k5 - homeostasis

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    Acid-Base BalanceNormal pH of body fluids

    Arterial blood is 7.4

    Venous blood and interstitial fluid is 7.35Intracellular fluid is 7.0

    Alkalosis or alkalemia arterial blood pH

    rises above 7.45 Acidosis or acidemia arterial pH dropsbelow 7.35 (physiological acidosis)

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    Bica rbonate B uff er System A mixture of carbonic acid (H 2CO 3) and itssalt, sodium bicarbonate (NaHC O 3)

    (potassium or magnesium bicarbonateswork as well)If strong acid is added:

    Hydrogen ions released combine with thebicarbonate ions and form carbonic acid (a weakacid)The pH of the solution decreases only slightly

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    Bica rbonate B uff er SystemIf strong base is added:

    It reacts with the carbonic acid to form sodium

    bicarbonate (a weak base)The pH of the solution rises only slightly

    This system is the only important ECFbuffer

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    P h osph ate B uff er SystemNearly identical to the bicarbonate systemIts components are:

    Sodium salts of dihydrogen phosphate (H 2PO 4 ),a weak acidM onohydrogen phosphate (HP O 42 ), a weakbase

    This system is an effective buffer in urineand intracellular fluid

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    P rotein B uff er SystemPlasma and intracellular proteins are thebodys most plentiful and powerful buffers

    Some amino acids of proteins have:Free organic acid groups (weak acids)Groups that act as weak bases (e.g., aminogroups)

    Amphoteric molecules are proteinmolecules that can function as both a weakacid and a weak base

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    P h ysiological B uff er SystemsThe respiratory system regulation of acid-base balance is a physiological buffering

    systemThere is a reversible equilibrium between:

    Dissolved carbon dioxide and water

    Carbonic acid and the hydrogen and bicarbonateionsCO 2 + H2O m H2CO 3 m H+ + HCO 3

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    P h ysiological B uff er SystemsDuring carbon dioxide unloading, hydrogenions are incorporated into water When hypercapnia or rising plasma H +occurs:

    Deeper and more rapid breathing expels morecarbon dioxideHydrogen ion concentration is reduced

    Alkalosis causes slower, more shallowbreathing, causing H + to increaseRespiratory system impairment causesacid-base imbalance (respiratory acidosisor respiratory alkalosis)

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    Renal Mec h anisms of Acid-Base

    BalanceChemical buffers can tie up excess acids or bases, but they cannot eliminate them from the

    bodyThe lungs can eliminate carbonic acid byeliminating carbon dioxideO nly the kidneys can rid the body of metabolicacids (phosphoric, uric, and lactic acids andketones) and prevent metabolic acidosisThe ultimate acid-base regulatory organs arethe kidneys

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    Renal Mec h anisms of Acid-Base

    BalanceThe most important renal mechanisms for regulating acid-base balance are:

    Conserving (reabsorbing) or generating newbicarbonate ionsExcreting bicarbonate ions

    Losing a bicarbonate ion is the same asgaining a hydrogen ion; reabsorbing abicarbonate ion is the same as losing ahydrogen ion

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    Renal Mec h anisms of Acid-Base

    BalanceHydrogen ion secretion occurs in the PCTand in type A intercalated cells

    Hydrogen ions come from the dissociationof carbonic acid

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    Rea bsor pti on of Bica rbo nateCarbon dioxide combines with water in tubulecells, forming carbonic acid

    Carbonic acid splits into hydrogen ions andbicarbonate ionsFor each hydrogen ion secreted, a sodium ionand a bicarbonate ion are reabsorbed by thePCT cellsSecreted hydrogen ions form carbonic acid;thus, bicarbonate disappears from filtrate at thesame rate that it enters the peritubular capillaryblood

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    Rea bsor pti on of Bica rbo nateCarbonic acidformed in filtratedissociates torelease carbondioxide andwater Carbon dioxidethen diffuses into

    tubule cells,where it acts totrigger further hydrogen ion

    secretion F igur e 26.12

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    G ene r atin g New Bica rbo nate I onsTwo mechanisms carried out by type Aintercalated cells generate new bicarbonate

    ionsBoth involve renal excretion of acid viasecretion and excretion of hydrogen ions or ammonium ions (NH

