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Role of exercise in changing the heart rate?

Hi, I'm currently learning about the effect of exercise on heart rate. I have a few textbooks which I generally use, however they present conflicting information.

So when you exercise, muscles respire at a faster rate and as a result, there is a higher concentration of CO2 in the blood which dissolves to form an acidic solution. This is detected by chemoreceptors in the arteries (e.g. carotid and aorta) and this decrease in pH causes the heart to pump at a faster rate to aid the removal of CO2 at the lungs (the faster heart rate means that the concentration gradient between O2 and CO2 in the lungs is maintained). [I think this bit is correct- could someone confirm?]

During exercise however, the increasing heart rate is also due to the stretch receptors present in the arteries and in the aorta of the heart. During exercise, the muscles contract harder and more frequently which causes more blood to be pushed into the atria of the heart. This means that there is a larger volume of blood in the atria which causes blood pressure to increase. This 'stretches' the stretch receptors which sends nervous impulses to the cardiovascular centre in the medulla oblongata, which sends an impulse via the parasympathetic nerve to the heart causing heart rate to decrease and blood pressure to also fall. [This makes sense, however by this logic, shouldn't there be a fall in blood pressure during exercise? Or there should be a slight increase in heart rate (as BP rises) but then this is regulated to control BP and so HR will subsequently fall?]

Thanks for your help!

Reply 1

schtim

Original post by Bazinga?

First off, let's clear one thing up-exercise physiology is complicated. There are lots of things which affect cardiac output during exercise, many of which are hotly debated by the leading scientists in the field, so don't feel too bad if you're a bit baffled, because so is everyone else. Bear in mind that the changes in cardiac output you see for a given period of exercise will be the result of lots of "tweaks" from lots of different physiological systems. Some inputs will act to increase cardiac output to supply more oxygen, glucose etc to the exercising muscles; some inputs will act to put the breaks on cardiac output so your blood pressure doesn't rise to a squillion mmHg and blow your head off. When you read in books about "X decreases cardiac output in exercise", if often shouldn't be taken as meaning "during exercise, the overall change in cardiac output is a decrease, because X is the most important factor"-often, it actually means "when we artificially kept everything else in an experiment the same, cardiac output decreased because X was the only thing making it change".

Long story short, the overall effects of all the different inputs combined means that heart rate increases with increasing exercise intensity (shocking, right?) up til a maximum heart rate, which is determined by the fastest rate at which your natural pacemaker tissue can fire electrical impulses.

I can't give a fuller explanation right now, but if you want to learn some of the really important stuff about hearts and exercise, try googling "Starling mechanism"-this isn't directly to do with heart rate as much as the amount of blood pumped per heartbeat, but it's one of the most important concepts to know about if you're interested in what the heart does during exercise.