The adrenal gland

what is the process of adrenal glands

the adrenal glands

he adrenal glands he two adrenal glands are named for their position in the body above (ad meaning "near") the kidneys (renal mean-g "kidney"). Each of these triangular glands has two parts 'with two different functions. The adrenal cortex  is the outer, yellowish portion of each adrenal gland. he word cortex comes from a Latin word meaning "bark" d is often used to refer to the outer covering of a tissue,organ, or gland. The adrenal medulla is the inner, reddishprortion of the gland and is surrounded by the cortex. Notsurprisingly, the word medulla comes from a Latin word caning "marrow" or "middle." 

The adrenal cortex 

As you may recall, the anterior pituitary gland secretes the hormone ACTH, adrenocorticotropic hormone. This hormone, as its name implies, stimulates the adrenal cortex to secrete a group of hormones known as corticosteroids. These steroid hormones act on the nucleus of target cells, triggering the cell's hereditary material to produce certain proteins. The two main types of corticosteroids produced by the adrenal cortex are the mineralocorticoids and the glucocorticoids. 

The mineralocorticoids are involved in the regulation of the levels of certain ions within the body fluids. The most important of this group of hormones is aldosterone. It affects tubules within the kidneys, stimulating them to reabsorb sodium ions and water from the urine that is being produced, putting these substances back into the blood-stream. The secretion of aldosterone is triggered when the volume of the blood is too low, such as during dehydration or blood loss. Special cells in the kidneys "monitor" the blood pressure. When the blood pressure drops, these cells secrete an enzyme that begins a chain of reactions ending with the secretion of aldosterone. Conversely, when the blood pressure is within a normal range, the cell "detectors" in the kidneys are not stimulated, the release of aldosterone is not triggered, and the kidney tubules are not stimulated to conserve sodium and water. 

 The glucocorticoids affect glucose metabolism, causing molecules of glucose to be manufactured in the body from non-carbohydrates such as proteins. This glucose enters the bloodstream, is transported to the cells, and is used for energy as part of body's reaction to stress.

Almost everyone is familiar with the term stress. And almost everyone can give examples of stressful situations: their boss "chewing them out" in front of co-workers, their kids fighting constantly with one another, or their sustaining a physical injury. The stress reaction was first described in 1936 by Hans Selye, a researcher who has since become the acknowledged authority on stress. Dr. Selye explained how the body typically reacted to stress—any disturbance that affects the body—and called this reaction the general adaptation syndrome. Over a prolonged period of stress, the body reacts in three stages: (1) the alarm reaction, (2) resistance, and (3) exhaustion. Contrary to maintaining homeostasis within the body, the general adaptation syndrome works to help the body "gear up" to meet an emergency. During the alarm reaction the body goes into quick action. Imagine that you just entered your place of work and your boss confronted you, accusing you—in front of the office staff—of making a costly mistake. Your body reacts with a quickening pulse, increased blood flow, and an increased rate of chemical reactions within your body. Why does your body react in this way? Although the adrenal cortex is involved in the stress reaction, the beginning of the story lies in an understanding of the middle section of the adrenal glands, the adrenal medulla.

The adrenal medulla 

The adrenal medulla is different from most other endocrine tissue in that its cells arc derived from cells of the peripheral nervous system and are specialized to secrete hormones. These cells arc triggered by the        au

tonomic nervous system, which controls involuntary “automatic" responses. The other major nervous tissues With direct endocrine function are the secretory portion of the hypothalamus in the brain and the posterior Pituitary just under the hypothalamus.

The two principal hormones made by the adrenal medulla are adrenaline and noradrenaline (also called epi. nephrine and norepinephrine). These two hormones are pl.'. manly responsible for the alarm reaction. The hypothalamus is responsible for sending the "alarm signal"  to the adrenal medulla. The hypo. thalamus picks up the alarm signal as it monitors changes in the emotions and carries it as nerve signals on tracts of neurons that connect the hypothalamus with the emotional centers in the cerebral cortex. It can therefore sense when the body perceives an emotional stress. It can also sense physical stress, such as cold, bleeding, and poisons in the body. The hypothalamus reacts to stress by readying the body for "fight or flight," first triggering the adrenal medulla to dump adrenaline and noradrenaline into the bloodstream. These hormones cause the heart rate and breathing to quicken, the rate of chemical reactions to increase, and glucose (stored in the liver) to be dumped into the bloodstream. In general, the actions of adrenaline and noradrenaline in-crease the amounts of glucose and oxygen available to the organs and tissues most used for defense: the brain, heart, and skeletal muscles.

