Fluid, Electrolyte and AcidBase Homeostasis

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Chapter 27

• Fluid, Electrolyte and Acid-Base Homeostasis
• Lecture Outline

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Chapter 27Fluid, Electrolyte and Acid-Base
Homeostasis

• Body fluid
• all the water and dissolved solutes in the bodys
  fluid compartments
• Mechanisms regulate
• total volume
• distribution
• concentration of solutes and pH
• Regulatory mechanisms insure homeostasis of body
  fluids since their malfunction may seriously
  endanger nervous system and organ functioning.

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FLUID COMPARTMENTS AND FLUID BALANCE
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Balance Between Fluid Compartments
Volume of fluid in each is kept constant. Since
water follows electrolytes, they must be in
balance as well

• Only 2 places for exchange between compartments
• cell membranes separate intracellular from
  interstitial fluid.
• only in capillaries are walls thin enough for
  exchange between plasma and interstitial fluids

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Introduction

• In lean adults body fluids comprise about 55-60
  (Figure 27.1) of total body weight.
• Water is the main component of all body fluids.
• About two-thirds of the bodys fluid is located
  in cells and is called intracellular fluid (ICF).
• The other third is called extracellular fluid
  (ECF).
• About 80 of the ECF is interstitial fluid and
  20 is blood plasma.
• Some of the interstitial fluid is localized in
  specific places, such as lymph cerebrospinal
  fluid gastrointestinal tract fluids synovial
  fluid fluids of the eyes (aqueous humor and
  vitreous body) and ears (endolymph and
  perilymph) pleural, pericardial, and peritoneal
  fluids between serous membranes and glomerular
  filtrate in the kidneys.

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Membranes

• Selectively permeable membranes separate body
  fluids into distinct compartments.
• Plasma membranes of individual cells separate
  intracellular fluid from interstitial fluid.
• Blood vessel walls divide interstitial fluid from
  blood plasma.
• Although fluids are in constant motion from one
  compartment to another, the volume of fluid in
  each compartment remains fairly stable another
  example of homeostasis.

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Fluid and Solute Balance

• Fluid balance means that the various body
  compartments contain the required amount of
  water, proportioned according to their needs.
• Fluid balance, then, means water balance, but
  also implies electrolyte balance the two are
  inseparable.
• Osmosis is the primary way in which water moves
  in and out of body compartments. The
  concentrations of solutes in the fluids is
  therefore a major determinant of fluid balance.
• Most solutes in body fluids are electrolytes,
  compounds that dissociate into ions.

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Body Water Gain and Loss (Figure 27.2)

• 45-75 body weight
• declines with age since fat contains almost no
  water
• Gain from ingestion and metabolic water formed
  during aerobic respiration dehydration
  synthesis reactions (2500 mL/day)
• Normally loss gain
• urine, feces, sweat, breathe

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Dehydration Stimulates Thirst

• Regulation of fluid gain is by regulation of
  thirst.

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Regulation of Water Gain

• Metabolic water volume depends mostly on the
  level of aerobic cellular respiration, which
  reflects the demand for ATP in body cells.
• The main way to regulate body water balance is by
  adjusting the volume of water intake.
• When water loss is greater than water gain,
  dehydration occurs (Figure 27.3).
• The stimulus for fluid intake (gain) is
  dehydration resulting in thirst sensations one
  mechanism for stimulating the thirst center in
  the hypothalamus is the renin-angiotensin II
  pathway, which responds to decreased blood volume
  (therefore, decreased blood pressure) (Figure
  27.3).
• Drinking occurs ? body water levels return to
  normal

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Regulation of Water and Solute Loss

• Although increased amounts of water and solutes
  are lost through sweating and exhalation during
  exercise, loss of body water or excess solutes
  depends mainly on regulating how much is lost in
  the urine (Figure 27.4).
• Under normal conditions, fluid output (loss) is
  adjusted by
• antidiuretic hormone (ADH)
• atrial natriuretic peptide (ANP)
• aldosterone
• all of which regulate urine production.
• Table 27.1 summarizes the factors that maintain
  body water balance.

