomach, pancreas, small intestine, adipose, kidneys, gonads.

17.2b Stimulation of Hormone Synthesis and Release

  • Regulated by:

    • Endocrine reflexes triggered by hormonal, humoral, or nervous stimuli.

17.3 Circulating Hormones

  • Categories:

    • Steroids: Lipid-soluble (e.g., estrogen, cortisol).

    • Biogenic Amines: Modified amino acids (e.g., catecholamines, melatonin).

    • Proteins: Water-soluble chains of amino acids.

    • Local Hormones: Do not circulate in blood and bind to release or neighboring cells.

17.4 Transport in the Blood

  • Lipid-soluble hormones: Require a carrier protein.

  • Water-soluble hormones: Travel freely but some may use carriers.

17.5 Action of Hormones

  • Lipid-soluble hormones:

    • Can diffuse across target cell membrane, bind intracellular receptors.

  • Water-soluble hormones:

    • Bind to surface receptors, initiate signal transduction pathways.

17.6 Hormone Interactions

  • Different hormones can bind to a cell simultaneously:

    • Synergistic: One hormone reinforces another.

    • Permissive: One requires another’s activity.

    • Antagonistic: One opposes another’s action.

17.7 Hypothalamus and Pituitary Gland

  • Roles of the Hypothalamus:

    • Controls pituitary which regulates other endocrine organs.

    • Secretes releasing and inhibiting hormones.

17.8 Thyroid Gland

  • Function:

    • Controls metabolism.

    • Hormones: T3 and T4.

    • Regulation involves TRH and TSH through negative feedback.

17.9 Adrenal Glands

  • Regions:

    • Adrenal medulla (epinephrine/norepinephrine) and cortex (steroids).

    • Cortisol release regulated by CRH and ACTH.

17.10 Pancreas

  • Endocrine function:

    • Alpha cells (glucagon), Beta cells (insulin).

    • Regulates blood glucose levels through negative feedback.

17.11 Other Endocrine Structures

  • Pineal gland secretes melatonin, regulating sleep.

  • Parathyroid glands secrete PTH, increasing blood calcium levels.

  • Kidney releases EPO, increasing red blood cell production.

  • Liver produces insulin-like growth factors.

17.12 Aging and Endocrine System

  • Changes with aging:

    • Secretory activity wanes, reducing hormone levels and efficiency of functions.