The thyroid gland

what is the prolactin

Prolactin is another hormone secreted by the anterior pituitary. Prolactin works with estrogen, progesterone, and other hormones to stimulate the mammary glands in the breasts to secrete milk after a woman has given birth to a child. During the menstrual cycle, milk is not produced and secreted because prolactin levels in the bloodstream are very low. Late in the menstrual cycle, however, as the levels of progesterone and estrogen fall, the pituitary is stimulated by the hypothalamus to secrete some prolactin. This rise in prolactin, although not sufficient to cause milk production, does cause the breasts of some women to feel sore before menstruation. After menstruation, estrogen levels begin to rise, and prolactin secretion is once again inhibited. 

Melanocyte-stimulating hormone (MSH)

 acts on cells in the skin called melanocytes, which synthesize a pigment called melanin. This pigment is taken up by epideimal cells in the skin, producing skin colorations from pale yellow(in combination with another pigment called carotene) to black. Variations are caused by the amount of pigment the melanocytes produce; this variation is genetically deter-mined and is an inherited characteristic.

The posterior pituitary 

The posterior lobe of the pituitary stores and releases two hormones that are produced by the hypothalamus: antidiuretic hormone (ADH) and oxytocin. ADH helps control the volume of the blood by regulating the amount of water reabsorbed by the kidneys. For example, receptors in the hypothalamus can detect a low blood volume by detecting when the solute concentration of the blood is high. When the hypothalamus detects such a situation, it triggers its specialized neurosecretory cells to make ADH. This hormone is transported within axons to the posterior pituitary, which releases the hormone into the bloodstreambinds to target cells in the collecting ducts of the nephrons of the kidneys, increasing their permeability. More water then moves out of these ducts and back into the blood, resulting in a more concentrated urine. ADH also acts on the smooth muscle surrounding arterioles. As these muscles tighten, they constrict the arterioles, an action that helps raise the blood pressure. Alcohol supresses ADH release, which is why excessive drinking leads to the production of excessive quantities of urine and eventually to dehydration.

 Oxytocin is another hormone of the posterior pituitary: it is produced in the hypothalamus and transported within axons to the posterior pituitary for secretion. In women, oxytocin is secreted during the birth process, triggered by a stretching of the cervix of the uterus at the beginning of the birth process. Oxytocin binds to target cells of the uterus, enhancing the contractions already taking place. The mechanism of oxytocin secretion is an example of a positive feed-back loop in which the effect produced by the hormone enhances the secretion of the hormone. For this reason, oxytocin is used by physicians to induce uterine contractions when labor must be brought on by external means. Oxytocin also targets muscle cells around the ducts of the mammary glands, allowing a new mother to nurse her child. The suck-ling of the infant triggers the production of more oxytocin, which aids in the nursing process and helps contract the uterus to its normal size.

what is the thyroid gland of human

The thyroid gland

Sitting like a large butterfly just below the level of the voice box, the thyroid gland can be thought of as your "metabolic switch." This gland secretes hormones that determine the rate of the chemical reactions of your body's cells. Put sim-ply, thyroid hormones determine how fast bodily processes take place.
 The thyroid hormones are thyroxine (T4) and triiodothyronine (T3). These hormones are called amines: single, modified amino acids. They are not considered to be "true" peptide hormones, however, because they act on the DNA of target cells as steroid hormones do. They are also unique because an inorganic ion—iodine—is part of their structures.

