Endocrine system - endocrine glands and hormones

Endocrine System 
The endocrine system consists of a number of glands lo­cated throughout the body that produce and secrete sub­stances called hormones into the blood. A hormone is a chemical that influences or controls the activity of a specific tissue or organ. Like the nervous system, the endocrine sys­tem coordinates the functioning of body parts. The nervous system is fast acting and utilizes nerve impulses that travel rapidly along nerve fibers. The endocrine system is slower acting because it takes time to deliver hormones to target organs, but the action of hormones is longer lasting.
Hormone Action 
Hormones can be divided into two categories: (1) peptides (used here to include amino acids, polypeptides, and pro­tein hormones) and (2) steroids (complex rings of carbon and hydrogen atoms). Peptide hormones act
as messengers that stimulate cells by binding to specific receptors in the plasma membrane. The hor­mone receptor complex activates an enzyme that produces cyclic AMP (cAMP), a compound derived from ATP. Cyclic AMP, serving as a so-called second messenger, then acti­vates other enzymes that carry out various cellular activi­ties. Other peptide hormones utilize calcium (Ca2+) as a second messenger. In contrast to a two-messenger system, steroid hormones (which are lipid-soluble) pass through the plasma membrane and bind to receptors present in the cytoplasm. This hormone receptor complex moves into the nucleus, where the complex activates cer­tain genes involved in protein synthesis.
Endocrine Glands 
All hormones are carried throughout the body by the blood, but each one affects only a specific body part or parts, appropriately termed the target organ(s).Endocrine glands are ductless glands that secrete hormones directly into the blood. Exocrine glands, on the other hand, secrete their products into ducts that transport them to their desti­nation. For example, the salivary glands secrete saliva into ducts that enter the mouth.
Hormones affect metabolism, appearance, and behav­ior. They help maintain homeostasis by keeping the fluid, electrolyte, and glucose balance of the blood within nor­mal limits. Hormone secretion by endocrine glands is of­ten regulated by negative feedback.
Negative Feedback 
In a self-regulating negative feedback mechanism, an adap­tive response dampens or even cancels the stimulus that brought about the response. An endocrine gland can be sensitive to either the condition it is regulat­ing or to the blood level of the hormone it is producing. For example, when blood is concentrated, the hypothala­mus produces a hormone that causes blood dilution, which then is a signal to the hypothalamus to stop produc­ing the hormone. On the other hand, the pituitary gland produces a hormone that stimulates the thyroid gland.
When the blood level of a hormone produced by the thy­roid rises, the pituitary gland no longer stimulates the thyroid gland.
The endocrine glands secrete their hormones into the bloodstream for transport to target organ cells, vvhere the hormones are received by receptors. Hormone secretion by endocrine glands is often regulated by negative feedback.