I don't think any of the top answers are good at all. I suspect part of the reason is how poorly we understand muscle fatigue.
Many answers talk about lactate and how it builds up in muscles but not the heart. However they don't say anything about how lactate is related to fatigue.
A lot of the answers talk about the high mitochondria concentration in the heart. This allows it to use more oxygen and produce more ATP. That's fine, but again, what is the connection between mitochondria and fatigue?
One example to show these answers are inadequate: You can walk or jog at an easy effort for a long time and eventually your muscles will get tired and sore, without ever going anaerobic or lactic -- i.e. without your leg muscles ever needing more energy than your mitochondria can provide or ever producing more lactic acid than your bloodstream can clear away immediately.
So mitochondria and lactic acid can't be the full story.
I don't know the true answer and I'm not a biologist, but I suspect the heart has evolved to never be the weakest link. Maybe exercise always strengthens the heart at a higher rate than the other muscles. Or maybe part of the answer is that we don't have nerves to feel soreness/tiredness in the heart? (I have seen research that the heart does experience temporary damage from e.g. marathons, so it does get tired in some sense. Of course then it gets stronger.)
Another part of the answer is that fatigue is largely in our heads. Your body wants you to minimize energy spending, probably because in cavemen days food was so hard to obtain. You will feel tired long before you hit real physical limits. If you've ever taken amphetamines, it becomes obvious. Suddenly, you can spend all night dancing more energetically than you ever did without feeling tired, work longer than ever, etc. Not suggesting that this is in any way sustainable long-term, it isn't, but for me, it really highlighted how my body is physically able to go several times beyond the point where I feel tired and uncomfortable. It's just that some core instincts really don't want me to spend energy liberally.
I don't know if it was a top answer when you looked 8 hours ago, but a cardiovascular physiologist answered (14 hours ago) a very thorough, detailed answer with second-order explanations. It's now _the_ top comment, here is a direct link with tracking removed:
The comments bridge these gaps. Production of lactic acid allows for the production of ATP in the absence of oxygen. It’s like a buffer for peak loads. The heart’s load is heavy, but relatively steady. It doesn’t have the ability to have peaks in output. It also can’t afford to not be supplied with a constant supply of lot of oxygen to feed the mitochondria.
As a bit of a side rant: biology is complicated, that doesn’t mean good work hasn’t been done. I notice many people discredit the field of biology, but eventually make fools of themselves when talking about it.
i'll take the word of a Cardiovascular Physiologist in that reddit thread over some dude on hacker news who really doesn't know what he's talking about
> You can walk or jog at an easy effort for a long time and eventually your muscles will get tired and sore,
Perhaps this is because your lungs are getting tired--I wonder if there's lactate buildup in the lungs. I'm no biologist, but my understanding is our muscles need oxygen, to get this oxygen we need to do many things--breathe more air (v02 max), more efficiently distribute the oxygen through the body (heart rate and stroke volume), and finally the oxygen is delivered to the mitochondria in our muscles, so increased vascularity (vein size), would help.
Our muscles also need other supplies of energy--glycogen to produce atp or ketosis. I suspect if we're glycogen dependent we get tired a lot faster. There are many hyper marathon runners that are in ketosis, like Goggins:
> He was able to run 101 miles in 19 hours and 6 minutes—despite never having attempted to run a marathon previously
I think if the heart gets tired, you die, so the brain probably stops that from happening. You can die of exhaustion, many atheletes just die early from heart attacks.
Anecdotally, if I don't get lactate buildup in my muscles, since being in ketosis I don't really get tired. I went from a 5 minute bike being difficult, to a 1 hour bike and not getting tired near the end.
So it's been a while since I've studied this subject, but it works as most of our needs: physical activity activates some areas of our brain (in this case mostly the parietal lobe and the cerebellum) whereas other areas have their activity reduced. What follows in our senses is a response to the increased physical demand, through tiredness and soreness. Cardiac muscles and skeletal muscles communicate with different areas of the brain (consider how fear causes your heart rate to increase, for example), which are related to different sensations.
As we were told during high school biology: cardiovascular muscle contains vast amounts of mitochondria as compared to other muscle types. Mitochondria are good. Therefore, more and stronger mitochondria = more efficient working muscle tissue.
One of my favorite professors holds a PhD in kinesiology (exercise physiology concentration) and wrote his dissertation on the mitochondria. As he stated many times in class: if you want to have a superhuman composition, acquire more mitochondria. he was always willing to bet his kids entire college tuition funds if you could show him a pill that would produce more mitochondria.
Aren't mitochondria captive cells from early evolutionary times?
