Chapter 11
Acid-Base Balance During
Exercise
EXERCISE PHYSIOLOGY
Theory and Application to Fitness and Performance,
6th edition
Scott K. Powers & Edward T. Howley
Acids, Bases, and pH
• Acid
– Molecule that can liberate H+ ions
• Raises H+ concentration
– Lactic acid
• Base
– Molecule that is capable of combining with H+ ions
• Lowers H+ concentration
– Bicarbonate
• pH
– Measure of H+ ion concentration
pH = -log10[H+]
pH of Blood
• Normal
– pH = 7.4±0.05
• Acidosis
– pH < 7.4
• Alkalosis
– pH > 7.4
• Abnormal pH can disrupt normal body
function and affect performance
The pH Scale
Figure 11.1
Acidosis and Alkalosis
Figure 11.2
Sources of H+ Ions During Exercise
• Volatile acids
– Carbon dioxide
CO2 + H2O  H2CO3  H+ + HCO3-
• Fixed acids
– Sulfuric acid
– Phosphoric acid
• Organic acids
– Lactic acid
Sources of Hydrogen Ions Due to
Metabolic Processes
Figure 11.3
Sport and Muscle Acid-Base Balance
Risk of Acid-Base
Sport
Disturbance
Baseball
Low
Basketball
Low-to-moderate
Boxing
Low-to-moderate
Cross-country skiing
Low
Football (American)
Low
100-meter sprint
Low
100-meter swim
Low
400-meter run
High
800-meter run
High
1,500-meter run
Moderate-to-high
5,000-meter run
Moderate
10,000-meter run
Low-to-moderate
Marathon run
Low
Soccer
Low-to-moderate
Weight lifting (low repetitions)
Low
Volleyball
Low
Importance of Acid-Base Regulation
During Exercise
• Failure to maintain acid-base balance may
impair performance
– Inhibit ATP production
– Interfere with muscle contraction
• Acid-base balance maintained by buffers
– Release H+ ions when pH is high
– Accept H+ ions when pH is low
Acid-Base Buffer Systems
• Intracellular
– Proteins
– Phosphate groups
– Bicarbonate
• Extracellular
– Bicarbonate
– Hemoglobin
– Blood proteins
• Bicarbonate buffering system
CO2 + H2O  H2CO3  H+ + HCO3-
Acid-Base Buffer Systems
Buffer System
Bicarbonate
system
Constituents
Sodium bicarbonate
(NaHCO3)
Carbonic acid
(H2CO3)
Actions
Converts strong acid
into weak acid
Converts strong
base into weak base
Phosphate
system
Sodium phosphate
(Na2HPO-4)
Converts strong acid
into weak acid
Protein system
COO- group of a
molecule
Accepts hydrogens in the
presence of excess
acid
NH3 group of a
molecule
Accepts hydrogens in the
presence of excess
acid
Table 11.2
Regulation of Acid-Base Balance
• Lungs
– When H+ concentration increases (low pH)
• Increases ventilation
• CO2 is “blown off” and pH increases
• Kidneys
– Regulate blood bicarbonate concentration
– Important in long-term acid-base balance
• Not significant in acid-base balance during exercise
Regulation of Acid-Base Balance
During Exercise
• Lactic acid production depends on:
– Exercise intensity
– Amount of muscle mass involved
– Duration of exercise
• Blood pH
– Declines with increasing intensity exercise
• Muscle pH
– Declines more dramatically than blood pH
• Muscle has lower buffering capacity
Changes in Arterial Blood and Muscle pH
During Exercise
Figure 11.4
Regulation of Acid-Base Balance During
Exercise
• Buffering of lactic acid in the muscle
– 60% through intracellular proteins
– 20–30% by muscle bicarbonate
– 10–20% from intracellular phosphate
groups
• Buffering of lactic acid in the blood
– Bicarbonate is major buffer
• Increases in lactic acid accompanied by
decreases in bicarbonate and blood pH
– Hemoglobin and blood proteins play minor
role
Changes in Blood
Lactic Acid, HCO3-,
and pH During
Exercise
Figure 11.5
Regulation of Acid-Base Balance
During Exercise
• First line
– Cellular buffers
• Proteins, bicarbonate, and phosphate groups
– Blood buffers
• Bicarbonate, hemoglobin, and proteins
• Second line
– Respiratory compensation
• Increased ventilation in response to increased H+
concentration
Lines of Defense Against pH Change During
Intense Exercise
Figure 11.6