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Regulation of Water and Solute Loss

• Elimination of excess water or solutes occurs
  through urination
• Consumption of very salty meal demonstrates
  function of three hormones
• Demonstrates how
• water follows salt
• excrete Na and water will follow and decrease
  blood volume

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Movement of Water Between Body Fluid Compartments

• A fluid imbalance between the intracellular and
  interstitial fluids can be caused by a change in
  their osmolarity.
• Most often a change in osmolarity is due to a
  change in the concentration of Na.
• When water is consumed faster than the kidneys
  can excrete it, water intoxication may result
  (Figure 27.5).
• Repeated use of enemas can increase the risk of
  fluid and electrolyte imbalances. (Clinical
  Application)

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Hormone Effects on Solutes

• Angiotensin II and aldosterone promote
  reabsorption of Na and Cl- and an increase in
  fluid volume
• stretches atrial volume and promotes release of
  ANP
• slows release of renin formation of angiotensin
  II
• increases filtration rate reduces water Na
  reabsorption
• decreases secretion of aldosterone slowing
  reabsorption of Na and Cl- in collecting ducts
• ANP promotes natriuresis or the increased
  excretion of Na and Cl- which decreases blood
  volume

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Hormone Regulation of Water Balance

• Antidiuretic hormone (ADH) from the posterior
  pituitary
• stimulates thirst
• increases permeability of principal cells of
  collecting ducts to assist in water reabsorption
• very concentrated urine is formed
• ADH secretion shuts off after the intake of water
• ADH secretion is increased
• large decrease in blood volume
• severe dehydration and drop in blood pressure
• vomiting, diarrhea, heavy sweating or burns

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Movement of Water

• Intracellular and interstitial fluidsnormally
  have the same osmolarity,so cells neither swell
  nor shrink
• Swollen cells of water intoxicationbecause Na
  concentration of plasmafalls below normal
• drink plain water faster than kidneys canexcrete
  it
• replace water lost from diarrhea or vomitingwith
  plain water
• may cause convulsions, coma death unless oral
  rehydration includes small amount salt in water
  intake

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ELECTROLYTES IN BODY FLUIDS

• Electrolytes serve four general functions in the
  body.
• Because they are more numerous than
  nonelectrolytes, electrolytes control the osmosis
  of water between body compartments.
• maintain the acid-base balance required for
  normal cellular activities.
• carry electrical current, which allows production
  of action potentials and graded potentials and
  controls secretion of some hormones and
  neurotransmitters. Electrical currents are also
  important during development.
• cofactors needed for optimal activity of enzymes.
• Concentration expressed in mEq/liter or
  milliequivalents per liter for plasma,
  interstitial fluid and intracellular fluid

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Concentrations of Electrolytes in Body Fluids

• To compare the charge carried by ions in
  different solutions, the concentration is
  typically expressed in milliequivalents/liter
  (mEg/Liter), which gives the concentration of
  cations or anions in a solution.
• The chief difference between plasma and
  interstitial fluid
• plasma contains quite a few protein anions
• interstitial fluid has hardly any since plasma
  proteins generally cannot move out of impermeable
  blood vessel walls
• plasma also contains slightly more sodium ions
  but fewer chloride ions than the interstitial
  fluid. In other respects, the two fluids are
  similar.

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Concentrations of Electrolytes in Body Fluids

• Intracellular fluid (ICF) differs considerably
  from extracellular fluid (ECF), however.
• Figure 27.6 compares the concentrations of the
  main electrolytes and protein anions in plasma,
  interstitial fluid, and intracellular fluid.

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Comparison Between Fluid Components

• Plasma contains many proteins, but interstitial
  fluid does not
• producing blood colloid osmotic pressure
• Extracellular fluid contains Na and Cl-
• Intracellular fluid contains K and phosphates
  (HPO4 -2)

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Sodium (Na) is the most abundant extracellular
ion.