I googled and from nature.com
"Mitochondria and chloroplasts likely evolved from engulfed prokaryotes that once lived as independent organisms. At some point, a eukaryotic cell engulfed an aerobic prokaryote, which then formed an endosymbiotic relationship with the host eukaryote, gradually developing into a mitochondrion."
> If the heart is a muscle, why doesn't it get tired?
Because it's illogical to assume muscles fatigue. They physically don't get to the point where they can no longer operate -- under normal everyday use and exercise.
What does happen, that may seem like fatigue, is a gradual shifting of fuel sources by muscle cells depending on exertion levels.
Suffice to say, it goes like this:
- First: Creatine Phosphate is the first energy source in your muscles, the most powerful one, and the least plentiful. It's the first to go during exercise and is partly why you focus on 1-5 reps for strength-focused lifting (that's roughly how long it takes to get used up)
- Second: Glycogen in your muscles and liver. This is fairly plentiful, fairly powerful, and rapidly mobilizeable. After your creatine phosphate stores are emptied, glycogen takes over. This is still a very powerful fuel source, but its metabolism creates a negative feedback loop on itself. You can only sustain moderate exertion (see: sprinting or 8-15 reps moderate weight) for around 60-90 seconds, before glycogen is no longer easily accessible (note: accessible, not depleted. It's almost impossible to deplete in a single workout)
- Finally: Fat. Once you've exhausted glycogen, your body turns to fatty acid oxidation. This is the least powerful but most plentiful. In normal exercise you use a mix of fat and glycogen depending on how hard you exert yourself. Harder: more glycogen. Easier: more fat. If you want to know how it feels like to run on only fat: do a marathon -- then hit the wall. What you experience is literally the complete depletion of glycogen, and a transition into "low-power" mode as your body starts running on the only fuel source it has left: fat. You have months of this fuel source on your body, you won't run out. You can keep walking for days without having eaten anything or slept, but you won't be able to run at any pace that resembles a jog. I know, because I have.
What's the point of all this? It's to illustrate that muscle "exhaustion" is a misnomer. Muscle exhaustion is, in reality, a depletion of power-generating fuels leading to a state of minimal exertion.
Coincidentally, that's the mode the heart operates in 24/7.
Coincidentally, the heart has an asinine amount of mitochondria to fuel non-stop fatty acid oxidation.
The heart doesn't fatigue because it's not physically possible. It doesn't need creatine phosphate or glycogen (very very little) to pump blood. It's not a strenuous task.
An aside: if you had as much mitochondria in all your cells, as you do in your heart's, you would waste away.
Then I guess the obvious question a non-physiologist would ask is, why don't all our muscles already have vast amounts of mitochondria? If it's that beneficial with no major downsides then surely evolution would have stumbled on that configuration already?
He would have lost his sons college thition then. Mitochondrial _inhibition_ is actually more correlated with longevity than the other way around.
If more mitochondria meant more efficient working muscle tissue, then that's what we would have evolved, there's always a compromise. Are you sure you can keep the same power output for a given mass and volume of muscle if you keep increasing mitochondrial numbers? Even the reddit answer clearly says that the heart muscle is rate limited by oxygenation (they have more mitochondria but are also highly vascularized to power the mitochondria). More mitochondria===more power doesn't mean it's better for us, physiology is rarely that simple.
There's three types of muscles: smooth, skeletal, and cardiac. They share a lot in common but are very distinct to each other. Might not be an apples to apples comparison because fatigue is a notion taken from our experience of skeletal muscles which is not cardiac muscle. It would be interesting to compare the energy spend of each, or rather the power each is capable of. As enduring as the heart is its likely limited in its power output because its just circulating fluid against resistance and gravity. I don't know how much energy is required for that. Bench pressing a hundred pounds, what's the power in that and how many heart beats would equal that work?
I'd actually be interested to know how many mitochondria are there in the heart of other species when compared to humans, because the mere assertion that the heart doesn't get tired because otherwise we would die does little to explain the mechanisms behind it.
This. Beating heart on an ultrasound is widely considered the first true sign of a healthy pregnancy. Almost before anything else, the heart develops and immediately begins beating, and does so until you die.
There's a reason why the cliche X is the beating heart of Y.
Atrial fib is no joke. I don't know who needs to hear this, but if you are considering becoming an endurance athlete, and especially if you are considering transitioning to ultra-endurance, go see a fucking doctor before, after and during.
Everybody always wonders about the heart and never about the intestines.
Shit in shit out, day after day after day. If not for you guts never tiring, your heart wouldn't get any fuel, you'd get a fistula and die of sepsis, you'd back up like a lot of people's pipes this week because they're flushing unflushable wipes.