 Your body uses iodine in the food you eat to help make the thyroid hormones; the 3 or 4 in each hormone name refers to the number of atoms of iodine in each hormone. Foods such as seafood and iodized salt are good sources of dietary iodine. If the diet contains an insufficient amount of iodine, the thyroid gland enlarges. This condition is called a hypothyroid goiter. The hypothalamus and the thyroid gland work together to keep the proper level of thyroid hormone circulating in the bloodstream. This level is detected by the hypothalamus. A low level of thyroid hormones stimulates the hypothalamus to secrete a releasing factor—a chemical message—to the anterior pituitary. This message tells the pituitary torelease more TSH. The thyroid responds, thereby raising the blood level of T3 and T4 back to normal. This mechanism of action is an example of a negative feed-back loop in which the effect produced by stimulation of a gland "shuts down" the stimulus. Shutdown occurs when a sufficient effect has been produced, similar to the mechanism of a thermostat. In your home, your furnace is triggered to go on when the temperature goes below the thermostat setting. The furnace stays on until the house heats up to the desired level. The thermostat then signals the furnace to turn off.

In certain disease conditions the amount of thyroid hormones in the bloodstream cannot be regulated properly. If the thyroid produces too much of the thyroid hormones, a person may feel as though the "engine is racing," with such symptoms as a rapid heartbeat, nervousness, weight loss, and protrusion of the eyes

. This condition is called hyperthyroidism. On the other hand, if the thyroid produces too little of the thyroid hormones, a person may feel "run down," with such symptoms as weight gain and slow growth of the hair and fingernails. This condition is called hypothyroidism. Various factors can be the under-lying cause of such problems; often medication or surgery can correct the situation.

In addition to secreting the thyroid hormones, the thyroid gland secretes a hormone called calcitonin, or CT. This hormone works to balance the effect of another hormone called parathyroid hormone, or PTH. PTH regulates the concentration of calcium in the b
loodstream. Calcium is an important structural component in bones and teeth and aids in the proper functioning of nerves and muscles.

What is  The parathyroid glands

the parathyroid glands 

The parathyroid glands Embedded in the posterior side of the thyroid are the para-thyroid glands. Most people have two parathyroids on each of the two lobes of the thyroid. These are the glands that secrete PTH, which works antagonistically to CT to help maintain the proper blood levels of various ions, primarily calcium. Two of the many problems related to abnormal calcium levels in the blood are kidney stones and osteoporosis. If calcium levels in the blood remain high, tiny masses of calcium may develop in the kidneys. These masses, called kidney stones, can partially block the flow of the urine from a kidney. If calcium levels in the blood remain low, calcium may be removed from the bones, a disorder known as osteoporosis. Osteoporosis is most common in middle-aged and elderly women, who have stopped secreting estrogen at menopause . Estrogen stimulates bone cells to take calcium from the blood to build hone tissue.

PTH and CT work in the following way to keep calcium at an optimum level in the blood: If the calcium level is too low, PTH stimulates the activity of osteoclasts, or bone-destroying cells. These cells liberate calcium from the bones and put it into the bloodstream. PTH also stimulates the kidneys to reabsorb calcium from urine that is being formed and stimulates cells in the intestines to absorb an increased amount of calcium from digested food. CT acts antagonistically to PTH. When the level of calcium in the blood is too high, less PTH is secreted by the parathyroids and more a is secreted by the thyroid. The CT inhibits the release of calcium from bone and speeds up its absorption, decreasing the levels of calcium in the blood. These interactions of PTH and CT are an example of a negative feedback loop that does not involve the hypothalamus or pituitary gland. The level of calcium in the blood directly stimulates the thyroid and parathyroid glands .

the pituitary gland

How to controlling pituitary

The pituitary gland

The pituitary gland The pituitary is a powerful gland that secretes nine different hormones. Although it secretes so many hormones, it is amazingly tiny—about the size of a marble. The pituitary "marble" hangs from the underside of the brain, supported and cradled within a bony depression of the sphenoid bone. 

Controlling the pituitary: The hypothalamus

 The pituitary secretes seven major hormones from its larger front portion, or lobe, the anterior pituitary. It secretes two from its rear lobe, the posterior pituitary. The secretion of these hormones is regulated by a mass of nerve cells that lies directly above the pituitary, making up a small part of the "floor" of the brain. This regulatory nervous tissue, the hypothalamus, is connected to the pituitary by a stalk of tissue  . The hypothalamus uses information it gathers from the peripheral nerves and other parts of the brain to stimulate or inhibit the secretion of hormones from the anterior pituitary. In this way, the by pothalamus acts like a production manager, receiving in-  formation about the needs of the company's customers and regulating the production of products to satisfy those needs. The hypothalamus accomplishes its management job by producing releasing hormones that affect the secretion of specific hormones from the anterior pituitary. The hypothalmus also produces two hormones that do not regulate hormonal release in the pituitary. When they are needed by the body, the hypothalamus signals the pituitary to release them. 

