Respiratory Changes During Exercise, Oxygen Debt,

ByDr. Mudassar Ali Roomi

2 main respiratory changes in exercise:

• 1) increase in pulmonary ventilation• 2) increase in both rate & depth of respiration.

Regulation of Respiration during exercise:

• What causes intense ventilation during exercise?

O2 consumption in moderate & severe exercise:

• In healthy athlete alveolar vent. is directly proportional to oxygen metabolism.

• The arterial PO2, PCO2 and pH remain almost normal.

Conclusion: Hypoxia, hypercapnia & acidosis have no role in inducing hyperventilation during exercise!!

4 main factors that increase rate of respiration during exercise:

1. Anticipatory increase in rate of ventilation: When a person intends to perform exercise

impulses from cerebral cortex skeletal muscle to initiate contraction & simultaneously collateral impulses respiratory centre increase ventilation.

• 2. Impulses from proprioceptors: (receptors for position & movement present around joints, in the muscles, tendons and joint ligaments).

This is the major stimulus for respiratory centre during exercise.

• 3. Increase in temperature: During exercise metabolism increases

body temperature increases stimulates respiration directly & indirectly.

4. Chemical factors:– Decrease in PO2– Increase in PCO2– Increase in H+ conc.

• The effect of PO2, PCO2 & H+ is minimum to stimulate respiration in exercise because there is increased ventilation so PO2 & PCO2 remain in normal limits.

Metabolic systems during exercise:

3 types:

1) Phosphagen system: consist of ATP & Creatine phosphate in muscle (ATP

can maintain muscle contraction for 5-6 sec; energy from creatine phosphate can sustain contraction for another 10 sec)

2) Glycogen-Lactic Acid System: (another 30-40 sec) Glucose stored as glycogen in the muscle undergoes glycolysis ATP.

3) Aerobic System: (For long long time) Nutrients, Glucose, Amino Acids Fatty Acids are oxidized. It is the ultimate source of energy.

Changes in Respiration during Exercise:

1) Normal respiratory minute volume (RMV) at rest = 500 x 12 = 6 L / min

– in severe exercise:

RMV = up to 100 – 110 L / min2) Maximum Breathing Capacity (MBC): Up to 150 – 170 L / min

• 3) Oxygen Consumption (O.C): It is the percentage of arterial blood which gives its O2 while passing through the tissues.

– 250 ml / min (at rest)– may increase to 4-5 L / min in exercise

• 4) Utilization Co-efficient (U.C): 25% (at rest) 75 – 85 % in severe exercise

• 5) Diffusion Capacity for O2:– At rest: 20 – 30 ml / mm Hg / min– in exercise: 65 ml / mm Hg / min

• 6) Chemical parameters in skeletal muscles:– PO2 decreases,– PCO2, H+, Temp increases Right hand shift of

oxy-Hb dissociation curve easy dissociation of O2 to supply skeletal muscle.

• 7) Effect on Respiratory Quotient (RQ): – In moderate exercise: RQ remains about 1.

– In severe exercise: May increase up to 1.5-2 due to extra CO2 formation

– After severe exercise: RQ falls up to 0.5.

Interrelation between chemical & nervous factors in

control of respiration during exercise: • At the onset of exercise alveolar vent. increases

instantaneously, without an initial increase in arterial PCO2

• There is initial decrease in arterial PCO2 due to great increase in alv. Vent.

• Conclusion: brain anticipatory stim. of resp. at the onset of exercise.

Neuro-genic drive from respiratory centre during heavy exercise

• Arterial PCO2 remain normal (40 mm Hg) at rest & during heavy exercise.

• If PCO2 does change from 40, there is stim. of vent. above 40 & depression of vent. below 40.

• This shift in exercise is partly a learned response that involves cerebral cortex.

Conclusion: Neurogenic factor shifts the curve about

20- fold in upward direction so that vent. Matches the rate of CO2 release keeping normal level of Arterial PCO2

Oxygen Debt:

Definition: Extra amount of oxygen, that must be supplied

to body after exercise, in order to restore metabolic system back to pre-exercise state.

• During exercise oxygen consumption is increased by skeletal muscle.

Oxygen is present:• In combination with Hb• In myoglobin &• In dissolved form

Oxygen used in severe exercise:

0.3 L

O2 combined with Myoglobin

1 L

O2 combined with Hemoglobin

0.5 L

O2 in

alveolar air

0.25 L

O2 in

dissolved form

TOTAL OXYGEN = 2 L (approx.) This much oxygen must be repaid.

Debts:

• To restore phosphagen & glycogen system: 2 L is required.

• To restore Aerobic system: 8 L is required.

• So, a total of 10-12 L oxygen is used in exercise & is paid in 90 min after exercise respiratory rate remain increased for 90 min after exercise to repay oxygen debt = 10-12 L.