To work alongside the Nervous
System in regulating and maintaining homeostasis.
The NS regulates body activities by responding rapidly using nerve impulses
The endocrine system regulates and responds by releasing hormones.
Function - Helps maintain homeostasis
HOW???
Regulate chemical composition and volume of internal environment Help regulate metabolism and energy balance
Help regulate contraction of smooth and cardiac muscle fibers and secretion of glands
Regulate some activities of immune system
Play role in smooth, sequential integration of growth and development
Contribute to basic process of reproduction
How does the endocrine system keep its fingers in
EVERYTHING???
What induces hormone release???
Mechanisms of Hormone Release
-humoral: in response to changing levels of ions or nutrients in the blood
-neural: stimulation by nerves
- hormonal: stimulation received from other hormones
General Characteristics
System of ductless glands that secrete hormones
Hormones are “messenger molecules”
Circulate in the blood
exceptions: paracrine, autocrine hormones
Act on distant target cells
exceptions: paracrine, autocrine hormones
Target cells respond to the hormones for which they have receptors The effects are dependent on the programmed response of the target cells
Hormones are just molecular triggers
Circulating Hormones
Paracrine Hormones
Autocrine Hormones
Lipid-soluble
Steroids
Lipids structurally similar to cholesterol
Synthesized from cholesterol
Released by male and female reproductive organs, adrenal glands and kidneys
Thyroid hormones
Nitric oxide (gas) – both hormone and neurotransmitter What is the significance of lipid soluble????
Steroid Hormones
Water-soluble Hormones
Amines
Structurally similar to amino acids
Released by adrenal medulla, thyroid, and pineal gland
Most are synthesized from amino acid tyrosine
Water-soluble Hormones
Peptide hormones
Chains of amino acids (30 49 each)
Largest class and includes all hormones secreted by hypothalamus, pituitary gland, heart, thymus, digestive tract, and pancreas
Oxytocin
Water-soluble Hormones
Eicosanoids
Small molecules with 5- carbon ring at one end
Includes prostaglandins and leukotrienes, important paracrine factors
Derived from a 20 carbon fatty acid
For hormone to affect target cell, it must first interact with appropriate receptor
Hormone receptors may be on cell membrane or inside cell
Water soluble hormones- cannot penetrate cell membrane
Hence, receptors for catecholamines, peptide hormones, and eicosanoids are found on cell membranes Lipid soluble hormones pass into cell and bind to receptors in cell nucleus
Action of Lipid Soluble Hormones
1. Released from transport protein in blood Note: receptor is in the cell
2. Diffuses into cell binds to receptor in nucleus 3. Activates gene expression
End result: synthesis of new proteins that alter cell activity
Mechanism of action of the water-soluble hormones
Hormone (1st messenger) released from endocrine gland bloodstream
Travels thru the body, passes out of capillary and binds to cells with the appropriate receptor.
Receptor binding induces appearance/ action of a 2nd messenger
2nd messengers include: enzyme activators, inhibitors, or cofactors (net result is change in rates of various metabolic reactions)
Most important second messengers are cyclic AMP, cyclic GMP, and calcium ions
Hormones and cell membranes
Binding of hormone to receptors may lead to release of many (thousands) of 2nd messengers--called amplification
G-protein- common link between 1st and 2nd messengers
