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ACID BASE BALANCE &
BODY FLUID
Ani Retno Prijanti
Renal and Body Fluids ModuleJuni 2008
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Continuous Mixing of Body Fluids
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Water Balance and ECFOsmolality
To remain properly hydrated, water intake
must equal water output
Water intake sourcesIngested fluid (60%) and solid food
(30%)
Metabolic water or water of oxidation(10%)
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Water Balance and ECFOsmolality
Water output
Urine (60%) and feces (4%)
Insensible losses (28%), sweat (8%)
Increases in plasma osmolality triggerthirst and release of antidiuretic hormone(ADH)
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Water Intake and Output
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Regulation of Water Intake
The hypothalamic thirst center is stimulated:
By a decline in plasma volume of 10%15% By increases in plasma osmolality of 12%
Via baroreceptor input, angiotensin II, and
other stimuli
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Regulation of Water Intake
Thirst is quenched as soon as we begin to
drink water
Feedback signals that inhibit the thirst centersinclude:
Moistening of the mucosa of the mouth and
throatActivation of stomach and intestinal stretch
receptors
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Regulation of Water Intake: ThirstMechanism
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Regulation of Water Output
Obligatory water losses include:
Insensible water losses from lungs and skin
Water that accompanies undigested food residues infeces
Sensible water loss of 500ml in urine
Kidneys excrete 900-1200 mOsm of solutes to maintainblood homeostasis
Urine solutes must be flushed out of the body in water
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Influence and Regulation of ADH
Water reabsorption in collecting ducts is proportional toADH release
Low ADH levels produce dilute urine and reduced volume
of body fluids
High ADH levels produce concentrated urine
Hypothalamic osmoreceptors trigger or inhibit ADH release
Factors that specifically trigger ADH release includeprolonged fever; excessive sweating,vomiting, ordiarrhea; severe blood loss; and traumatic burns
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Electrolyte BalanceElectrolytes are salts, acids, and bases, but
electrolyte balance usually refers only to saltbalance
Salts are important for:Neuromuscular excitabilitySecretory activityMembrane permeability
Controlling fluid movements
Salts enter the body by ingestion and are lostvia perspiration, feces, and urine
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Sodium in Fluid and ElectrolyteBalance
Changes in plasma sodium levels affect:
Plasma volume, blood pressure
ICF and interstitial fluid volumes
Renal acid-base control mechanisms arecoupled to sodium ion transport
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Regulation of Sodium Balance:Aldosterone
Sodium reabsorption65% of sodium in filtrate is reabsorbed in the
proximal tubules
25% is reclaimed in the DCT
When aldosterone levels are high, all remainingNa+ is actively reabsorbed
Water follows sodium if tubule permeability hasbeen increased with ADH
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Regulation of Sodium Balance:Aldosterone
The renin-angiotensin mechanism triggers the
release of aldosterone
This is mediated by the juxtaglomerular apparatus, whichreleases renin in response to:
Sympathetic nervous system stimulationDecreased filtrate osmolality
Decreased stretch (due to decreased blood
pressure)
Renin catalyzes the production of angiotensin II, whichprompts aldosterone release
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Regulation of Sodium Balance:Aldosterone
Low aldosterone cause Na excretion and water willfollow it
High aldosterone levels will cause Na absorption.
For the water to be absorbed ADH must also be present
Adrenal cortical cells are also directly stimulated to
release aldosterone by elevated K+ levels in the ECF
Aldosterone brings about its effects (diminishedurine output and increased blood volume) slowly
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What is the acid base balance
Acid-base balance is defined by theconcentration of hydrogen ions.
In order to achieve homeostasis, theremust be a balance between the intake or
production of hydrogen ions and the netremoval of hydrogen ions from the body.
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An Acid
Molecules containing hydrogen atoms thatcan release hydrogen ions in solutions arereferred to as an acid.
An example of an acid is hydrochloric acid(HCL)
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A Base
A base is an ion that can accept ahydrogen ion.
