47.1 Regulation is often accomplished by chemical messengers.
Types of Regulatory Molecules
• A hormone is a regulatory chemical secreted into the blood by an endocrine gland and carried in the bloodstream. (p. 992)
• Paracrine regulators act in the organ in which they were produced and released. (p. 992)
Endocrine Glands and Hormones
• Hormones secreted by the endocrine glands are categorized as polypeptides, glycoproteins, amines, and steroids. (p. 993)
• The secretory activity of many endocrine glands is controlled by the nervous system. (p. 993)
Paracrine Regulation
• Paracrine regulators are numerous, and include cytokines, growth factors, neurotrophins, and prostaglandins. (p. 994)
• Prostaglandins help regulate many systems, including the immune, reproductive, digestive, respiratory, circulatory, and urinary systems. (pp. 994-995)
47.2 Lipophilic and polar hormones regulate their target cells by different means.
Hormones That Enter Cells
• Hormones can be divided into lipophilic (lipid-soluble) and lipophobic (hydrophilic) categories. (p. 996)
• Lipophilic hormones pass through the target cell's plasma membrane and bind to intracellular receptor proteins, thereby regulating target cell genes. (pp. 996-997)
Hormones That Do Not Enter Cells
• Lipophobic hormones cannot pass through plasma membranes, and thus use messengers within the target cells, such as cyclic AMP, IP3, and Ca++, for mediation. (pp. 998-999)
47.3 The hypothalamus controls the secretions of the pituitary gland.
The Posterior Pituitary Gland
• The posterior pituitary contains axons that originate within the hypothalamus and extend along the pituitary. (p. 1000)
• Antidiuretic hormone (ADH) is a posterior pituitary peptide that stimulates water retention by the kidneys. Oxytocin is a posterior pituitary peptide that stimulates the milk-ejection reflex in mammary glands. Both are produced by neuron cell bodies located in the hypothalamus. (p. 1000)
The Anterior Pituitary Gland
• The hormones produced and secreted by different cell types in the anterior pituitary gland include growth hormone, adrenocorticotropic hormone, thyroid-stimulating hormone, luteinizing hormone, follicle-stimulating hormone, prolactin, and melanocyte-stimulating hormone. (pp. 1001-1002)
• The hypothalamus and the anterior pituitary gland are influenced by negative feedback inhibition from the endrocrine glands they stimulate. (pp. 1003-1004)
47.4 Endocrine glands secrete hormones that regulate many body functions.
The Thyroid and Parathyroid Glands
• The thyroid gland secretes thyroxine and triiodothyronine, which stimulate oxidative respiration, and in children, they promote growth and stimulate maturation of the central nervous system. (p. 1005)
• Parathyroid hormone raises blood Ca++ levels by stimulating bone dissolution. (pp. 1005-1006)
The Adrenal Glands
• Sympathetic neurons stimulate the adrenal medulla to secrete epinephrine and norepinephrine to trigger alarm responses. (p. 1007)
• The adrenal cortex is stimulated to secrete glucocorticoids by ACTH and mineralocorticoid by angiotensin II. (p. 1007)
• Cortisol helps regulate blood glucose levels, while aldosterone regulates blood sodium and potassium levels. (p. 1007)
The Pancreas
• Beta cells of the islets of Langerhans secrete insulin, which lowers blood glucose levels, while alpha cells secrete glucagon, which raises blood glucose levels. (p. 1008)
Other Endocrine Glands
• In molting insects, metamorphosis and molting are regulated by ecdysone and juvenile hormone. (p. 1009)
• Gonadal sex steroids regulate reproduction, while melatonin from the pineal gland regulates circadian rhythms, and thymus hormones help regulate the vertebrate immune system. (pp. 1009-1010)