So here's to the real unsung smooth muscles of the body. We salute you.
Isn't it funny how many meanings the word "why" has?
My first explanation, on hearing the question, was: because, evolutionarily speaking, no organism would have relied so heavily on the heart if it got tired easily.
Or looking on it another way, if the heart was easily fatigued, we'd be all dead, or be very different.
There have been a few astrophysics shows where they talked about 'why are we here?', with varying definitions of 'why' or special emphasis on here (as opposed to elsewhere).
In most of them, someone says words to the effect of "well if we weren't here, we wouldn't be having this conversation". Survivor bias, in a very literal sense.
Wouldn't that increase mean arterial pressure? Would the heart reduce its output as a response to the external pressure? And what's with their claims of it clearing "trash" from your brain?
Doesn't it though?As we grow older, the heart rate usually slows down.And since it is a muscle, it shows that working out can improve heart rate and other vascular conditions.
"Tired" not as in the other muscles, but you can't function at 100% for long period of times.
I hate to spread a wacky idea, but I ran across something on Facebook the other day that I initially wanted to disprove but then started to seem plausible. The general consensus is that the heart is a pump and generally the sole driver of circulation in mammals. There is an alternative theory, which seems easy to disprove, that the heart is more of a regulator and operates more like a hydraulic ram. I can post more links later. Curious if anyone has heard of that and if there is a rebuttal to the idea.
Edit #2, from the paper: "Gladwin et al. estimated that 25% to 30% of basal human blood flow can be attributed to red blood cell-induced production of nitric oxide by vascular endothelium."
> The general consensus is that the heart is a pump and generally the sole driver of circulation in mammals.
It's certainly not the general consensus that the heart is the sole driver of circulation in mammals. Your heart drives the flow of blood out through your arteries; it's much less effective at driving blood back through your veins.
I routinely (especially in the summer in the Southwest) experience near immediate hypotension after finishing a (running) race. The last time it happened, the medic explained that my legs add significant circulatory capacity— when they’re moving. I now walk around after races until vasoconstriction happens, and I fair much better.
I don’t know enough to corroborate the ram pump theory, but I don’t doubt that there are other mechanisms aiding circulation
I always think of it as a peristaltic pump: big tube, squeeze one side, squeeze the other to circulate blood.
This paper might be of interest: "A review of selected pumping systems in nature and engineering--potential biomimetic concepts for improving displacement pumps and pulsation damping" https://www.ncbi.nlm.nih.gov/pubmed/26335744
Isn't a heart attack basically the heart muscle getting too fatigued to keep going?
It doesn't seem to ben the only muscle that doesn't get tired - e.g. if it weren't for sleeping I suspect the neck muscles could hold up the head indefinitely. The diaphragm obviously never gets tired either, unless you have asthma.
I can also type pretty much indefinitely without feeling muscle fatigue in my fingers. climbing on the other hand will exert them enough to quickly fatigue them.
[+] [-] bo1024|6 years ago|reply
Many answers talk about lactate and how it builds up in muscles but not the heart. However they don't say anything about how lactate is related to fatigue.
A lot of the answers talk about the high mitochondria concentration in the heart. This allows it to use more oxygen and produce more ATP. That's fine, but again, what is the connection between mitochondria and fatigue?
One example to show these answers are inadequate: You can walk or jog at an easy effort for a long time and eventually your muscles will get tired and sore, without ever going anaerobic or lactic -- i.e. without your leg muscles ever needing more energy than your mitochondria can provide or ever producing more lactic acid than your bloodstream can clear away immediately.
So mitochondria and lactic acid can't be the full story.
I don't know the true answer and I'm not a biologist, but I suspect the heart has evolved to never be the weakest link. Maybe exercise always strengthens the heart at a higher rate than the other muscles. Or maybe part of the answer is that we don't have nerves to feel soreness/tiredness in the heart? (I have seen research that the heart does experience temporary damage from e.g. marathons, so it does get tired in some sense. Of course then it gets stronger.)
[+] [-] tachyonbeam|6 years ago|reply
[+] [-] dotancohen|6 years ago|reply
https://www.reddit.com/r/askscience/comments/fm2z8x/if_a_hea...
[+] [-] willis936|6 years ago|reply
As a bit of a side rant: biology is complicated, that doesn’t mean good work hasn’t been done. I notice many people discredit the field of biology, but eventually make fools of themselves when talking about it.
https://www.reddit.com/r/askscience/comments/fm2z8x/if_a_hea...
