CHAPTER 41 – ANIMAL HORMONES
41.1 What Are Hormones and How Do They Work?
-Control and regulation require information.
-In multi-cellular animals, most of this information is transmitted as electric signals and as
The electrical signals are impulses generated by the nervous system, conducted along
cell processes of nerve cells to their targets on specific cells.
The chemical signals are hormones, secreted by cells of the endocrine system into
the extracellular fluid.
-The two informational systems of the body are the nervous and endocrine systems.
Hormones can act locally or at a distance
-Endocrine Cells: The cells that secrete hormones.
-Target Cells: The cells that have receptors for hormones.
-Circulating Hormones: Hormones secreted into the extracellular fluid can diffuse into the
blood, which distributes them throughout the body so they can activate target cells far from
the site of release.
Testosterone is a circulating hormone.
-Some hormones are released in such tiny quantities, or are so rapidly inactivated by enzymes,
or taken up so efficiently by local cells that they never diffuse into the blood in sufficient
amounts to act on distant cells.
-Paracrine Hormones: Hormones that affect only target cells near their release site.
e.g., histamine, one of the mediators of inflammation.
-The most local action of a hormone can have is when there are receptors on the same cell that
-When a hormone influences the cell that released it, it is said to have autocrine function.
Such functions can provide negative feedback to control rates of secretion.
-Some endocrine cells exist as single cells within a tissue.
-Hormones of the digestive tract are secreted by isolated endocrine cells in the wall of the
stomach and small intestines.
-Endocrine Glands: Hormones that are secreted by aggregations of endocrine cells forming
“endocrine”: the glands do not have ducts that lead to the outside of the body; they
secrete their products directly into the extracellular fluid.
A single endocrine gland may secrete several different hormones.
Exocrine Glands are contrasting; they have ducts that carry their products to the
surface of the skin (e.g., sweat glands) or to the surface of the body passageway that
leads to the outside of the body (salivary glands).