Chapter 11Acid-Base Balance During

Exercise

EXERCISE PHYSIOLOGYTheory and Application to Fitness and Performance,

6th edition

Scott K. Powers & Edward T. Howley

pH = -log10[H+]

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 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

• Fixed acids– Sulfuric acid– Phosphoric acid

• Organic acids– Lactic acid

CO2 + H2O H2CO3 H+ + HCO3-

Sources of Hydrogen Ions Due to Metabolic Processes

Figure 11.3

Sport and Muscle Acid-Base Balance

Risk of Acid-BaseSport DisturbanceBaseball LowBasketball Low-to-moderateBoxing Low-to-moderateCross-country skiing LowFootball (American) Low100-meter sprint Low100-meter swim Low400-meter run High800-meter run High1,500-meter run Moderate-to-high5,000-meter run Moderate10,000-meter run Low-to-moderateMarathon run LowSoccer Low-to-moderateWeight lifting (low repetitions) LowVolleyball Low

Table 11.1

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 systemCO2 + H2O H2CO3 H+ + HCO3

-

Acid-Base Buffer SystemsBuffer System Constituents ActionsBicarbonate Sodium bicarbonate Converts strong acid

system (NaHCO3) into weak acidCarbonic acid Converts strong

(H2CO3) base into weak base

Phosphate Sodium phosphate Converts strong acid

system (Na2HPO-4) into weak acid

Protein system COO- group of a Accepts hydrogens in the molecule presence of excess

acid

NH3 group of a Accepts hydrogens in the molecule 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