ACID BASE

BALANCE

ACID BASE BALANCE

General Principles

– Dynamic relationship which reflects the

concentration of hydrogen (H+) ions

– Hydrogen ions are acids which must be

maintained within strict limits

» Any deviation from norm affects biochemical

events

– Measured in a negative log (pH) which is

inversely proportional to the hydrogen ion

concentration

» High H+ ion concentration = Low pH

» Low H+ ion concentration = High pH

THE pH SCALE

Most frequently used measure of acid base balance

Ranges from 1-14

– 1 means only hydrogen ions present

– 14 means no hydrogen ions present

– pH of water is 7.0 (neutral)

pH of human body is 7.35-7.45

pH compatible with life = 6.9-7.8

– pH of < 7.35 indicates a state of acidosis

– pH of > 7.45 indicates a state of alkalosis

– Variation of 0.4 in either direction can be fatal

Acid-Base Balance

A pH below 7.35 is

referred to as

acidosis

A pH above 7.45 is

referred to as

alkalosis

Body constantly produces acids through

metabolism

– Carbon dioxide

– Metabolic acids (lactic and pyruvic acid)

Acids must be constantly eliminated from the body

– As acids increase, bodily functions decrease

– Example

» Diminishes myocardial contractility

» Reduces vascular response to catecholamines

» Interferes with the actions of

pharmacological agents

BUFFER SYSTEM Fastest acting defense mechanism (seconds)

Chief buffer is bicarbonate

– Bicarbonate combines with excess hydrogen ions to form

carbonic acid in a dynamic relationship:

HCO3 + H+ <--------> H2CO3

– For every molecule of carbonic acid, there are 20 molecules

of bicarbonate

– Any change in 20:1 ratio is immediately corrected to

maintain pH

» An increase in H+ causes an increase in H2CO3

» A decrease in H+ causes a decrease in H2CO3

Bicarbonate Buffer System

Fastest mechanism

Hydrogen will bind with bicarbonate

Results in formation of carbonic acid

acidcarbonicionebicarbonationHydrogen

COHHCOH 323

Carbonate Buffer System

An increase in hydrogen ions leads to an increase

in carbonic acid.

A deficit in hydrogen ions (alkalosis); carbonic

acid will dissociate into bicarbonate ion and

hydrogen ion

323

323

COHHCOH:AcidDecreased

COHHCOH:AcidIncreased

BUFFER SYSTEM (CONT.)

Carbonic acid is a weak, volatile acid

which must be eliminated

Enzyme carbonic anhydrase

catalyzes the carbonic acid to

convert to carbon dioxide and water:

H2CO3--------->CO2 + H2O

CO2 and H2O are easily eliminated

by the lungs and kidneys

Functions within minutes (1-3)

Lungs eliminate excess CO2 by

increasing respirations, causing a

decrease in H+ ion and an increase in

pH

Lungs can retain more CO2 by

decreasing respirations, causing an

increase in H+ ions and a decrease in

pH

Respiratory Mechanisms Increased respirations cause increased elimination of CO2

and H2O.

– An increase in pH

Decreased respirations cause retention of CO2 and H2O.

– Decreasing pH

Slowest onset and generally important in

long term maintenance of acid-base

balance (hours to days)

Function

– Kidneys retain bicarbonate, causing a

decrease in H+ ions and an increase in pH

– Kidneys excrete bicarbonate, causing an

increase in H+ ions and a decrease in pH

Respiratory Acidosis

Caused by abnormal retention of CO2 from

impaired ventilation due to problems

occurring in the lungs or respiratory center

of the brain

Respiration = CO2 + H2O H2CO3 H+ + HCO3

-

Caused by increased respiration and

excessive elimination of CO2

The CO2 level is decreased and the pH is

increased.

Respiratory Alkalosis

Respiration = CO2 + H2O H2CO3 H+ + HCO3

-

ACID-BASE

DERANGEMENTS (CONT.)

Metabolic acidosis

– Increased production of acids (lactic and pyruvic)

– Can also result from diarrhea, vomiting, diabetes and

medications (ASA)

– pH is decreased and CO2 level is normal

– Treatment is aimed at improving ventilations to

eliminate CO2

– Sodium bicarbonate with documented metabolic

acidosis

» Rarely administered during initial resuscitative

strategies

ACID-BASE

DERANGEMENTS (CONT.)

Metabolic alkalosis

– Caused by increased vomiting, overdosing on

antiacids, diuretics

– pH is increased and CO2 remains normal

– Treatment is aimed at underlying cause

Combinations

– Any combination of acid-base derangements

– Only ABG can accurately diagnose problem

Thank you!