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Hormones are the chemical signaling molecules produced by the endocrine glands and secreted directly into the bloodstream. They travel through the blood to distant tissues and organs, where they can bind to specific cell sites called receptors. By binding to receptors, hormones trigger various responses in the tissues containing the receptors. The pituitary gland or hypophysis is a small gland about 1 centimeter in diameter or the size of a pea. It is nearly surrounded by bone as it rests in the sella turcica, a depression in the sphenoid bone. The gland is connected to the hypothalamus of the brain by a slender stalk called the infundibulum. There are two distinct regions in the gland: the anterior lobe (adenohypophysis) and the posterior lobe (neurohypophysis). The activity of the adenohypophysis is controlled by releasing hormones from the hypothalamus. The neurohypophysis is controlled by nerve stimulation.
Hormones of the Anterior Lobe (Adenohypophysis) Growth hormone is a protein that stimulates the growth of bones, muscles, and other organs by promoting protein synthesis. This hormone drastically affects the appearance of an individual because it influences height. If there is too little growth hormone in a child, that person may become a pituitary dwarf of normal proportions but small stature. An excess of the hormone in a child results in an exaggerated bone growth, and the individual becomes exceptionally tall or a giant. Thyroid-stimulating hormone, or thyrotropin, causes the glandular cells of the thyroid to secrete thyroid hormone. When there is a hypersecretion of thyroid-stimulating hormone, the thyroid gland enlarges and secretes too much thyroid hormone. Adrenocorticotropic hormone (ACTH) reacts with receptor sites in the cortex of the adrenal gland to stimulate the secretion of cortical hormones, particularly cortisol. Gonadotropic hormones react with receptor sites in the gonads, or ovaries and testes, to regulate the development, growth, and function of these organs. Prolactin hormone promotes the development of glandular tissue in the
female breast during pregnancy and stimulates milk production after the birth
of the infant. Hormones of the Posterior Lobe (Neurohypophysis) Antidiuretic hormone promotes the reabsorption of water by the kidney tubules, with the result that less water is lost as urine. This mechanism conserves water for the body. Insufficient amounts of antidiuretic hormone cause excessive water loss in the urine. Oxytocin causes contraction of the smooth muscle in the wall of the uterus. It also stimulates the ejection of milk from the lactating breast. The pineal gland, also called pineal body or epiphysis cerebri, is a small cone-shaped structure that extends posteriorly from the third ventricle of the brain. The pineal gland consists of portions of neurons, neuroglial cells, and specialized secretory cells called pinealocytes. The pinealocytes synthesize the hormone melatonin and secrete it directly into the cerebrospinal fluid, which takes it into the blood. Melatonin affects reproductive development and daily physiologic cycles.
About 95 percent of the active thyroid hormone is thyroxine, and most of the remaining 5 percent is triiodothyronine. Both of these require iodine for their synthesis. Thyroid hormone secretion is regulated by a negative feedback mechanism that involves the amount of circulating hormone, hypothalamus, and adenohypophysis. If there is an iodine deficiency, the thyroid cannot make sufficient hormone. This stimulates the anterior pituitary to secrete thyroid-stimulating hormone, which causes the thyroid gland to increase in size in a vain attempt to produce more hormones. But it cannot produce more hormones because it does not have the necessary raw material, iodine. This type of thyroid enlargement is called simple goiter or iodine deficiency goiter. Calcitonin is secreted by the parafollicular cells of the thyroid gland. This hormone opposes the action of the parathyroid glands by reducing the calcium level in the blood. If blood calcium becomes too high, calcitonin is secreted until calcium ion levels decrease to normal. Four small masses of epithelial tissue are embedded in the connective tissue capsule on the posterior surface of the thyroid glands. These are parathyroid glands, and they secrete parathyroid hormone or parathormone. Parathyroid hormone is the most important regulator of blood calcium levels. The hormone is secreted in response to low blood calcium levels, and its effect is to increase those levels. Hypoparathyroidism, or insufficient secretion of parathyroid hormone, leads to increased nerve excitability. The low blood calcium levels trigger spontaneous and continuous nerve impulses, which then stimulate muscle contraction.
