Reading Essentials for Biology Glencoe (794133), страница 99
Текст из файла (страница 99)
It also receivesmessages from the internal organs. When a change in homeostasis is detected, the hypothalamus stimulates the pituitary gland.READING ESSENTIALSChapter 35435NameDateClassSection35.3The Endocrine System, continued1. Evaluate What is therelationship between thehypothalamus and thepituitary gland?________________________________________________The pituitary gland (pih TEW uh ter ee) is the main gland ofthe endocrine system.The pituitary gland is located in the skull, just beneath thehypothalamus.
The hypothalamus controls the pituitary gland.The two are connected by nerves and by blood vessels. When thehypothalamus receives messages, the pituitary gland releases itsown chemicals, or it stimulates other glands to release theirchemicals. The pituitary gland controls endocrine glands, including the thyroid gland, the adrenal glands, and glands associatedwith reproduction.________________________AThe endocrine glands secrete chemicals called hormones intothe bloodstream.
Remember that a hormone is a chemical that isreleased in one part of an organism that affects another part ofthe organism. Hormones carry information to other cells in thebody. Hormones give these other cells instructions regardingmetabolism, growth, development, andLow blood sugarbehavior. Once glands release the horis detected.mones, the hormones travel in thebloodstream. Hormones then attachthemselves to target cells. Target cellshave specific binding sites for hormones.B HypothalamusThese binding sites are located eitherstimulates pituitaryon the plasma membranes or in theto release hGH.nuclei of these cells. The binding siteson target cells are called receptors.How does human growthhormone (hGH) work?PituitaryD Increased sugar in bloodsignals back to hypothalamusto slow stimulation of pituitary.C hGH stimulates liver to convertglycogen into glucose andreleases glucose into blood.436Chapter 35Human growth hormone, hGH, provides a good example of an endocrinesystem hormone.
When your body isactively growing, blood glucose levelsare slightly lowered because the growing cells use up the sugar. The hypothalamus detects this low blood glucoselevel. The hypothalamus then stimulates the production and release ofhGH from the pituitary gland into thebloodstream.
The hormone hGH bindsto receptors on the plasma membranesof liver cells. This, in turn, stimulatesREADING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.How do hormones travel?NameDateClassSection35.3The Endocrine System, continuedthe liver cells to release glucose into your blood. Your cells needthis glucose to keep growing.Negative Feedback ControlIf homeostasis is disrupted, the body responds. The endocrineglands are stimulated. Endocrine glands can be stimulated by thenervous system, by changes in blood chemistry, or by other hormones.
One type of internal feedback mechanism generally controls adjustments to the endocrine system. This is called a negativefeedback system. In a negative feedback system, the hormones,or their effects, are fed back to suppress or slow the original signal. Once homeostasis is reached, the signal stops. The hormoneis no longer released.How does the feedback system work?Copyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Most of the endocrine glands operate under a negative feedback system.
A gland synthesizes and secretes its hormone. Thehormone travels in the blood to the target cells. The neededresponse occurs in these target cells. Information concerning thehormone level or its effect on these target cells is fed back. Thefeedback is usually sent to the hypothalamus or the pituitary glandto regulate, or change, the gland’s production of the hormone.How do hormones controlled by the negativefeedback system work?Antidiuretic hormone, ADH, is one of the hormones that iscontrolled by a negative feedback system.
If you have lost waterbecause your body has been sweating, you will feel thirsty. Youfeel thirsty because the water content of your blood is reduced.The hypothalamus is able to sense the concentration of water inyour blood. The hypothalamus determines that your body isdehydrated. It responds by stimulating the pituitary gland torelease antidiuretic (AN tih di yuh reh tihk) hormone (ADH).Antidiuretic hormone (ADH) reduces the amount of water inyour urine. The hormone binds to receptors in the kidney cells.In the kidneys, the hormone ADH promotes the reabsorptionof water.
