short term responses to challenge: exercise

short term responses to challenge: exercise

1. respiratory respnse

   • what cases hyperventillation during exercise?
     • Not a fall in PO2 or a rise in PCO2 as these are virtually unchanged during exercise (treadmill)
       • in fact, PO2 may rise a little during moderate exercise and PCO2 may actually fall during severe exercise (due to CO2 being “blown off”)
     • he H+ concentration in arterial blood remains unchanged (unless exercise is maximal)
     • Oscillations inPO2, PCO2 and H+ values may have some effect, but no overt changes are evident.
     • there is only an association between ventilation rate and changes in o2 and co2 concentrations
   • at rest
     • respiratory rate is 12 breaths per minute
     • heart rate is 72 beats per minute
     • Thus, blood leaving the lungs going to the left atrium will show oscillations in PO2 and PCO2 on a beat to beat basis.
   • during exercise
     • these variations are likely to be accentuated and this may be sufficient to influence the discharge of peripheral chemoreceptors.
     • resp are goes up to around 30 breaths per minute and rap gets more spikey
   • other known factors which covers exercise hyperpnoea
     • increased body temperature
       • heat generated by excising muscles
     • neural input from limbs
       • (joint and muscle sensory receptors activated by movement). This may partly explain sudden changes in the respiratory pattern at the beginning and end of exercise.
     • secretion of adrenaline and no addrealline
       • (sympathetic system stress response).
     • input from higher centres
       • abrupt changes in ventilation happen at beginning and end of exercise – a conditioned response in part. Also, the motor outflow from the cerebral cortex to the limbs may also impinge upon the respiratory centre.

2. cardiovascular responses

   • Exercise results in major changes in haemodynamics
     • As muscle activity increases, metabolites are produced causing local dilatation of arterioles.
       • This leads to fall in Total Peripheral Resistance (TPR)
         • therefore in order to maintain blood pressure CO must rise
   • Recall that BP = CO x TPR
     • BP= arterial blood pressure and CO= cardiac output
   • Systolic blood pressure (SBP) rises but diastolic blood pressure (DBP) tends to decline slightly, so that overall there is no fall in BP.
     • since mean arterial BP = DBP + (SBP-DBP)/3
   • One potential problem with the Starling mechanism is that in normal subjects there is no clear increase in heart diameter during exercise – if anything, diameter decreases with moderate exercise. Thought to be due to the increase sympathetic drive to heart leading to increased force of contraction, ejecting more blood per beat, thus reducing end-systolic volume.
   • Evidence that the nervous system is involved can be obtained from the observation that heart rate increases in anticipation of exercise
     • indicating that there is a central command from the higher brain centres leading to activation of the sympathetic system
       • this is an example of a feed forward mechanism(conditioned response)
   • The baroreceptor reflex is also likely to be involved, undertaking its usual role in maintaining arterial blood pressure
     • However, this reflex does not “drive” the response to exercise (as sometimes suggested in older textbooks).
   • Activation of the sympathetic system not only affects the heart (increasing HR and SV) but also constricts blood vessels in other vascular beds (GI tract, kidneys), diverting blood to the exercising muscles.
   • Venoconstriction (constriction of veins) also occurs
     • further increasing venous return. This is further assisted by the contracting skeletal muscles “squeezing” veins (‘muscle pump’)
   • The lower intra-thoracic pressure during exercise (due to deeper breathing) “draws” blood back to heart, enhancing venous return (‘respiratory pump’).
   • The sympathetic system also promotes sweating (for heat loss) and mobilisation of glucose (for energy).