• Most abundant extracellular ion
• accounts for 1/2 of osmolarity of ECF
• Average daily intake exceeds normal requirements
• Hormonal controls
• aldosterone causes increased reabsorption Na
• ADH release ceases if Na levels too low--dilute
  urine lost until Na levels rise
• ANP increases Na and water excretion if Na
  levels too high
• Excess Na in the body can result in edema.
  Excess loss of Na causes excessive loss of
  water, which results in hypovolemia, an
  abnormally low blood volume. (Clinical
  Application)

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Edema, Hypovolemia and Na Imbalance

• Sodium retention causes water retention
• edema is abnormal accumulation of interstitial
  fluid
• Causes of sodium retention
• renal failure
• hyperaldosterone
• Excessive loss of sodium causes excessive loss of
  water (low blood volume)
• due to inadequate secretion of aldosterone
• too many diuretics

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Chloride (Cl-) is the major extracellular anion.

• Regulation of Cl- balance in body fluids is
  indirectly controlled by aldosterone. Aldosterone
  regulate sodium reabsorption the negatively
  charged chloride follows the positively charged
  sodium passively by electrical attraction.

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Chloride (Cl-) is the major extracellular anion.

• Most prevalent extracellular anion
• Moves easily between compartments due to Cl-
  leakage channels
• Helps balance anions in different compartments
• Regulation
• passively follows Na so it is regulated
  indirectly by aldosterone levels
• ADH helps regulate Cl- in body fluids because it
  controls water loss in urine
• Chloride shift across red blood cells with buffer
  movement
• It plays a role in forming HCl in the stomach.

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Potassium (K) is the most abundant cation in
intracellular fluid.

• It is involved in maintaining fluid volume,
  impulse conduction, muscle contraction.
• Exchanged for H to help regulate pH in
  intracellular fluid
• The plasma level of K is under the control of
  mineralocorticoids, mainly aldosterone.
• Helps establish resting membrane potential
  repolarize nerve muscle tissue
• Control is mainly by aldosterone which stimulates
  principal cells to increase K secretion into the
  urine
• abnormal plasma K levels adversely affect
  cardiac and neuromuscular function

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Bicarbonate (HCO3-) is a prominent ion in the
plasma.

• It is a significant plasma anion in electrolyte
  balance.
• It is a major component of the plasma acid-base
  buffer system.
• Concentration increases as blood flows through
  systemic capillaries due to CO2 released from
  metabolically active cells
• Concentration decreases as blood flows through
  pulmonary capillaries and CO2 is exhaled
• Kidneys are main regulator of plasma levels
• intercalated cells form more HCO3- if levels are
  too low
• excrete excess in the urine

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Calcium (Ca2), the most abundant ion in the
body, is principally an extracellular ion.

• It is a structural component of bones and teeth.
• Important role in blood clotting,
  neurotransmitter release, muscle tone nerve and
  muscle function
• Regulated by parathyroid hormone
• stimulates osteoclasts to release calcium from
  bone
• increases production of calcitriol (Ca2
  absorption from GI tract and reabsorption from
  glomerular filtrate)

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Magnesium (Mg2) is primarily an intracellular
cation.

• It activates several enzyme systems involved in
  the metabolism of carbohydrates and proteins and
  is needed for operation of the sodium pump.
• It is also important in neuromuscular activity,
  neural transmission within the central nervous
  system, and myocardial functioning.
• Several factors regulate magnesium ion
  concentration in plasma. They include hypo- or
  hypercalcemia, hypo- or hypermagnesemia, an
  increase or decrease in extracellular fluid
  volume, an increase or decrease in parathyroid
  hormone, and acidosis or alkalosis.

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Phosphate

• Present as calcium phosphate in bones and teeth,
  and in phospholipids, ATP, DNA and RNA
• HPO4 -2 is important intracellular anion and acts
  as buffer of H in body fluids and in urine
• mono and dihydrogen phosphate act as buffers in
  the blood
• Plasma levels are regulated by parathyroid
  hormone calcitriol
• resorption of bone releases phosphate
• in the kidney, PTH increase phosphate excretion
• calcitriol increases GI absorption of phosphate

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Review

• Table 27.2 describes the imbalances that result
  from the deficiency or excess of several
  electrolytes.

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Clinical Application

• Individuals at risk for fluid and electrolyte
  imbalances include those dependent on others for
  fluid and food needs those undergoing medical
  treatment involving intravenous infusions,
  drainage or suction, and urinary catheters, those
  receiving diuretics, and post-operative
  individuals, burned individuals, individuals with
  chronic disease, and those with altered states of
  consciousness.

