Homesostasis - Physiology - Lecture Notes | Study notes Physiology

HOMEOSTASIS

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Humanbodyismadeupofonehundredbillionsofcells.Inotherofallcellstoworkproperly,together,in

harmony,itisnecessarytobecreatedanenvironmentwhoseparameterstobemaintainedconstant,

independentoftheexternalenvironmentchanges.Thenotionofinternalenvironmentwasintroducedby

ClaudeBernardin1865.Hestatedthatmaintainingconstantparameterofenvironmentisnecessaryfor

functionofthecells.

Thenotionofhomeostasiswasintroducedin1932byWalterCannon,namelymaintainingcon stantthe

parametersoftheinternalenvironmentofthehumanbody.Thenormalfunctionofcellsdepends,ofthe

normalpartialpressureofoxygen,ofcarbondioxide,maintainingconstantbodytemperature,bloodpressure ,

osmolarity,pH,concentrationofdifferentchemicalsubstances.Alotofmechanismsintervene.These

mechanismsarenamedhomeostaticmechanism.Allorgansofthehumanbodyparticipatetoperformthe

homeostasis,forinstance,thedigestivetractsuppliesthenutritivesubstancesbyabsorptionofthese

substancesandtheirdigestion,lungsprovidesthebodywithoxygenandremovecarbondioxide,kidneys

removealotofwasteproduct,speciallyforproteinmetabolism,andcardiovascularsystembymeansofblood

circulationcontributestothesupplyofallcellinfoodandtheirwasteproducttoberemoved.Theactivityofall

theseorgansisregulatedbythenervoussystemandtheendocrinesystem.

Theorganhasalotofhomeostaticmechanismwhichintervenesinthesametime,orinsuccession,themore

importantistheparameter,themorehomeostaticmechanismsareinvolvedinkeepingitconstant.

Homeostaticmechanismsareoftwotypes,namelyfeedbackmechanism,andfeedforwardmechanisms.

FEEDBACK:INTHEIRTURNARENEGATIVEFEEDBACKMECHANISM,AND

POSITIVEFEEDBACKMECHANISM

NEGATIVEFEEDBACKMECHANISMS:Arecharacterizedbyreducingtheoutputortheactivityofanorgan

orasystemtoitsinitialactivity.Example:ifacertainparamete risincreased(glucose)bynegativefeedback

mechanism,theglucoseisbroughtbacktoitsnormalvalue.Ifit’sreducedtheorganismincreasesitbacktoits

normalvalue.Themechanismisintheoppositedirectiontothevariationofthemechanism.Deviationfrom

thenormalvalueofaparameter(glucose,bodytemperature,…)inducedbyastimulusissensedbyadetector

orasensor,orareceptor,isfelt.Theinformationistransmittedtoacontroller,orandoperator,oracenter,

wherethesignalortheinformationwillbecomparedtoanormalvaluenamedsetpoint.Ifthereisadifference

thevalueandsetpoint,thenthecontrollerwillgenerateanerrorsignalthatwillbetransmittedtoaneffector

inordertobringbacktheparametertothevalueofsetpoint.Eachcomponentofnegativefeedback

mechanismcontrolsthenextcomponent.Ifthebloodpressuredecreasesundertheactionofbleeding,thisis

sensedbysomedetectorscalledbaroreceptoratthelevelorcarotidsinusandaorticarch,theyareableto

sensethevariationofbloodpressure,theydischargesignalstransmittedtomedullaoblongata,ifthepressure

isnotthesetpoint,thenthecenteremitanerrorsignaltobringbackthenormalvalue,theheartwillstartto

contractwithahigherintensity,avasoconstrictionoccurs,andthevenousreturnisalsoincreased,allthis

increasesthebloodpressureandbringitbacktothesetpoint.Theeffectivecommunicationbetweensensor

andeffectorkeepstheoscillationoftheregulatedparametertoaminimum.Thistypeofmechanismscannot

preventanewchangeoftheparameter,oncetheyarebroughtbacktotheinitiallevel.Similarfeedback

mechanismtomaintainingbloodpressureexistinmaintainingconstantpH,osmolarity,glucoseconcentration

inblood,oxygenpressure,carbondioxidepressure.