[+] [-] huffmsa|6 years ago|reply
The heart is actually "resting" 60-70% of the time, the time between contractions, and it fires in a staggered fashion.
The high level of mitochondria means it can produce the ATP it needs during these short rests and is ready to go the next time the SA node zaps it.
[+] [-] ryder9|6 years ago|reply
[+] [-] solinent|6 years ago|reply
Perhaps this is because your lungs are getting tired--I wonder if there's lactate buildup in the lungs. I'm no biologist, but my understanding is our muscles need oxygen, to get this oxygen we need to do many things--breathe more air (v02 max), more efficiently distribute the oxygen through the body (heart rate and stroke volume), and finally the oxygen is delivered to the mitochondria in our muscles, so increased vascularity (vein size), would help.
Our muscles also need other supplies of energy--glycogen to produce atp or ketosis. I suspect if we're glycogen dependent we get tired a lot faster. There are many hyper marathon runners that are in ketosis, like Goggins:
> He was able to run 101 miles in 19 hours and 6 minutes—despite never having attempted to run a marathon previously
I think if the heart gets tired, you die, so the brain probably stops that from happening. You can die of exhaustion, many atheletes just die early from heart attacks.
Anecdotally, if I don't get lactate buildup in my muscles, since being in ketosis I don't really get tired. I went from a 5 minute bike being difficult, to a 1 hour bike and not getting tired near the end.
[+] [-] avoz|6 years ago|reply
[+] [-] docbrown|6 years ago|reply
One of my favorite professors holds a PhD in kinesiology (exercise physiology concentration) and wrote his dissertation on the mitochondria. As he stated many times in class: if you want to have a superhuman composition, acquire more mitochondria. he was always willing to bet his kids entire college tuition funds if you could show him a pill that would produce more mitochondria.
[+] [-] loeg|6 years ago|reply
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3737409/
aka
https://en.wikipedia.org/wiki/SR9009
[+] [-] justinator|6 years ago|reply
[+] [-] chrisco255|6 years ago|reply
[+] [-] peter_retief|6 years ago|reply
[+] [-] heartisnt001|6 years ago|reply
> If the heart is a muscle, why doesn't it get tired?
Because it's illogical to assume muscles fatigue. They physically don't get to the point where they can no longer operate -- under normal everyday use and exercise.
What does happen, that may seem like fatigue, is a gradual shifting of fuel sources by muscle cells depending on exertion levels.
Suffice to say, it goes like this:
- First: Creatine Phosphate is the first energy source in your muscles, the most powerful one, and the least plentiful. It's the first to go during exercise and is partly why you focus on 1-5 reps for strength-focused lifting (that's roughly how long it takes to get used up)
- Second: Glycogen in your muscles and liver. This is fairly plentiful, fairly powerful, and rapidly mobilizeable. After your creatine phosphate stores are emptied, glycogen takes over. This is still a very powerful fuel source, but its metabolism creates a negative feedback loop on itself. You can only sustain moderate exertion (see: sprinting or 8-15 reps moderate weight) for around 60-90 seconds, before glycogen is no longer easily accessible (note: accessible, not depleted. It's almost impossible to deplete in a single workout)
- Finally: Fat. Once you've exhausted glycogen, your body turns to fatty acid oxidation. This is the least powerful but most plentiful. In normal exercise you use a mix of fat and glycogen depending on how hard you exert yourself. Harder: more glycogen. Easier: more fat. If you want to know how it feels like to run on only fat: do a marathon -- then hit the wall. What you experience is literally the complete depletion of glycogen, and a transition into "low-power" mode as your body starts running on the only fuel source it has left: fat. You have months of this fuel source on your body, you won't run out. You can keep walking for days without having eaten anything or slept, but you won't be able to run at any pace that resembles a jog. I know, because I have.
What's the point of all this? It's to illustrate that muscle "exhaustion" is a misnomer. Muscle exhaustion is, in reality, a depletion of power-generating fuels leading to a state of minimal exertion.
Coincidentally, that's the mode the heart operates in 24/7.
Coincidentally, the heart has an asinine amount of mitochondria to fuel non-stop fatty acid oxidation.
The heart doesn't fatigue because it's not physically possible. It doesn't need creatine phosphate or glycogen (very very little) to pump blood. It's not a strenuous task.
An aside: if you had as much mitochondria in all your cells, as you do in your heart's, you would waste away.
[+] [-] jldugger|6 years ago|reply
Did he mention whether faculty get free family tuition or not?
