Acid-Base Balance
By: Hannah Coakley
2/27/2014
Quick Review: Acids
Acids are compounds which function as
hydrogen (H+) donors
in biochemical equations/solutes
The more free H+ ions available for donation,
the more acidic the compound
Many foods that are “acidic” in taste are
actually metabolized into basic compounds in
the body
Quick Review: Bases
Bases are compounds which can
accept H+ ions.
This is accomplished by having an
excess of OH- (hydroxide) ions
The terms “basic” and “alkaline” are used
interchangeably
Quick Review: pH
pH is the –log of H+ concentration in any given
solute
Its range spans from 0 – 14
A lower pH implies a high H+ concentration
(acidic). A higher pH implies a low H+
concentration (basic)
The pH Scale
Role of pH in the Body
Intra and extracellular pH levels are tightly regulated:
Acids & Bases in the Body
The body naturally produces more H+ than OH- ions
This occurs in several ways:
-- The metabolism of fats (fatty acids) and
proteins (amino acids)
-- The byproduct of cellular respiration: where
carbonic acid breaks down into CO2 and H20 to be
breathed out by the lungs
The Chemical Buffer System
Works to regulate pH by taking up or releasing
H+ ions accordingly
Protects neutrality, usually by pairing a weak
acid with a base
Also functions by substituting a strong acid or
base for a weak one
Methods of Excretion
The other primary way to maintain pH
homeostasis is through the excretion of
excess acids or bases.
Respiratory excretion of CO2 using rate and
depth of breath
Renal excretion, which eliminates acids and
can also regulate the amount of circulating
bicarbonate (HCO3-)
pH Balance Visualized
Defining Acidosis and Alkalosis
Acidosis:
• pH< 7.35
• Primary effect is in suppression of the CNS 
decreased Ca binding to protein, high I-Cal
• Weakness, coma, death
Alkalosis:
• pH > 7.45
• Primary effect is in overstimulation of CNS & PNS 
increased Ca binding to protein, low I-Cal
• Lightheadedness, spasms/tetany, death
Metabolic vs Respiratory
Metabolic Acidosis  loss of relative
concentration of bicarbonate ion (< 22 mEq/L)
(Symptoms: Headache, lethargy, N/V/D, coma)
Metabolic Alkalosis  excess of relative
concentration of bicarbonate ion (> 26 mEq/L)
(Symptoms: electrolyte depletion, tetany, slow and/or
shallow breathing, tachycardia)
Metabolic vs Respiratory
Respiratory Acidosis  carbonic acid excess
leading to hypercapnia (pCO2 > 45 mm Hg)
(Symptoms: warm, flushed skin  vasodilation,
breathlessness, hypoventilation, disorientation, tremors)
Respiratory Alkalosis  carbonic acid deficit
leading to hypocapnia (pCO2 < 35 mm Hg)
(Symptoms: dizziness, lightheaded, numbness of
extremities)
Compensatory Mechanisms
Depending upon the primary acid-base
imbalance, the body will compensate using a
secondary mechanism in order to return pH
homeostasis to the body
Compensation: Metabolic
Metabolic Acidosis 
Increased Ventilation to eliminate excess CO2
(Hyperventilation)
 K+ & PO4 shifting from ICF to ECF to function as
a buffer (H+ shifts into the cells)
Metabolic Alkalosis 
 Decreased ventilation (Hypoventilation)
limited by constraints of hypoxia
Compensation: Respiratory
Respiratory Acidosis 
 Kidneys eliminate excess H+ ions, retain
Bicarbonate ions
Respiratory Alkalosis 
 Kidneys conserve H+ ions and excrete excess
Bicarb ions
 K+ shift from ECF to ICF to increase
circulating H+ (sudden low serum K+)
Calculating the Anion Gap
Interpreting the Anion Gap
If the Anion gap is > than 26 mEq/L this is
considered normochloremic acidosis
If the anion gap is WNL (6 – 12 mEq/L), this is
considered hyperchloremic acidosis
Understanding Base Excess
Mixed Acid-Base Disorders
More than one acid/base disturbance can occur
concomitantly in the body
If unexpected lab values are noted, there is
good reason to suspect a mixed acid-base
disorder
Treatment Strategies: Metabolic
Acidosis
If Hyperchloremic:
IV-Lactate solution is given, this is converted
to bicarb in the liver, thus raising the relative
concentration of bicarbonate in the blood
(shift of K+ back into ICF and may cause a need for it to be repleted)
If Normochloremic:
Identify and correct sources of excess acids
Treatment Strategies: Metabolic
Alkalosis
Saline Responsive (urine Cl- < 10 mEq/L)
IV-NaCl solution is given to physiologically
replace the excess bicarbonate ions in the
blood with Cl
** Administration of KCl is also essential, as
adequate K+ buffer in the ECF is essential to
fully correcting the alkalosis
Treatment Strategies: Respiratory
Acidosis
-- Treat the underlying dysfunction or disease
-- Restore appropriate ventilation
-- Add IV-lactate to aid in compensatory
bicarb production
-- Ensure that the patient is not being overfed, as
this will prolong acidosis (via excess CO2
production)
Treatment Strategies: Respiratory
Alkalosis
-- Treat underlying dysfunction or disease
-- Attempt to slow respiration
-- Add IV-Cl to aid in compensatory replacement of
excess bicarb
-- Replete K+ as needed, since K+ shifts
intracellularly in exchange for H+ in the ECF
Correcting Acid/Base Imbalance:
Step By Step
1) Analyze the pH
2) Analyze the pCO2
3) Analyze the HCO3
4) Match the pCO2 or the HCO3 with the pH
5) Assess AG and BE
6) Assess directionality/compensation
7) Analyze the pCO2 and O2 saturation
Check Your Knowledge
Scenario #1a:
pH: acidic // CO2: high // HCO3: high
What is the primary imbalance? Is there
evidence of compensation?
#1b: What if HCO3 was normal? Low?
Take Home:
What is the RD’s Role?
Monitoring
Treatment (Repletion of electrolytes and
fluids)
Maintenance (appropriate TF or TPN)
Thank You!
References
Brantley, Susan. The ABCs of ABGs. Support Line. UT Medical Center, Knoxville, TN.
Langley, Ginger. Fluid, Electrolytes, and Acid-Case Disorders. A.S.P.E.N Nutrition
Support Core Curriculum, 2007.
Gilmore, Diane M. Acid Base Balance and Imbalance. Arkansas State University, Dept
of Pathophysiology, 2012.
Ebihara, L. & West, John. Acid-Base Balance, A Respiratory Approach. Repiratory
Physiology, The Essentials. 2011
Jaber, Bertrand. Metabolic Acidosis. Tufts University Open Courseware, Renal
Pathophysiology. 2007.
Kibble, Jonathan D.& Colby R. Halsey, Medical Physiology: The Big Picture. 2009.
Skujor, Mario & Mira Milas. Endocrinology. Cleveland Clinic: Center for Continuing
Education. 2013