The pituitary is a tiny gland that hangs from the  underside of the brain. The secretion of its many diverse hormones is controlled by a mass of nerve cells lying directly above it called the hypothalamus. The hypothalamus stimulates or inhibits the secretion of hormones from the pituitary by means of re-leasing hormones. In addition, the hypothalamus produces two hormones that it stores in the pituitary. 

The anterior pituitary

The seven hormones produced by the anterior pituitary regulate a wide range of bodily functions . Four of these hormones are called tropic hormones. The word tropic comes from a Greek word meaning "turning" and refers to the ability of tropic hormones to turn on or stimulate other endocrine glands. Of the four tropic hormones, two are gonadotropins. The gonads are the male and female sex organs, the testes and the ovaries. The gonadotropins are hormones that affect these sex organs (considered endocrine glands because they secrete sex hormones). The two gonadotropins are follicle stimulating hormone (FSH) and luteinizing hormone (LH). In females, FSH targets the ovaries and triggers the maturation of one egg each month. In addition, it stimulates cells in the ovaries to secrete female sex hormones called estrogens. In men, FSH targets the testes and triggers the production of sperm. LH stimu
lates cells in the testes to produce the male sex hormone testosterone. In females, a surge of LH near the middle of the menstrual cycle stimulates the release of an egg. In addition, LH triggers the development of cells within the ovaries that produce another female sex hormone—progesterone. (See Chapter 20 for the organs and processes of the reproductive system.)

  The two other tropic hormones are adrenocorticotropic hormone (ACTH) and thyroid-stimulating hormone (TSH). ACTH triggers the adrenal cortex to produce certain steroid hormones. The adrenal glands are located on top of the kidneys (see Figure 19-1). Each of these two glands has two distinct parts: an outer cortex and an inner medulla. ACTH stimulates the adrenal cortex to produce hormones that regulate the production of glucose from "non-carbohydrates" such as fats and proteins. Others regulate the balance of sodium and potassium ions in the blood. Still others contribute to the development of the male secondary sexual characteristics. TSH triggers the thyroid gland to produce the three thyroid hormones.. This endocrine gland is located on the front of the neck, just below the voice box (see Figure 19-1). Its hormones control normal growth and development and are essential to proper metabolism.
The front portion of the pituitary, the anterior pituitary, secretes seven hormones. Of these seven, four stimulate other endocrine glands and are called tropic hormones.
Growth hormone (GH) is produced by the anterior pituitary and works with the thyroid hormones to control normal growth. GH increases the rate of growth of the skeleton by causing cartilage cells and bone cells to reproduce and lay down their inter cellular matrix. In addition, GH stimulates the deposition of minerals within this matrix. GH also stimulates the skeletal muscles to grow in both size and number. In the past, children who did not produce enough GH did not grow to an average height; this condition is called hypo-pituitary dwarfism. However, in the past decade, scientists have been able to use the techniques of genetic engineering  to insert the human GH gene into bacteria to produce human GH. Currently.


 Sandy Allen is shown here with her mother, brother, an dog. Giantism is caused by the over secretion of growth hormone.

this laboratory-made hormone is being used successfully treating growth disorders caused by hyposecretion (UN production) of GH in children. The opposite problem also occur: during the growth years, some children pr too much GH. This hypersecretion (overproduction) cause the long bones to grow unusually long (Figure 19-and result in a condition known as giantism. In adults, h persecretion of GH causes the bones of the hands and fa to thicken, resulting in a condition known as acromegaly .

Human Hormones

what is the hormone

Shooting up in the locker room with anabolic steroids has caused the downfall of many winners. These controversial drugs are really synthetic hormones, chem. icals that affect the activity of specific organs or tissues. The various hormones of the body all affect their "target" tissues in unique ways. Anabolic steroids affect the body in ways similar to the male sex hormone testosterone and stimulate the buildup of muscles. But along with building a championship body, anabolic steroids strikingly change the body's metabolism.