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Acid-Base Balance

• The overall acid-base balance of the body is
  maintained by controlling the H concentration of
  body fluids, especially extracellular fluid.
• Homeostasis of H concentration is vital
• proteins 3-D structure sensitive to pH changes
• normal plasma pH must be maintained between 7.35
  - 7.45
• diet high in proteins tends to acidify the blood
• 3 major mechanisms to regulate pH
• buffer system
• exhalation of CO2 (respiratory system)
• kidney excretion of H (urinary system)

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Actions of Buffer Systems

• Prevent rapid, drastic changes in pH
• Change either strong acid or base into weaker one
• Work in fractions of a second
• Found in fluids of the body
• 3 principal buffer systems
• protein buffer system
• carbonic acid-bicarbonate buffer system
• phosphate buffer system

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Protein Buffer System

• Abundant in intracellular fluids in plasma
• hemoglobin very good at buffering H in RBCs
• albumin is main plasma protein buffer
• Amino acids contains at least one carboxyl group
  (-COOH) and at least one amino group (-NH2)
• carboxyl group acts like an acid releases H
• amino group acts like a base combines with H
• some side chains can buffer H
• Hemoglobin acts as a buffer in blood by picking
  up CO2 or H

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Carbonic Acid-Bicarbonate Buffer System

• Acts as extracellular intracellular buffer
  system
• bicarbonate ion (HCO3-) can act as a weak base
• holds excess H
• carbonic acid (H2CO3) can act as weak acid
• dissociates into H ions
• At a pH of 7.4, bicarbonate ion concentration is
  about 20 times that of carbonic acid
• Can not protect against pH changes due to
  respiratory problems

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Phosphate Buffer System

• Most important intracellularly, but also acts to
  buffer acids in the urine
• Dihydrogen phosphate ion acts as a weak acid that
  can buffer a strong base
• Monohydrogen phosphate acts a weak base by
  buffering the H released by a strong acid

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Exhalation of Carbon Dioxide

• The pH of body fluids may be adjusted by a change
  in the rate and depth of respirations, which
  usually takes from 1 to 3 minutes.
• An increase in the rate and depth of breathing
  causes more carbon dioxide to be exhaled, thereby
  increasing pH.
• A decrease in respiration rate and depth means
  that less carbon dioxide is exhaled, causing the
  blood pH to fall.
• The pH of body fluids, in turn, affects the rate
  of breathing (Figure 27.7).
• The kidneys excrete H and reabsorb HCO3- to aid
  in maintaining pH.

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Exhalation of Carbon Dioxide

• pH modified by changing rate depth of breathing
• faster breathing rate, blood pH rises
• slow breathing rate, blood pH drops
• H detected by chemoreceptors in medulla
  oblongata, carotid aortic bodies
• Respiratory centers inhibited or stimulated by
  changes is pH

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Kidney Excretion of H

• Metabolic reactions produce 1mEq/liter of
  nonvolatile acid for every kilogram of body
  weight
• Excretion of H in the urine is only way to
  eliminate huge excess
• Kidneys synthesize new bicarbonate and save
  filtered bicarbonate
• Renal failure can cause death rapidly due to its
  role in pH balance

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Regulation of Acid-Base Balance

• Cells in the PCT and collecting ducts secrete
  hydrogen ions into the tubular fluid.
• In the PCT Na/H antiporters secrete H and
  reabsorb Na (Figure 26.13).
• The apical surfaces of some intercalated cells
  include proton pumps (H ATPases) that secrete
  H into the tubular fluid and HCO3 antiporters
  in their basolateral membranes to reabsorb HCO3
  (Figure 27.8).
• Other intercalated cells have proton pumps in
  their basolateral membranes and Cl/HCO3
  antiporters in their apical membranes.
• These two types of cells help maintain body fluid
  pH by excreting excess H when pH is too low or
  by excreting excess HCO3 when the pH is too
  high.
• Table 27.3 summarizes the mechanism that
  maintains pH of body fluids.