An example of a base is is the bicarbonateion.( HCO3-)
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How is Acid-Base balancemeasured
Hydrogen ion concentration is expressedon a logarithm scale using pH units(part/percentage hydrogen).
7.0 being neutral
Body systems carefully control pH of thebody within the range of 7.35-7.45
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pH
A low pH corresponds to a high hydrogenion concentration
The term Acidosis refers to the additionof excess hydrogen ions and the body hasa pH that falls below 7.35
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pH
A high pH corresponds to a low hydrogenconcentration
The term Alkalosis refers to excessremoval of hydrogen ions from the bodyand has a pH that rises above 7.45
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How the Body defends againstfluctuations in pH
Three Systems in the body:
1.Buffers in the blood
2.Respiration through the lungs
3.Excretion by the kidneys
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Buffers in the Blood
Buffers are substances that neutralizeacids or bases
Bicarbonate which is a base and carbonicacid in the body fluids protect the bodyagainst changes in acidity
These buffer systems serve as a first lineof defense against changes in the acid-base balance
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Respiration through the lungs
Carbon Dioxide which is formed duringcellular metabolism forms carbonic acid inthe blood decreasing the pH
When the pH drops respiration rateincreases this hyperventilation increasesthe amount of CO2 exhaled thereby
lowering the carbonic acid concentrationand restoring homeostasis
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Excretion by the Kidneys
The kidneys play the primary role inmaintaining long term control of Acid-Basebalance
The kidney does this by selecting whichions to retain and which to excrete
The kidneys adjust the bodys Acid-Basebalance
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The Importance of the BodysBuffering Systems
Can be quickly realized if one considers the lowconcentration of hydrogen ions in the body fluidsand the relatively large amounts of acids
produced by the body each day Example: 80 milliequvilalants of hydrogen is
either ingested or produced each day bymetabolism.
Whereas the hydrogen ion concentration of thebody fluids normally is only about .0004meq/L
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Acid-Base Balance
Normal pH
7.35 7.45
Alkalosis or alkalemia arterial blood pHrises above 7.45
Acidosis or acidemia arterial pH drops
below 7.35 (physiological acidosis)
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Sources of Hydrogen Ions
Most hydrogen ions originate from cellular
metabolism
Breakdown of phosphorus-containing proteinsreleases phosphoric acid into the ECF
Anaerobic respiration of glucose produces lacticacid
Fat metabolism yields organic acids andketone bodies
Transporting carbon dioxide as bicarbonatereleases hydrogen ions
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Hydrogen Ion Regulation
Concentration of hydrogen ions is regulatedsequentially by:
Chemical buffer systems act within seconds
The respiratory center in the brain stem actswithin 1-3 minutes
Renal mechanisms require hours to days toeffect pH changes
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Chemical Buffer Systems
Strong acids all their H+ is dissociatedcompletely in water
Weak acids dissociate partially in water and
are efficient at preventing pH changes
Strong bases dissociate easily in water andquickly tie up H+
Weak bases accept H+ more slowly(e.g.,HCO3 and NH3)
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Strong and Weak Acids
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Chemical Buffer Systems
One or two molecules that act to resist pHchanges when strong acid or base is added
Three major chemical buffer systemsBicarbonate buffer systemPhosphate buffer systemProtein buffer system
Any drifts in pH are resisted by the entirechemical buffering system
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Bicarbonate Buffer System
A mixture of carbonic acid (H2CO3) and its salt,sodium bicarbonate (NaHCO3) (potassium or
magnesium bicarbonates work as well)
Ifstrong acid is added:
Hydrogen ions released combine with the
bicarbonate ions and form carbonic acid (a
weak acid)
The pH of the solution decreases only
slightly
HCl + NaHCO3 = H2CO3 + NaCl
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Bicarbonate Buffer System
Ifstrong base is added:
