7.2 Review of Anatomy and Physiology of the Endocrine System (68/84) -- Health Alterations

7.2 Review of Anatomy and Physiology of the Endocrine System The endocrine system includes several glands, including the pineal, hypothalamus, pituitary, thyroid, parathyroid, and adrenal glands, as well as the pancreas, ovaries, and testes. See Figure 7.1 for an illustration of these organs of the endocrine system.[1] Endocrine glands secrete hormones as chemical signaling. Hormones are transported via the bloodstream throughout the body, where they bind to receptors on target cells, triggering a characteristic response. This long-distance communication is the fundamental function of the endocrine system. Pineal Gland The pineal gland is a small cone-shaped structure that extends posteriorly from a ventricle of the brain. The pineal gland produces the hormone melatonin and secretes it directly into the cerebrospinal fluid, which carries it into the blood. Melatonin affects reproductive development and daily circadian rhythms.[2] Thymus Gland The thymus gland is located in the mediastinum. It is responsible for production of T lymphocytes for the body’s immune response. Hypothalamus and Pituitary The hypothalamus can be viewed as the body’s control center. The hypothalamus connects to the pituitary gland by the stalk-like infundibulum. The pituitary gland is about the size of a pea and consists of an anterior and posterior lobe. Each secretes different hormones in response to signals from the hypothalamus. See Figure 7.2 for an illustration of the hypothalamus–pituitary complex.[3] The hypothalamus–pituitary complex releases hormones that stimulate secretion of other hormones by other endocrine glands and also produce direct responses in target tissues. Its main function is to maintain a stable state called homeostasis. See Figure 7.3[4] for an illustration of the effects of the hormones released by the anterior and posterior pituitary glands. Posterior Pituitary The posterior pituitary gland secretes two hormones produced by the hypothalamus called oxytocin and antidiuretic hormone (ADH). - Oxytocin stimulates labor contractions and lactation after delivery. - Antidiuretic hormone (ADH) regulates blood osmolarity by targeting the kidneys to increase water reabsorption. Blood osmolarity refers to the concentration of sodium and other solutes in the blood. Blood osmolarity levels change in response to fluid intake, salty food ingestion, head injuries, disease, and side effects of medications. Blood osmolarity is constantly monitored by osmoreceptors in the hypothalamus. As blood osmolarity increases beyond normal levels, the posterior pituitary is stimulated to release ADH. For example, high blood osmolarity can occur if someone eats a very salty meal or becomes dehydrated from inadequate intake of fluids. Osmoreceptors sense the high blood osmolarity and trigger the sensation of thirst, while also stimulating the posterior pituitary to release ADH. ADH targets the kidneys to increase water reabsorption. As more water is reabsorbed by the kidneys