Cells in the lining of the stomach and small intestine secrete hor-mones that stimulate the production of digestive juices from the stom-ach, pancreas, and liver. This secretion occurs when food is present in the digestive system, but not at other times. Hormones secreted from the small intestine also help regulate the rate at which food passes from the stomach into the small intestine .
Prostaglandins are widely distributed in tissues of the body, where they function as intercellular signals. Unlike most hor-mones, they are usually not transported long distances in the blood but function mainly as autocrine or paracrine chemical signals (see table 10.1). Thus, their effects occur in the tissues where they are produced. Some prostaglandins cause relaxation of smooth muscle, such as dilation of blood vessels. Others cause contraction of smooth muscle, such as contraction of the uterus during the deliv-ery of a baby. Because of their action on the uterus, prostaglandins have been used medically to initiate abortion. Prostaglandins also play a role in inflammation. They are released by damaged tissues and cause blood vessel dilation, localized swelling, and pain. Prostaglandins produced by platelets appear to be neces- sary for normal blood clotting. The ability of aspirin and related substances to reduce pain and inflammation, to help prevent the painful cramping of uterine smooth muscle, and to treat headache is a result of their inhibitory effect on prostaglandin synthesis.
The kidneys secrete the hormone erythropoietin (ĕ-rith′ rō-poy′ ĕ-tin) in response to reduced oxygen levels in the kidney. Erythropoietin acts on bone marrow to increase the production of red blood cells .
In pregnant women, the placenta is an important source of hormones that maintain pregnancy and stimulate breast develop-ment. These hormones are estrogen, progesterone, and humanchorionic gonadotropin (gō′ nad-ō-trō′ pin), which is similar instructure and function to LH.
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