It reacts with the carbonic acid to form
sodium bicarbonate (a weak base)
The pH of the solution rises only slightly
NaOH + H2CO3 = NaHCO3 + H2O
This system is the only important ECF buffer
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Phosphate Buffer System
Nearly identical to the bicarbonate systemIts components are:
Sodium salts of dihydrogen phosphate(H2PO4), a weak acid
Monohydrogen phosphate (HPO42), a weak base
HCl + Na2HPO4 = NaH2PO4 + NaCl
NaOH + NaH2PO4 = Na2HPO4 + H2O
This system is an effective buffer in urine andintracellular fluid
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Protein Buffer System
Plasma and intracellular proteins are the bodys mostplentiful and powerful buffers
Some amino acids of proteins have:
Organic acid groups (weak acids) COOH (carboxyl)
R-COOHRCOO- + H+Groups that act as weak bases NH2 (amino)
R-NH2 R-NH3
Amphoteric molecules are protein molecules thatcan function as both a weak acid and a weak base
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Physiological Buffer Systems
The respiratory system regulation of acid-basebalance is a physiological buffering system
There is a reversible equilibrium between:Dissolved carbon dioxide and water
Carbonic acid and the hydrogen and
bicarbonate ions
CO2 + H2O H2CO3 H+ + HCO3
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Physiological Buffer Systems
During carbon dioxide unloading, hydrogen ions
are incorporated into water
When hypercapnia or rising plasma H+ occurs:Deeper and more rapid breathing expels more
carbon dioxideHydrogen ion concentration is reduced
Alkalosis causes slower, more shallow breathing, causingH+ to increase
Respiratory system impairment causes acid-baseimbalance (respiratory acidosis or respiratory alkalosis)
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Renal Mechanisms of Acid-Base Balance
Chemical buffers can tie up excess acids orbases, but they cannot eliminate them from thebody
The lungs can eliminate carbonic acid (volatileacid) by eliminating carbon dioxide
Only the kidneys can rid the body of metabolic
acids (phosphoric, uric, and lactic acids andketones) and prevent metabolic acidosis
The ultimate acid-base regulatory organs arethe kidneys
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Renal Mechanisms of Acid-Base Balance
The most important renal mechanisms forregulating acid-base balance are:
Conserving (reabsorbing) or generating new
bicarbonate ionsExcreting bicarbonate ions
Losing a bicarbonate ion is the same as
gaining a hydrogen ion (the blood becomesacidic); reabsorbing a bicarbonate ion is thesame as losing a hydrogen ion (the bloodbecomes alkaline)
R l M h i f A id
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Renal Mechanisms of Acid-Base Balance
Hydrogen ion secretion occurs in the PCTand in the collecting ducts
Hydrogen ions come from the dissociation of
carbonic acid
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Reabsorption of Bicarbonate
Carbon dioxide combines with water in tubule cells, forming
carbonic acid
Carbonic acid splits into hydrogen ions and bicarbonateions
For each hydrogen ion secreted, a sodium ion and abicarbonate ion are reabsorbed by thePCT cells
Secreted hydrogen ions form carbonic acid;thus, bicarbonate disappears from filtrate at thesame rate that it enters the peritubular capillaryblood
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Reabsorption of Bicarbonate
Carbonic acid formed infiltrate dissociates torelease carbon dioxideand water
Carbon dioxide then
diffuses into tubulecells, where it acts totrigger further hydrogenionsecretion
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Generating New BicarbonateIons
Two mechanisms carried out collecting ducts
cells generate new bicarbonate ions
Both involve renal excretion of acid via
secretion and excretion of hydrogen ions or
ammonium ions (NH4+)
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Hydrogen Ion Excretion
Dietary hydrogen ions must be counteracted bygenerating new bicarbonate
The excreted hydrogen ions must bind to buffers in theurine (phosphate buffer system)
Collecting duct cells actively secrete hydrogen ions intourine, which is buffered and excreted
Bicarbonate generated is:
Moved into the interstitial space via a cotransport systemPassively moved into the peritubular capillary blood
H drogen Ion E cretion
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Hydrogen Ion Excretion
In response to acidosis:
Kidneys generatebicarbonate ions and
add them to the blood
An equal amount ofhydrogen ions are
added to the urine
A i I E ti
