Hormones are organic substances that are secreted by plants and animals and that function in the regulation of physiological activities and in maintaining homeostasis. They carry out their functions by evoking responses from specific organs or tissues that are adapted to react to minute quantities of them. The classical view of hormones is that they are transmitted to their targets in the bloodstream after discharge from the glands that secrete them. This mode of discharge (directly into the bloodstream) is called endocrine secretion. The meaning of the term hormone has been extended beyond the original definition of a blood-borne secretion, however, to include similar regulatory substances that are distributed by diffusion across cell membranes instead of by a blood system. . Among animals, the hormones of the vertebrates–particularly those of humans and other mammals–are the best known. Most vertebrate hormones originate in specialized tissues, called endocrine tissues, and are carried to their targets through the bloodstream. Endocrine glands. A major endocrine gland in vertebrates is the pituitary, which consists of two distinct sections: the anterior pituitary (or adenohypophysis) and the posterior pituitary (or neurohypophysis). The anterior pituitary is sometimes called the “master gland,” because it secretes several hormones that affect the other endocrine glands. For example, the anterior pituitary hormones thyrotropin and adrenocorticotropic hormone (ACTH) regulate endocrine activity in the thyroid and the outer region (cortex) of the adrenal glands, respectively. The anterior pituitary also secretes hormones that affect the sex glands. One of these is follicle-stimulating hormone (FSH), which stimulates egg production in the ovaries and sperm production in the testes. Another is luteinizing hormone (LH). In females, LH works in conjunction with FSH to regulate the female reproductive cycle and the secr…

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