 As we have seen before that oxygen consumption during exercise can increase from 250 ml per minute at rest to approximately 4 to 5 litre per minute, so that is like 18 to 20 times the increase in oxygen consumption is occurring, how is that possible? That is because of increase in the ventilation, remember increase in ventilation where there is increase in the respiratory rate as well as increase in the depth of the ventilation. So as ventilation increases there is increase in oxygen consumption and if we draw a graph of oxygen consumption versus the ventilation we see a linear relationship as oxygen consumption is increasing ventilation also increases and linear relationship here also you see the slope oxygen consumption increases approximately 18 to 20 times and ventilation also increases approximately 18 to 20 times and what we see that the pulmonary ventilation at maximal exercise to be around 100 litres per minute ok, so that much it can increase to approximately 100 litres per minute, but remember that the maximum we can ventilate that is the maximum voluntary ventilation, what is this maximum voluntary ventilation? That is the maximum ventilation which can be achieved with the maximum rate and maximum depth of ventilation, this is approximately 150 to 170 litres per minute, understanding so at maximal exercise pulmonary ventilation is approximately 100 litres per minute, so still at that point of maximal exercise there is a reserve of approximately 50 litres per minute, why I am talking all this thing? This is because respiratory system is never the limiting factor for exercise in physiological condition, this is not the limiting factor because we have a huge reserve for respiratory system, so limiting factor actually is the cardiovascular system that we will see in a short while, coming back to respiratory system where we talked about oxygen consumption, where we talked about pulmonary ventilation, we also talked about the pulmonary reserve, what are the other changes occurring? Other change occurring is increase in the oxygen diffusing capacity that means more oxygen can diffuse, how is it possible? That is because at rest not all the pulmonary capillaries are open, there are some reserve capillaries which are like collapsed state, okay and whenever there is increase in the cardiac output, all the cardiac output goes from the right ventricle to lungs, right so lungs have to accommodate more cardiac output from the lungs, from the right ventricle, that time these capillaries open up and that is known as capillary recruitment, capillary recruitment, also the existing capillaries which are there, they also increase in diameter that is known as capillary distention, so capillary recruitment and capillary distention increases the surface area across which the oxygen can diffuse and that increases the oxygen diffusing capacity in lungs, then fifth change occurs in ODC curve, what happens in ODC curve? ODC curve actually shifts to right at the tissues, see at the level of the muscles which are exercising what will happen? There will be increase in temperature, there will be increase in the hydrogen ions, right, there will be increase in the carbon dioxide and if you remember all these factors cause the ODC curve to shift to right and hence at the level of the muscles there will be increased unloading of oxygen thus increasing the oxygen delivery to the tissues which require it most, so these are the various respiratory system changes which occur during exercise, one concept you should remember here that during exercise oxygen levels never go down and carbon dioxide levels never rise, understanding even though our body is requiring more oxygen and carbon dioxide is being produced at a more higher rate, still we don't see this, then you may wonder that how ventilation is increasing, if that is the case, it simply means that peripheral chemoreceptors are not the main reason why this ventilation is increasing, it is because from our higher centers from our cortex the information is coming and also from the exercising muscles that is the proprioceptors information is coming to the respiratory centers and there is stimulation of the respiration, so it is more like a feed forward mechanism preparing the body beforehand that exercise is happening rather waiting for oxygen levels to go down and carbon dioxide to rise and you may understand that it may not be very good for the exercise, now let's move on to the last part that is the cardiovascular adjustments during exercise, well during exercise obviously because of increase in the sympathetic activity there is increase in the cardiac output because of increase in heart rate and increase in the stroke volume both increase, now this cardiac output can increase up to five to six times during exercise and in marathon runners it has been seen that it can increase up to seven to eight times also, now people who are exercising for a long time what happens that there is hypertrophy of the heart, okay so muscle mass of the heart is increasing so this heart can pump with greater power so there will be increased stroke volume okay because it is pumping with a greater power, now at rest this increased stroke volume what it will lead to it will lead to unnecessary increase in cardiac output but we know that cardiac output is equal to heart rate into stroke volume, so if muscle mass has more power and it is causing increase in stroke volume why not decrease the heart rate so that the cardiac output at rest it will become normal, so that is why trained athletes trained athletes have resting bradycardia okay resting bradycardia because the increase in cardiac output you see that will lead to increase in blood pressure, baroreflex will be active and there will be activation of the parasympathetic system that leads to decrease in heart rate, so resting bradycardia occurs because of increased parasympathetic activity what is known as increase in vagal tone, so that is one concept we should remember, now when we are exercising which of these factors heart rate or stroke volume contributes more to exercise, so let us see it in graph manner, so I will not write the values here but just for the concept I am telling you suppose on exercise axis it is cardiac output in liters per minute and on y axis it is the stroke volume and also heart rate is there okay, now whenever we start to exercise stroke volume is the one which increases first maximally and later on it is the heart rate which contributes much to the increased cardiac output, so both have started increasing in the beginning try to understand this both have started increasing but you see the increase in the stroke volume it is much faster, so this reaches maximum value much much faster compared to the heart rate, so why this concept is important, see in trained athletes because of increased vagal tone there is resting bradycardia, so this portion it will be much lower right for the heart rate and that reserve increase in heart rate which can occur is much more because the maximum achievable heart rate will be the same for trained athletes and for the untrained people but the reserve of the heart rate is much more plus also the stroke volume will be more, so obviously the maximum of the stroke volume which is reached is much more, so that is how this resting bradycardia is very beneficial in case of trained athletes, now I was before referring to why that respiratory system is not the limiting factor in case of exercise it is cardiovascular system, see cardiac output is equal to heart rate into stroke volume, now during maximal exercise what happens is stroke volume and heart rate both reach up to 90% of maximum achievable okay, so obviously cardiac output also reaches to 90% of maximum which can be reached isn't it, was it the same case in case of respiration, no we had 50 litres of reserve actually respiratory system reaches only 65% of maximum okay, so the reserve in cardiovascular system is getting finished, so cardiovascular system is the limiting factor for the maximum exercise which can be attained and this depends on something known as VO2 max, what is the VO2 max that is the maximum amount of oxygen which can be utilized during aerobic exercise, so the limiting factor for VO2 max is actually cardiovascular system and not respiratory system in physiological foundations, now one question which is frequently asked is that what would be the changes in VO2 max with training, actually it has been found that VO2 max does not change much with training, even after three, five, seven weeks of training there was an increase in VO2 max of approximately only 10% and furthermore VO2 max could not be increased, so it has been thought that maybe that again this VO2 max is dependent on a particular athlete only, it cannot be changed much with exercise, so that was in brief about exercise we talk about the strength, the power, endurance we talked about the muscle metabolic systems and how they are important for different types of exercise, then we also covered oxygen depth and the recovery of the muscle glycogen and which diet is best for the recovery and finally we covered about the cardiovascular and respiratory changes occurring during the exercise, thanks for watching the video, if you liked it do press the like button share the video with others and don't forget to subscribe to the channel Physiology Open, thank you.