    4+)

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    Hyd rog en I on Exc r etionDietary hydrogen ions must be counteractedby generating new bicarbonate

    The excreted hydrogen ions must bind tobuffers in the urine (phosphate buffer system)Intercalated cells actively secrete hydrogenions into urine, which is buffered and

    excretedBicarbonate generated is:M oved into the interstitial space via a cotransportsystem

    Passively moved into the peritubular capillary blood

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    Hyd rog en I on Exc r etionIn response toacidosis:

    Kidneysgeneratebicarbonate ionsand add them to

    the blood An equal amountof hydrogen ionsare added to the

    urine F igur e 26.13

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    Amm onium

    Ion Exc r etion

    F igur e 26.14

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    Bica rbonate I on Secr etionWhen the body is in alkalosis, type Bintercalated cells:

    Exhibit bicarbonate ion secretionReclaim hydrogen ions and acidify the bloodThe mechanism is the opposite of type Aintercalated cells and the bicarbonate ion

    reabsorption processEven during alkalosis, the nephrons andcollecting ducts excrete fewer bicarbonateions than they conserve

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    Respi r ator y Acid osis and Alkal osisResult from failure of the respiratory system tobalance pH

    PCO 2 is the single most important indicator of respiratory inadequacyPCO 2 levels

    Normal P CO 2 fluctuates between 35 and 45 mmHgValues above 45 mm Hg signal respiratoryacidosisValues below 35 mm Hg indicate respiratoryalkalosis

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    Respi r ator y Acid osis and Alkal osisRespiratory acidosis is the most commoncause of acid-base imbalance

    O ccurs when a person breathes shallowly, or gasexchange is hampered by diseases such aspneumonia, cystic fibrosis, or emphysema,impaired activity of diaphragm muscle and

    impaired respiratory control in the brain stemRespiratory alkalosis is a common result of hyperventilation and is caused by low levelof oxygen in the plasma, meningitis, headinjury and anxiety

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    Meta bolic Acid osis All pH imbalances except those caused byabnormal blood carbon dioxide levelsM

    etabolic acid-base imbalance bicarbonate ion levels above or belownormal (22-26 mEq/L)M etabolic acidosis is the second most

    common cause of acid-base imbalanceTypical causes are ingestion of too much alcoholand excessive loss of bicarbonate ions,such asdiarrhea, and excessive vomittingO ther causes include accumulation of lactic acid,shock, ketosis in diabetic crisis, starvation, and+

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    Meta bolic Alkal osisRising blood pH and bicarbonate levelsindicate metabolic alkalosis

    Typical causes are:Vomiting of the acid contents of the stomachHypokalemiaIntake of excess base (e.g., from antacids)Constipation, in which excessive bicarbonate isreabsorbed

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    Respi r ator y and RenalC

    ompensati ons Acid-base imbalance due to inadequacy of a physiological buffer system is

    compensated for by the other systemThe respiratory system will attempt to correctmetabolic acid-base imbalancesThe kidneys will work to correct imbalancescaused by respiratory disease

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    Respi r ator yC

    ompensati onIn metabolic acidosis:

    The rate and depth of breathing are elevated

    Blood pH is below 7.35 and bicarbonate level islow As carbon dioxide is eliminated by the respiratorysystem, P CO 2 falls below normal

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    Respi r ator yC

    ompensati onIn metabolic alkalosis:

    Compensation exhibits slow, shallow breathing,

    allowing carbon dioxide to accumulate in theblood

    Correction is revealed by:High pH (over 7.45) and elevated bicarbonate ionlevelsRising P CO 2

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    RenalC

    ompensati onTo correct respiratory acid-base imbalance,renal mechanisms are stepped up

    Acidosis has high P CO 2 and highbicarbonate levels

    The high P CO 2 is the cause of acidosisThe high bicarbonate levels indicate the kidneysare retaining bicarbonate to offset the acidosis

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    RenalC

    ompensati on Alkalosis has Low P CO 2 and high pH

    The kidneys eliminate bicarbonate from the body

    by failing to reclaim it or by actively secreting it

    InterActive Physiology :Fluid, Electrolyte, and Acid/Base Balance: Acid/Base Homeostasis

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