acidbasebalance ani

7/27/2019 Acidbasebalance Ani

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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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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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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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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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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

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