                                                                 

 Female athletes on steroids experience side effects such as shrinking breasts, a deepening voice, and an increase in body hair. Male athletes find that their testicles shrink. some users also experience life-threatening kidney and liver damage. Youngsters who take these drugs risk stunting their growth, since anabolic steroids cause bones to stop growing prematurely. And a great deal of controversy still surrounds claims that anabolic steroids can cause psychological effects such as "steroid rage," a state of mind in which users attack people and things around them. Today, scientists still lack solid scientific data regarding all aspects and consequences of anabolic steroid use. How-ever, it is clear that their use is risky at the least—and may put users in the cemetery rather than in the winner's circle.

  what is the human endocrine system

Endocrine gland and their hormone

A hormone is a "chemical messenger" sent by a gland to other cells of the body. Traditionally, animal hormones have been described by scientists as the chemical products of glands that travel within the bloodstream to all parts of the body, causing an effect on specific cells, or target organs, far removed from that gland. Glands are individual cells or groups of cells that secrete substances. Their secretory portions are made up of specialized epithelial cells (see Chapter 9). The glands that secrete hormones spill these chemicals directly into the bloodstream and are called endocrine, or "ductless," glands. Glands that secrete other substances such as digestive enzymes or sweat route their secretions to specific destinations by means of ducts. In this way, for example, the digestive enzyme pancreatic amylase flows directly from the pancreas to the small intestine and goes nowhere else. Glands having associated "ductwork" are called exocrine glands.

Today, most scientists have expanded their definition of hormones to include any chemical produced by one cell that causes an effect in another. Included in this description, then, are substances such as neurotransmitters chemicals produced by the axon end of a nerve cell that travel to and bind with the dendrite end of an adjacent nerve cell, con-tributing to the propagation of the nerve impulse along that neuron (see Chapter 15). Such chemicals are often called local hormones because they affect neighboring target cells. The human body produces many local hormones; they are described later in this chapter.

 Hormones are "chemical messengers" secreted by cells that affect other cells. Hormones that travel within the bloodstream and affect cells in another part of the body are called endocrine hormones. Hormones that do not travel within the bloodstream but only affect cells lying near the secretory cells are called local hormones.

The 10 different endocrine glands of the human body make over 30 different hormones. Together, these glands are called the endocrine system (Figure 19-1). The endocrine system works with the nervous system to integrate the functioning of the various tissues, organs, and organ systems of the body. The nervous system sends messages to muscles and glands, regulating muscular contraction and glandular secretion. The hormones of the endocrine system, on the other hand, carry messages to virtually any type of cell in the body. The messages of the endocrine hormones are varied but can be grouped into four categories: 

1. Regulation: Hormones control the internal environment of the body by regulating the secretion and excretion of various chemicals in the blood, such as salts and acids.

 2. Response:Hormones help the body respond to changes in the environment and cope with physical and psycho-logical stress. 

3. Reproduction: Hormones control the female reproductive cycle and other reproductive processes essential to conception and birth and control the development of sex cells, the reproductive organs, and secondary sexual characteristics (those that make men and women different) in both sexes. 4. Growth and development: Hormones are essential to the proper growth and development of the body from conception to adulthood. 

Once molecules of a hormone are released into the bloodstream, they travel throughout the body. Although hormone molecules may pass billions of cells, specific hormones only affect specific cells called target cells. Hormones recognize target cells because they bind to receptor molecules embedded within the cell membrane or located within the cytoplasm of the cell. The binding of a hormone molecule to a receptor molecule activates a chain of events in the target cell that results in the effect of the hormone being expressed. 

Two major classes of endocrine hormones work within the human body: peptide hormones and steroid hormones. Peptide hormones are made of amino acids, but the amino acid chain length varies greatly from hormone to hormone. The smallest are actually modifications of the single amino acid tyrosine. Somewhat larger are short peptide hormones that are several amino acids in length. Polypeptide hormones have chain lengths of several dozen or more amino acids, such as the hormone insulin. Even larger are protein hormones that may have over 200 amino acids with carbohydrates attached at several positions. 