ADH also reduces the amount of water that is excretedin urine. Information about the blood water levels is constantlyfed back to the hypothalamus. The hypothalamus can then regulate the pituitary gland’s release of ADH. If the body becomesoverhydrated, or has too much water, the hypothalamus stopsstimulating the release of ADH.READING ESSENTIALS2. What does the hypothalamus do when you aredehydrated?________________________________________________________________________Chapter 35437NameDateClassSection35.3The Endocrine System, continuedHow do hormones that are controlled by a negativefeedback system control blood glucose levels?Another example of a negative feedback system involves theregulation of blood glucose levels.
Unlike most other endocrineglands, the pancreas is not controlled by the pituitary gland. Afteryou have finished eating a meal, your blood glucose levels arehigh. When the blood glucose levels are high, the pancreasreleases the hormone insulin. Insulin signals the liver and musclecells to take in glucose. This lowers the blood glucose levels.When the blood glucose levels become too low, the pancreasreleases another hormone called glucagon.
Glucagon signals cellsin the liver to release stored glycogen as glucose.Hormone Action3. What are the two groupsof hormones?________________________________________________Once hormones are released by an endocrine gland, they travelto target cells, and they cause a change. There are two basic typesof hormones. They are grouped according to how they act ontheir target cells. The two groups are steroid hormones andamino acid hormones.Hormones that are made from lipids, or fats, are called steroidhormones.
Steroid hormones are lipid-soluble. As a result, theycan diffuse freely into a cell through the cell’s plasma membrane.The hormones bind to a hormone receptor inside the cell. Thehormone, bound to its receptor, forms a hormone-receptor complex. The hormone-receptor complex travels to the cell nucleus.In the cell nucleus, this hormone-receptor complex starts theprocess for making specific messenger RNA (mRNA) molecules.The mRNA molecules move out to the cytoplasm.
The mRNAmolecules transport instructions from DNA in the nucleus for thesynthesis of the required proteins while in the cytoplasm.What are amino acid hormones?The second group of hormones is made from amino acids.Remember that amino acids can be strung together in chains.Proteins are made from long chains of amino acids. Some hormones are short chains of amino acids while other hormones arelong chains. Once amino acid hormones are secreted into thebloodstream, they bind to receptors. These receptors are embedded in the plasma membrane of the target cell.
From the plasmamembrane, they open ion channels in the membrane. Ions haveelectrical charges. These ion channels route signals down the438Chapter 35READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.What are steroid hormones?NameDateClassSection35.3The Endocrine System, continuedsurface of the membrane to activate enzymes within the cell. Theenzymes change the behavior of other molecules inside the cell.Copyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Adrenal Hormones and StressThe adrenal glands help prepare your body for stressful, difficult situations.
The adrenal glands are located on top of thekidneys. These glands consist of two parts—an inner portion andan outer portion. The outer portion secretes steroid hormones.These include glucocorticoids (glew ko KOR tuh koydz) andaldosterone (ahl DOS tuh rohn).These steroid hormones cause an increase in available glucose.They also raise blood pressure. As a result, they help the bodycombat fear, very hot or very cold temperatures, bleeding, infection, disease, and other common anxieties.The inner portion of the adrenal gland secretes two amino acidhormones. One is epinephrine (eh puh NEH frun). Epinephrineis often called adrenaline. The other hormone is called norepinephrine. Remember the fight-or-flight response? During thisresponse, the hypothalamus relays impulses to the nervous system. The nervous system stimulates the adrenal glands to increasethe output of both epinephrine and norepinephrine.
These twohormones stimulate heart rate, blood pressure, and the rate ofbreathing. They increase the efficiency of muscle contractionsand they also increase blood sugar levels.4. How are epinephrine andnorepinephrine involvedin the fight-or-flightresponse?________________________________________________________________________Thyroid and Parathyroid HormonesThe thyroid gland is located in the neck. This gland regulatesmetabolism, growth, and development. The main metabolic andgrowth hormone of the thyroid is thyroxine. Thyroxine affectsthe rate at which your body uses energy.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