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Acid-Base Imbalances

• The normal pH range of systemic arterial blood is
  between 7.35-7.45.
• Acidosis is a blood pH below 7.35. Its principal
  effect is depression of the central nervous
  system through depression of synaptic
  transmission.
• Alkalosis is a blood pH above 7.45. Its principal
  effect is overexcitability of the central nervous
  system through facilitation of synaptic
  transmission.

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Acid-Base Imbalances
Acidosis---blood pH below 7.35 Alkalosis---blood
pH above 7.45

• Compensation is an attempt to correct the problem
• respiratory compensation
• renal compensation
• Acidosis causes depression of CNS---coma
• Alkalosis causes excitability of nervous
  tissue---spasms, convulsions death

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Acid-Base Imbalances

• Compensation refers to the physiological response
  to an acid-base imbalance.
• Respiratory acidosis and respiratory alkalosis
  are primary disorders of blood PCO2.
• metabolic acidosis and metabolic alkalosis are
  primary disorders of bicarbonate concentration.
• A summary of acidosis and alkalosis is presented
  in Table 27.4.

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Diagnosis

• Diagnosis of acid-base imbalances employs a
  general four-step process.
• Note whether the pH is high or low relative to
  the normal range.
• Decide which value of PCO2 or HCO3- could cause
  the abnormality.
• Specify the problem source as respiratory or
  metabolic.
• Look at the noncausative value and determine if
  it is compensating for the problem.

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Summary of Causes

• Respiratory acidosis alkalosis are disorders
  involving changes in partial pressure of CO2 in
  blood
• Metabolic acidosis alkalosis are disorders due
  to changes in bicarbonate ion concentration in
  blood

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Respiratory Acidosis

• Cause is elevation of pCO2 of blood
• Due to lack of removal of CO2 from blood
• emphysema, pulmonary edema, injury to the
  brainstem respiratory centers
• Treatment
• IV administration of bicarbonate (HCO3-)
• ventilation therapy to increase exhalation of CO2

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Respiratory Alkalosis

• Arterial blood pCO2 is too low
• Hyperventilation caused by high altitude,
  pulmonary disease, stroke, anxiety, aspirin
  overdose
• Renal compensation involves decrease in excretion
  of H and increase reabsorption of bicarbonate
• Treatment
• breathe into a paper bag

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Metabolic Acidosis

• Blood bicarbonate ion concentration too low
• loss of ion through diarrhea or kidney
  dysfunction
• accumulation of acid (ketosis with
  dieting/diabetes)
• kidney failing to remove H from protein
  metabolism
• Respiratory compensation by hyperventilation
• Treatment
• IV administration of sodium bicarbonate
• correct the cause

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Metabolic Alkalosis

• Blood bicarbonate levels are too high
• Cause is nonrespiratory loss of acid
• vomiting, gastric suctioning, use of diuretics,
  dehydration, excessive intake of alkaline drugs
• Respiratory compensation is hypoventilation
• Treatment
• fluid and electrolyte therapy
• correct the cause

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Diagnosis of Acid-Base Imbalances

• Evaluate
• systemic arterial blood pH
• concentration of bicarbonate (too low or too
  high)
• PCO2 (too low or too high)
• Solutions
• if problem is respiratory, the pCO2 will not be
  normal
• if problem is metabolic, the bicarbonate level
  will not be normal

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Homeostasis in Infants

• More body water in ECF so more easily disrupted
• Rate of fluid intake/output is 7X higher
• Higher metabolic rate produces more metabolic
  wastes
• Kidneys can not concentrate urine nor remove
  excess H
• Surface area to volume ratio is greater so lose
  more water through skin
• Higher breathing rate increase water loss from
  lungs
• Higher K and Cl- concentrations than adults

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Impaired Homeostasis in the Elderly

• Decreased volume of intracellular fluid
• inadequate fluid intake
• Decreased total body K due to loss of muscle
  tissue or potassium-depleting diuretics for
  treatment of hypertension or heart disease
• Decreased respiratory renal function
• slowing of exhalation of CO2
• decreased blood flow glomerular filtration rate
• reduced sensitivity to ADH impaired ability to
  produce dilute urine
• renal tubule cells produce less ammonia to
  combine with H and excrete as NH4

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Questions?
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• end