16. Respiratory responses to exercise

Question 1

What is the key role of ventilation?

Answer 1

Maintain arterial O2 saturation and remove CO2.

Question 2

How are breathing frequency and tidal volume affected during exercise?

Answer 2

They increase

Question 3

What is the effect of heavy exercise on respiratory muscles?

Answer 3

They become significant consumers of oxygen, can fatigue and are a source of chemoreflex with effects on active muscle blood flow

Question 4

What is involved in the regulation of ventilation during exercise? (4 main dot points here)

Answer 4

Involves:

• central neural control
• feedback from muscle afferents
• mechanical feedback from lungs & limbs
• humoral factors

Question 5

What are the respiratory responses to exercise?

Answer 5

Primarily maintains arterial O2 saturation and CO2 removal. Impairment of exercise performance can occur through:

• accumulation of CO2
• decrease in arterial O2 saturation

Acid-base balance

• hyperventilation ⇒ increase in CO2 production (consuming H+ and leading to alkalosis)
• metabolic acid is a stimuli of breathing

Fluid & temperature balance

• loss of heat & fluid in air that we breathe out

Question 6

Ventilation during & after exercise

Answer 6

During exercise

• gradual increase until it gets to max minute ventilation (inc. dead space & alveolar ventilation)

After exercise

• rapid decrease (neuronal imput) followed by slower adjustment post-exercise (hormonal imput).

Question 7

Ventilation during incremental exercise

Answer 7

• Pre-VT1: Initially there is a linear relationship between ventilation and CO2, increase in CO2 production is from ox. metabolism.
• B/w VT1-VT2: classical anaerobic threshold where CO2 is from both ox. metabolism and carbonic anhydrase reaction
• Post-VT2: increases in CO2 is due to increases in adrenaline, heat, etc.

Question 8

Arterial PO2 vs. mixed venous PO2

Answer 8

Arterial PO2 = 95-100 mmHg

Mixed venous PO2 = 40 mmHg

Question 9

Arterial PCO2 vs. mixed venous PCO2

Answer 9

Arterial PCO2 = 40 mmHg

Mixed venous PCO2 = 46 mmHg

Question 10

Arterial pH & HR during exercise

Answer 10

As HR is a proxy for exercise intensity, we can say that as exercise intensity increases the lungs are effective at maintaining a stable PO2.

PCO2 has no change/however can have decrease when you hyperventilate, and consequently, arterial pH increases too (more alkaline).

Question 11

What happens to ventilation during prolonged exercise?

Answer 11

During prolonged ex, ventilation increases. The increase in ventilation is related to exercise intensity and is due to the increase in tidal volume, but also breathing frequency.

Over time, it’s the increase in breathing frequency that contributes to the progressive increase in ventilation.

• compliance of lung/chest wall limits tidal volume drift

RESPIRATION = TIDAL VOLUME x BREATHING FREQUENCY

Question 12

Pulmonary gas exchange during exercise

Answer 12

During exercise, there is:

• increased O2 extraction
• decreased mixed venous VO2
• higher cardiac output
  
  • CO to lung is also higher since L & R side of heart pump at the same rate.
  • HR increases ⇒ decrease in time spent in pulmonary circulation OR high altitude means smaller diffusion gradient of oxygen
    ⇒ suboptimal oxygenation of blood

Question 13

What causes exercise-induced arterial hypoxemia (EIAH)?

Answer 13

A-aDO2 (alveolar - arterial difference in oxygen):

• V/Q ventilation perfusion ratio mismatch (mostly non human - affects horses more)
• DIFFUSION LIMITATION: reach maximal CO, but transit time is too low to fully equilibrate gas exchange

Inadequate compensatory VE:

• expiratory flow limitation

O2 dissociation curve shift

Question 14

Relationship between exercise-induced arterial hypoxemia (EIAH) and locomotor muscle fatigue?

Answer 14

Decrease in arterial O2 saturation at higher intensities could have a significant negative effect on the performance of leg muscles

Question 15

What is the transdiaphragmatic pressure?

Answer 15

It is the difference between the esophageal pressure and the gastric pressure.

Question 16

How does the diaphragm contribute to air moving into lungs?

Answer 16

When the diaphragm contracts, a pressure difference is created between intrathoracic cavity and outside so air moves into the lungs.

Question 17

Does exercise induce diaphragmatic fatigue?

Answer 17

The diaphragm is stimulated by the phrenic nerve. It contains lots of ST fibres, capillaries, mt, and is very fatigue-resistant. Despite this, it can still fatigue with 15 min 90-95% VO2 max exercise.

Question 18

Why does the diaphragm fatigue?

Answer 18

It is working very hard, may increase to as much as 15% VO2 & CO during strenuous exercise.

This is because diaphragm is the only active muscle in inspiration, then as exercise intensity increases, expiration muscles can be recruited, these require O2 too.

Question 19

What can increased diaphragm & respiratory muscle activities result in?

Answer 19

At high intensities,

• chemoreflexes from respiratory muscles
• ⇒ sympathetically mediated vasoconstriction
• ⇒reduced active muscle blood flow

Question 20

Respiratory muscle metaboreflex

Answer 20

Fatiguing contractions of the diaphragm expiratory and accessory respiratory muscles cause:

• increased reflex activating metabolites
• increased group IV phrenic afferent discharge

Therefore brain has reflex actions to vasoconstrict active muscle:

• increased sympathetic efferent discharge, limb vasoconstriction
• decreased O2 transport
• increased locomotor muscle fatigue
• brain intensifies effort perceptions

Question 21

Control of ventilation

Answer 21

Respiratory neurons in hindbrain (pons, some in medulla) respond to:

• higher centres
• central drive (PCO2, H⁺, pH) in extracellular fluid, CSF
• chemoreceptors (PO2, PCO2, pH)
• stretch receptors in heart
• pain receptors
• body temperature
• hormones
• thermoreceptors (skin)
• pressure receptors
• mechanoreceptors, chemoreceptors(?) in muscle

Question 22

What factors control exercise hyperpnea (ventilation increase)?

Answer 22

• Motor cortical activation
  
  • same central command that increases cardiovascular response & HR also increases ventilation
• Muscle afferents (spindles, type III & IV)
  
  • unmyelinated nerve endings tend to respond to mechanical stimuli
• CO2 flux to the lung (chemoreceptor?)
• Increased K+, H+, lactate
  
  • influence type 3&4 afferents and other chemoreceptors
• Elevated catecholemines & temperature
  
  • contribute most at high intensities
• No role for O2
  
  • no hypoxic drive during exercise

Question 23

Muscle group 3 & 4 afferents & exercise hyperpnea

Answer 23

40-45% of the total ventilation at 50, 100 & 150W is attributed to activation to type 3 & 4 afferents

In an experiment involving blockage of impulses of legs back to spinal cord (but not motor nerves to legs), showed important feedback mechanisms from contracting muscle telling brain & lungs how much O2 needed

Question 24

What are the effects on exercise ventilation following training?

Answer 24

Comparing trained vs untrained, same O₂ uptake, but lower ventilation with trained subjects at the same power output, indicating increased O2 extraction.

• Reduced blood lactate/H+
• Lower plasma K+
• Lower plasma catecholamines
• Reduced activation of muscle afferents?
• Reduced central drive?