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HOMEOSTASIS

Human body is made up of one hundred billions of cells. In other of all cells to work properly, together, in harmony, it is necessary to be created an environment whose parameters to be maintained constant, independent of the external environment changes. The notion of internal environment was introduced by Claude Bernard in 1865. He stated that maintaining constant parameter of environment is necessary for function of the cells. The notion of homeostasis was introduced in 1932 by Walter Cannon, namely maintaining constant the parameters of the internal environment of the human body. The normal function of cells depends, of the normal partial pressure of oxygen, of carbon dioxide, maintaining constant body temperature , blood pressure , osmolarity , pH , concentration of different chemical substances. A lot of mechanisms intervene. These mechanisms are named homeostatic mechanism. All organs of the human body participate to perform the homeostasis, for instance, the digestive tract supplies the nutritive substances by absorption of these substances and their digestion, lungs provides the body with oxygen and remove carbon dioxide, kidneys remove a lot of waste product, specially for protein metabolism, and cardio vascular system by means of blood circulation contributes to the supply of all cell in food and their waste product to be removed. The activity of all these organs is regulated by the nervous system and the endocrine system. The organ has a lot of homeostatic mechanism which intervenes in the same time, or in succession, the more important is the parameter, the more homeostatic mechanisms are involved in keeping it constant. Homeostatic mechanisms are of two types, namely feedback mechanism, and feed forward mechanisms.

F EEDBACK : I N THEIR TURN ARE NEGATIVE FEEDBACK MECHANISM , AND

POSITIVE FEEDBACK MECHANISM

N EGATIVE F EEDBACK MECHANISMS : Are characterized by reducing the output or the activity of an organ

or a system to its initial activity. Example: if a certain parameter is increased (glucose) by negative feedback mechanism, the glucose is brought back to its normal value. If it’s reduced the organism increases it back to its normal value. The mechanism is in the opposite direction to the variation of the mechanism. Deviation from the normal value of a parameter (glucose, body temperature,…) induced by a stimulus is sensed by a detector or a sensor, or a receptor, is felt. The information is transmitted to a controller, or and operator, or a center, where the signal or the information will be compared to a normal value named set point. If there is a difference the value and set point, then the controller will generate an error signal that will be transmitted to an effector in order to bring back the parameter to the value of set point. Each component of negative feedback mechanism controls the next component. If the blood pressure decreases under the action of bleeding, this is sensed by some detectors called baroreceptor at the level or carotid sinus and aortic arch, they are able to sense the variation of blood pressure, they discharge signals transmitted to medulla oblongata, if the pressure is not the set point, then the center emit an error signal to bring back the normal value, the heart will start to contract with a higher intensity, a vasoconstriction occurs, and the venous return is also increased, all this increases the blood pressure and bring it back to the set point. The effective communication between sensor and effector keeps the oscillation of the regulated parameter to a minimum. This type of mechanisms cannot prevent a new change of the parameter, once they are brought back to the initial level. Similar feedback mechanism to maintaining blood pressure exist in maintaining constant pH, osmolarity, glucose concentration in blood, oxygen pressure, carbon dioxide pressure.

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P OSITIVE FEEDBACK MECHANISMS : They change instability and even death. This kind of mechanism is not often used by the organism, because there is a risk that increase of the changeable parameter to become uncontrolled, the risk is to increase the variation, that’s why positive feedback mechanism work together with negative feedback mechanism. The variation of the parameter is not brought back to the normal value, but it is increased or decreased more, depending on the direction of evolution of the parameter. Nevertheless there are some positive feedback mechanism which are useful for body, example the mechanism of coagulation of blood, or generation of nerve impulse, release of oxytocin during delivery to increase uterus contractions, ovulation. For example: the coagulation of blood is an enzymatic process by which a substance is activated and can activate the other substances who activates others. It is also a positive feedback mechanism that increases more the parameter. F EED FORWARD MECHANISMS : ARE INVOLVED IN PREVENTING THE ERRORS GENERATED BY PERTURBING FACTORS. T HAT ’ S WHY THEY TAKE CORRECTIVE ACTIONS THAT ANTICIPATE THE CHANGE. When the change with time is necessary, a common signal is generated which specify the target, in this way the gradual intervention of controller makes the parameter that is regulated to not be sensed. These mechanisms suppress the disturbance before it has the chance to affect the essential parameters of the system. Example : When a person is going to make a physical activity, muscles need more blood, more oxygen, more ATP, more blood, before starting activity it increases the blood supply at the level of muscle that will be involved in activity. These homeostatic mechanisms are not able to maintain all the time the parameters, that’s why they are over‐ helmed disease occurs. The human body becomes older, the efficiency of homeostatic systems decreases, so when we become older our internal environment becomes less regulated. docsity.com

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HOMEOSTASIS

F EEDBACK : I N THEIR TURN ARE NEGATIVE FEEDBACK MECHANISM , AND

POSITIVE FEEDBACK MECHANISM

N EGATIVE F EEDBACK MECHANISMS : Are characterized by reducing the output or the activity of an organ

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