The hypothalamus of the brain influences both portions of the adrenal gland but by different mechanisms. The adrenal cortex is regulated by negative feedback involving the hypothalamus and adrenocorticotropic hormone (ACTH); the medulla is regulated by nerve impulses from the hypothalamus. Hormones of the Adrenal Cortex The adrenal cortex consists of three different regions, with each region producing a different group or type of hormones. Chemically, all the cortical hormones are steroid. Mineralocorticoids are secreted by the outermost region of the adrenal cortex. The principal mineralocorticoid is aldosterone, which helps maintain blood pressure and the balance of water and salt in the body by helping the kidney retain sodium and excrete potassium. Glucocorticoids are secreted by the middle region of the adrenal cortex. The principal glucocorticoid is cortisol, which
The third group of steroids secreted by the adrenal cortex is the gonadocorticoids, or sex hormones. These are secreted by the innermost region. Male hormones, androgens, and female hormones, estrogens, are secreted in minimal amounts in both sexes by the adrenal cortex, but their effect is usually masked by the hormones from the testes and ovaries. In females, the masculinization effect of androgen secretion may become evident after menopause, when estrogen levels from the ovaries decrease. Hormones of the Adrenal Medulla The adrenal medulla develops from neural tissue and secretes two hormones, epinephrine and norepinephrine. These two hormones are secreted in response to stimulation by sympathetic nerve, particularly during stressful situations. A lack of hormones from the adrenal medulla produces no significant effects. Hypersecretion, usually from a tumor, causes prolonged or continual sympathetic responses.
Alpha cells in the pancreatic islets secrete the hormone glucagons in response to a low concentration of glucose in the blood. Beta cells in the pancreatic islets secrete the hormone insulin in response to a high concentration of glucose in the blood. Thymosin, produced by the thymus gland, plays an important role in the development of the body's immune system.
Testosterone secretion is regulated by a negative feedback system that involves releasing hormones from the hypothalamus and gonadotropins from the anterior pituitary.
Progesterone causes the uterine lining to thicken in preparation for pregnancy. Together, progesterone and estrogens are responsible for the changes that occur in the uterus during the female menstrual cycle.
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The endocrine system is a complex collection of hormone-producing glands
that control basic body functions such as metabolism, growth and sexual
development. The endocrine glands consist of: pituitary; pineal; thyroid and
parathyroids; adrenals; pancreas; thymus; testes (male); and ovaries (female).
The thyroid gland is a very vascular organ that is located in the
neck. It consists of two lobes, one on each side of the trachea,
just below the larynx or voice box. The two lobes are connected by a
narrow band of tissue called the isthmus. Internally, the gland
consists of follicles, which produce thyroxine
and triiodothyronine hormones. These hormones contain iodine.
The adrenal, or suprarenal, gland is paired
with one gland located near the upper portion of each kidney. Each
gland is divided into an outer cortex and an inner medulla. The cortex
and medulla of the adrenal gland, like the anterior and posterior
lobes of the pituitary, develop from different embryonic tissues and
secrete different hormones. The adrenal cortex is essential to life,
but the medulla may be removed with no life-threatening effects.
The pancreas is a long, soft organ that lies transversely along the
posterior abdominal wall, posterior to the stomach, and extends from
the region of the duodenum to the spleen. This gland has an exocrine
portion that secretes digestive enzymes that are carried through a
duct to the duodenum. The endocrine portion consists of the pancreatic
islets, which secrete glucagons and insulin.
Male sex hormones, as a group, are called
androgens. The principal androgen is testosterone, which is secreted
by the testes. A small amount is also produced by the adrenal cortex.
Production of testosterone begins during fetal development, continues
for a short time after birth, nearly ceases during childhood, and then
resumes at puberty. This steroid hormone is responsible for:
Two groups of female sex hormones are produced in the ovaries, the
estrogens and progesterone. These steroid hormones contribute to the
development and function of the female reproductive organs and sex
characteristics. At the onset of puberty, estrogens promotes: