M7(1) Ventilation and Acid-Base Balance

bicarbonate

HCO₃⁻ binds free H⁺ ions

• crucial for maintaining body’s pH

carbonic anhydrase

key enzyme that speeds up the conversion between carbon dioxide and water into carbonic acid

carbonic anhydrase reaction

CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺

carbonic acid

H₂CO₃

• spontaneously dissociates into bicarbonate and a hydrogen ion immediately after being catalyzed

acid base balance

the body's process of maintaining a stable level of acidity and alkalinity, known as pH, in the blood

• achieved via

  • bicarbonate buffer system

  • lungs

  • kidneys

bicarbonate buffer system

main chemical buffer in the blood

• HCO₃^- (bicarbonate) binds free H+ to prevent large pH changes

lungs

exhale CO₂ to remove acid from the body:

• If H⁺ increases → more CO₂ is produced via buffering → hyperventilation removes it → pH rises toward normal

kidneys

slow, long-term regulation (hours to days)

• reabsorb HCO₃⁻ and excrete H⁺ in urine

CO2 and ventilation

at rest, the main stimulus for breathing is arterial CO₂

• CO₂ diffuses into blood → reacts with H₂O → forms carbonic acid (H₂CO₃) → dissociates to H⁺ + HCO₃⁻

• Increased H⁺ (lower pH) is sensed by chemoreceptors in the brain and arteries → triggers increased ventilation

acidosis

an overproduction of acid that builds up in the blood or an excessive loss of bicarbonate from the blood

• often occurs when CO_2, accumulates in blood

acidosis response

in response to acidosis

• ventilation increases to:

  • remove CO₂ via lungs

  • reduce formation of H₂CO₃ → decreases H⁺ concentration

tissue carbonic anhydrase reaction

ATP production (Krebs cycle, pyruvate oxidation) → creates CO₂

CO₂ + H₂O → H₂CO₃ → HCO₃⁻ + H⁺

tissue reverse carbonic anhydrase reaction

lactic acid production (anaerobic glycolysis) → creates H+

HCO₃⁻ + H⁺ → H₂CO₃ → CO₂ + H₂O

lungs carbonic anhydrase reaction

HCO₃⁻ + H⁺ → H₂CO₃ → CO₂ + H₂O

intense exercise

several effects of intense exercise

• H+ accumulation

• pH regulation difficulty

H+ accumulation

H+ rises rapidly during intense exercises; 2 sources of H+ come during intense exercise

• CO₂ production due to inc metabolism → reacts w/ water → H₂CO₃ → H⁺ + HCO₃^-

• lactate production from anaerobic glycolysis → pyruvate → lactate + H⁺

pH regulation difficulty

pH regulation becomes harder with intense exercise

• Bicarbonate buffer system can only neutralize so much H⁺

• Ventilation can’t fully compensate if H⁺ production outpaces CO₂ removal

During repeated all-out bouts, blood lactate can rise above 30 mM, creating a strong metabolic acidosis

tolerance and risks

Humans can temporarily tolerate pH as low as ~7.0.

• If pH drops too far:

  • Nausea, headache, dizziness

  • Impaired muscle and enzyme function

this is why maximal or repeated sprint exercise is so physiologically stressful

oxygen cost of breathing

how much of your body’s total oxygen consumption is used just to breathe

• changes with minute ventilation (VE)

VO2 cost of breathing

• VO₂ cost of breathing = oxygen used by respiratory muscles (diaphragm, intercostals, accessory muscles).

• At rest:

  • Very low fraction of total VO₂ (<5%)

  • Respiratory muscles do most of the work

• As ventilation (VE) increases during exercise:

  • VO₂ cost of breathing rises, but usually remains <10–16% of total VO₂ for moderate exercise

  • Respiratory muscles work harder

• At extremely high VE (very intense exercise, like near maximal work):

  • Cost can rise significantly, reducing energy available for locomotion

O2 cost per volume of air breathed

At low VE: efficient — small O₂ per liter of air

At high VE: less efficient — more O₂ per liter of air

• Reason: respiratory muscles work harder, face increased resistance and elastic recoil of lungs/chest wall

respiratory muscle unloading

Respiratory muscle unloading (e.g., with inspiratory muscle training or assisted breathing) can:

• Reduce VO₂ cost of ventilation

• Reduce dyspnea sensation at ~90% maximal effort

• Improve endurance performance by freeing O₂ and blood flow for locomotor muscles

lungs limiting exercise performance

can ventilation limit exercise performance? ventilation is not usually the weak link in healthy individuals

1. adaptations to aerobic training

2. hyperventilation during exercise

3. breathing reserve

adaptations to aerobic training

Pulmonary structure and function don’t adapt as much as:

• Cardiovascular system (heart, blood volume, capillaries)

• Neuromuscular system (muscle fibers, mitochondria)

• This means even very fit individuals don’t see huge changes in lung size or maximal ventilation from training.

hyperventilation during exercise

During strenuous exercise, healthy people hyperventilate:

• Alveolar PCO₂ decreases slightly (CO₂ is blown off)

• Alveolar PO₂ increases slightly (helps maintain O₂ gradient for diffusion into blood)

• This is beneficial for oxygen transport because it maintains diffusion gradients even at high VO₂.

breathing reserve

Even at VO₂max, ventilation (VE) is only about 80% of maximal voluntary ventilation (MVV):

• There’s a reserve in breathing capacity

• Pulmonary system usually doesn’t limit O₂ transport in healthy people

Only in extreme cases (e.g., elite endurance athletes or pulmonary disease) might ventilation become a limiting factor.