The nervous system is not the only way the body controls bodily functions. It also uses chemical messengers. Some cells communicate directly with their contact neighbors through special junctions. This is usually to coordinate local activity such as ciliary movement or muscle contractions. Others release chemicals into the intercellular spaces that primarily affect cells in the same tissue. An example is the prostaglandins, a powerful fatty acid with many functions. Prostaglandins are released by damaged tissues and stimulate inflammation and the sensation of pain. Aspirin and other analgesics act by inhibiting the formation of prostaglandins and similar compounds. Many tissues issue chemicals that inhibit cell division locally. This prevents uncontrolled growth such as occurs in cancer tumors. Compounds such as these are called local hormones or paracrine factors. Some specialized cells produce chemicals that are excreted through ducts onto epithelial surfaces, such as inside the intestines or onto the skin. These cells are in structures called exocrine glands. Saliva and sweat are secretions of exocrine glands.
The focus in this section is on chemicals that are secreted by glands into the blood supply for the regulation of bodily function. These glands form the endocrine system, which consists of endocrine organs that produce hormones. The chemical messengers produced by the endocrine system are called hormones, which are chemical messengers that influence the response of cells and tissues at locations remote from the hormone-producing cells. The endocrine system is just as vital for proper functioning of the body as the nervous system is. Although its response time is not as fast, its effects can be long lasting. Of interest from an environmental point of view is the idea that some pollutants, including 2,3,7,8-TCDD (dioxin), mimic the female sex hormone estrogen. Such toxins are called xenoestrogens or endocrine disrupters.
Figure 9.5 shows some of the glands of the endocrine system. The pituitary gland has two main parts. The anterior pituitary produces a number of important hormones, the release of all of which is controlled by other hormones produced by the hypothalamus. The posterior pituitary does not produce its own hormones. However, it stores several hormones produced by the hypothalamus and releases them upon receiving a neural command from the hypothalamus.
The adrenal glands are located atop each kidney and also have two main parts. The outer part, or cortex, produces steroid hormones; the inner part, called the medulla, produces epinephrine and norepinephrine. Ninety-nine percent of the pancreas serves an exocrine function, producing digestive enzymes. The other 1% performs a critical endocrine function: controlling blood glucose.
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