Human endocrine system

Unable to pass through the cell membrane, peptide hormones bind to receptor molecules embedded in the cell membrane of target cells. The binding of hormone to a receptor triggers an increase in that cell's production of a compound referred to as a second messenger. A second messenger triggers enzymes that cause the cell to alter its the endocrine glands pictured  in this diagram secrete chemical messengers that travel through the bloodstream to affect other cells in the body

How peptide hormones work

functioning in response to the hormone  For example, prolactin stimulates cells of the mammary glands to produce milk. Target cells respond as enzymes "go into action" catalyzing reactions that produce the components of mother's milk. Other types of hormone responses include the secretion of substances from target cells and the closing or opening of certain "protein doors" within target cell membranes. Cyclic adenosine monophosphate (cyclic AMP for short), a "cousin" of ATP (see Chapter 6), acts as a second messenger to many cells. Besides cyclic AMP, other second messenger molecules have been discovered.

 Once inside the cell, peptide hormones bind to the cell membrane and trigger an increase of second messenger com-pounds within the cell, such as cyclic AMP. The second Messenger in turn activates enzymes that alter the cell's function in response to the hormonal message. 

                          How steroid hormones work

Steroid hormones are all made from cholesterol, a lipid synthesized by the liver. You know cholesterol as that "dietary devil" present in certain foods such as eggs, dairy products, and beef. A characteristic of steroid hormones is their set of carbon rings. Steroid hormones, being lipid soluble, pass freely through the lipid bilayer of the cell membrane. Once inside a cell, these hormones bind to receptor molecules located within the cytoplasm of target cells. Together, the hormone-receptor complex moves into the nucleus of the cell, causing the cell's hereditary material, or DNA, to trigger the production of certain proteins . In Steroid hormones are able to pass through the cell membrane without the aid of a receptor molecule. Inside the cell, they bind with receptor molecules. The hormone-receptor complex then enters the nucleus of the cell, where it acts on DNA to produce proteins. These proteins control physiological processes such as growth and development. 

A simple feedback loop

In response to a stimulus, an endocrine gland releases a specific hormone that acts on a specific target tissue. The effect of the hormone on the target tissue either causes the gland to release more of the hormone (positive feedback) or causes the gland to slow or stop its production of the hormone (negative feedback). 

response to the sex hormones estrogen or testosterone, for example, the proteins produced are those involved in such processes as the development and maintenance of female or male sexual characteristics. 

Two main classes of endocrine hormones are pep-tide hormones and steroid hormones. Both travel within the bloodstream to all parts of the body but affect only certain target cells. Peptide hormones bind to receptors on the cell membrane of target cells and ultimately trigger enzymes that alter cell functioning. Steroid hormones bind to receptors within the cytoplasm of target cells and ultimately cause the hereditary material of the cell to produce specific proteins.

The production of hormones is regulated by a mechanism called a feedback loop. In general, hormonal feedback loops work in the following way: endocrine glands are initially stimulated to release hormones. Stimulation of an endocrine gland occurs in one of three ways:

 1. Direct stimulation by the nervous system: The sensation of fear, for example, can cause the autonomic nervous system to trigger the release of the hormone adrenaline from the adrenal medulla. 2. Indirect stimulation by the nervous system by means of re-leasing hormones: The hypothalamus is a specialized portion of the brain that produces and secretes releasing hormones. Some releasing hormones stimulate the re-lease of other hormones; some prevent the release.

 3. The concentration of specific substances in the bloodstrecon. The blood level of a substance such as glucose or calcium. for example, may signal an endocrine gland to "turn ow, or "turn off"

 After an endocrine gland secretes its hormone into the bloodstream, the hormone travels throughout the body via the circulatory system and interacts with target tissues. The target tissues cause the desired effect to be produced. This "effect" acts as a new stimulus to the endocrine gland (Figure 19-4). Put simply, the body "feeds back" information to each endocrine gland after it releases hormone. In a positive feedback loop, the information that is fed back causes the gland to produce more of its hormone. In a negative feed. back loop, the feedback causes the gland to slow down or to stop the production of its hormone. Most hormones work by means of negative feedback loops. (Specific examples of feedback mechanisms and interactions are discussed throughout this chapter.)