lec12 anatomy

Endocrine glands are ductless glands: they secrete hormones directly into the bloodstream. (In contrast, exocrine glands secrete hormones into ducts, which, in turn, lead to a surface in some other location such as the body s surface—as in sweat glands—or the digestive tract—as in digestive enzymes.) Once in the bloodstream, each hormone travels throughout the body. In the process, the cells of many different organs are exposed to a particular hormone; however, only cells having receptors for that hormone (called target cells) will respond.
In other words, a hormone acts only on cells with receptors specific to that hormone; this is called specificity.Hormones can be classified as steroid or non-steroid.
1- Steroid hormones are synthesized from cholesterol; they include male
and female sex hormones as well as aldosterone (secreted by the adrenal cortex).
2-Nonsteroid, or protein-based, hormones are synthesized from amino acids.
They can be further divided into
1- protein hormones (such as insulin),
2-peptide hormones (such as antidiuretic hormone),
3- or amino acid derivative hormones (such as epinephrine and norepinephrine).
Hormones travel through the bloodstream to stimulate specific target cells. The target cells may lie anywhere in the body: close to the gland or far from it. Once a hormone reaches the target cell, it binds with a receptor to trigger changes within the cell. How this occurs , though, depends upon whether the hormone is a steroid or protein-based hormone.