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Structure and Function
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The endocrine system is made up of all the major glands, which act to regulate hormones in the body (Fig. 12-1). Various endocrine organs produce and secrete these hormones in order to maintain homeostasis, which is defined as the state of dynamic equilibrium. In other words, the hormones act together to keep the body’s internal environment healthy. Some hormones act directly on target organs; others stimulate certain glands to secrete yet different hormones.
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Hormone levels in the blood may vary according to bodily functions. Hormones usually work in pairs to maintain a healthy balance, with one acting to raise levels of other substances when needed and the other acting to lower levels when needed. For example, the hormones calcitonin and parathyroid hormone function in an opposite, yet complementary, fashion to maintain a healthy level of calcium in the blood.
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Endocrine glands are responsible for the sexual maturation of individuals from childhood to adolescence and into adulthood. Endocrine glands also play a role in the body’s ability to metabolize food and store energy.
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Learning Style Tip
Study Figure 12-1 and then create a fun and colorful 3-D model that similarly illustrates the pituitary gland, the hormones it produces, and the target organs the hormones affect. An example might be a hanging mobile with the pituitary gland at the top and labeled “hormone” strings, each of which dangles its respective target organ(s). Once you’ve completed it, show and explain your model to one or more classmates.
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The pituitary gland is a small, round, pea-sized structure attached to the lower surface of the hypothalamus in the brain. It is commonly called the master gland because it controls all of the other glands in the body. (Even so, the pituitary is actually controlled by the hypothalamus.) The pituitary gland is divided into an anterior lobe and a posterior lobe. These two parts function separately to produce many different hormones (Fig. 12-2). The anterior lobe secretes the following six hormones:
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Growth hormone (GH) promotes the growth of body structures, such as bones.
Thyroid-stimulating hormone (TSH) affects the growth and functioning of the thyroid gland.
Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) are referred to as gonadotropins because they act on the gonads—the ovaries in the female, to produce an ovum, and the testes in the male, to produce sperm.
Prolactin acts on the mammary glands to produce milk.
Adrenocorticotropic hormone (ACTH) acts on the adrenal glands to secrete glucocorticoids, including cortisol.
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