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Ammonium Ion Excretion
This method uses ammonium ions producedby the metabolism of glutamine in PCT cells
Each glutamine metabolized produces twoammonium ions and two bicarbonate ions
Bicarbonate moves to the blood and ammoniumions are excreted in urine
Ammonium ion Excretion
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Ammonium ion Excretion
Bi b t I S ti
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Bicarbonate Ion Secretion
When the body is in alkalosis, type B intercalated cells ofcollecting ducts :
Exhibit bicarbonate ion secretionReclaim hydrogen ions and acidify the blood
The mechanism is the opposite of type A intercalated cellsand the bicarbonate ion reabsorption process
Even during alkalosis, the nephrons and collecting ducts
excrete fewer bicarbonate ions than they conserve
R i t A id i d Alk l i
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Respiratory Acidosis and Alkalosis
Result from failure of the respiratory system tobalance pH
PCO2 is the single most important indicator of respiratory
inadequacy
PCO2 levelsNormal PCO2 fluctuates between 35 and 45mm HgValues above 45 mm Hg signal respiratory acidosis
Values below 35 mm Hg indicate respiratory alkalosis
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Respiratory Acidosis and Alkalosis
Respiratory acidosis is the most common cause ofacid-base imbalance
Occurs when a person breathes shallowly,
or gas exchange is hampered by diseasessuch as pneumonia, cystic fibrosis, or
emphysema
Respiratory alkalosis is a common result ofhyperventilation
M t b li A id i
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Metabolic Acidosis
All pH imbalances except those caused by abnormal bloodcarbon dioxide levels
Metabolic acid-base imbalance bicarbonate ion levelsabove or below normal (22-26 mEq/L)
Metabolic acidosis is the second most common cause ofacid-base imbalance
Typical causes are ingestion of too much
alcohol and excessive loss of bicarbonateionsOther causes include accumulation of lactic acid,shock, ketosis in diabetic crisis, starvation, andkidney failure
M t b li Alk l i
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Metabolic Alkalosis
Rising blood pH and bicarbonate levelsindicate metabolic alkalosis
Typical causes are:
Vomiting of the acid contents of thestomach
Intake of excess base (e.g., from antacids)
Constipation, in which excessivebicarbonate is reabsorbed
R i t d R l C ti
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Respiratory and Renal Compensations
Acid-base imbalance due to inadequacy of aphysiological buffer system is compensated for
by the other system
The respiratory system will attempt to
correct metabolic acid-base imbalances
The kidneys will work to correct imbalances
caused by respiratory disease
R i t C ti
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Respiratory Compensation
Metabolic acidosis has low pH:
Bicarbonate level is low
Pco2 is falling below normal to correctthe imbalance
The rate and depth of breathing are
elevated
R i t C ti
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Respiratory Compensation
Metabolic alkalosis has high pH:
High levels of bicarbonate
Correction is revealed by:
Rising PCO2
Compensation exhibits slow, shallow
breathing, allowing carbon dioxide toaccumulate in the blood
Renal Compensation
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Renal Compensation
To correct respiratory acid-base imbalance,renal mechanisms are stepped up
Respiratory Acidosis has low pHHas high PCO2 (the cause of acidosis)
In respiratory acidosis, the respiratory rateis often depressed and is the immediate
cause of the acidosisHigh bicarbonate levels indicate the kidneys
are retaining bicarbonate to offset theacidosis
Renal Compensation
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Renal Compensation
Respiratory Alkalosis has high pH
Low PCO2 (the cause of the alkalosis)
Low bicarbonate levels
The kidneys eliminate bicarbonate from
the body by failing to reclaim it or by
actively secreting it
Developmental Aspects
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Developmental Aspects
Water content of the body is greatest at birth (70-80%)and declines until adulthood, when it is about 58%
At puberty, sexual differences in body water contentarise as males develop greater muscle mass
Homeostatic mechanisms slow down with age
Elders may be unresponsive to thirst clues and are atrisk of dehydration
The very young and the very old are the most frequentvictims of fluid, acid-base, and electrolyte imbalances