[+] [-] hyperdunc|6 years ago|reply
[+] [-] hkmurakami|6 years ago|reply
[+] [-] ramraj07|6 years ago|reply
If more mitochondria meant more efficient working muscle tissue, then that's what we would have evolved, there's always a compromise. Are you sure you can keep the same power output for a given mass and volume of muscle if you keep increasing mitochondrial numbers? Even the reddit answer clearly says that the heart muscle is rate limited by oxygenation (they have more mitochondria but are also highly vascularized to power the mitochondria). More mitochondria===more power doesn't mean it's better for us, physiology is rarely that simple.
[+] [-] jxramos|6 years ago|reply
[+] [-] TheSpiceIsLife|6 years ago|reply
I’d imagine it’s at least a magnitude of more than the heart would expend in 5 seconds.
[+] [-] downerending|6 years ago|reply
It's like being Atlas--you're holding up the world. It's not a virtue, a flaw, a conceit, nor anything at all. It is because it must be.
[+] [-] avoz|6 years ago|reply
[+] [-] hadlock|6 years ago|reply
There's a reason why the cliche X is the beating heart of Y.
[+] [-] naasking|6 years ago|reply
They're asking why the heart can't get tired. You answered why we evolved such that the heart can't get tired, which is not the same question.
[+] [-] bad_user|6 years ago|reply
You can overwork your heart which can lead to lesions. Endurance athletes who exercise excessively can have heart problems.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3538475/
That muscles get tired and sore is a feature because it prevents damage.
[+] [-] hinkley|6 years ago|reply
Source: father had a stroke 20 years later.
[+] [-] huffmsa|6 years ago|reply
Shit in shit out, day after day after day. If not for you guts never tiring, your heart wouldn't get any fuel, you'd get a fistula and die of sepsis, you'd back up like a lot of people's pipes this week because they're flushing unflushable wipes.
So here's to the real unsung smooth muscles of the body. We salute you.
[+] [-] luc_|6 years ago|reply
[+] [-] jaked89|6 years ago|reply
[+] [-] 01100011|6 years ago|reply
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5215277/
It may be that the heart is multi-functional. It may serve as a regulator, a pump, or some combination of the two.
[+] [-] perlgeek|6 years ago|reply
My first explanation, on hearing the question, was: because, evolutionarily speaking, no organism would have relied so heavily on the heart if it got tired easily.
Or looking on it another way, if the heart was easily fatigued, we'd be all dead, or be very different.
[+] [-] hinkley|6 years ago|reply
In most of them, someone says words to the effect of "well if we weren't here, we wouldn't be having this conversation". Survivor bias, in a very literal sense.
[+] [-] 6510|6 years ago|reply
This is a pretty cool technology.
https://billionsinchange.in/en/solutions/renew/
It squeezes the legs to help the heart pump blood.
[+] [-] elric|6 years ago|reply
[+] [-] sebow|6 years ago|reply
"Tired" not as in the other muscles, but you can't function at 100% for long period of times.
[+] [-] 01100011|6 years ago|reply
Edit: The paper I was thinking of, if anyone is interested, is here https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5215277/
Edit #2, from the paper: "Gladwin et al. estimated that 25% to 30% of basal human blood flow can be attributed to red blood cell-induced production of nitric oxide by vascular endothelium."
[+] [-] thaumasiotes|6 years ago|reply
It's certainly not the general consensus that the heart is the sole driver of circulation in mammals. Your heart drives the flow of blood out through your arteries; it's much less effective at driving blood back through your veins.
[+] [-] ultrarunner|6 years ago|reply
I don’t know enough to corroborate the ram pump theory, but I don’t doubt that there are other mechanisms aiding circulation
[+] [-] rzzzt|6 years ago|reply
This paper might be of interest: "A review of selected pumping systems in nature and engineering--potential biomimetic concepts for improving displacement pumps and pulsation damping" https://www.ncbi.nlm.nih.gov/pubmed/26335744
[+] [-] unknown|6 years ago|reply
[deleted]
[+] [-] yawaramin|6 years ago|reply
[+] [-] ck2|6 years ago|reply
A beat is not guaranteed volume, a rested heart can move more blood at the same heartrate than a tired one.
[+] [-] havella|6 years ago|reply
[+] [-] econcon|6 years ago|reply
[+] [-] science4sail|6 years ago|reply
https://en.wikipedia.org/wiki/Athletic_heart_syndrome
[+] [-] ummonk|6 years ago|reply
It doesn't seem to ben the only muscle that doesn't get tired - e.g. if it weren't for sleeping I suspect the neck muscles could hold up the head indefinitely. The diaphragm obviously never gets tired either, unless you have asthma.
I can also type pretty much indefinitely without feeling muscle fatigue in my fingers. climbing on the other hand will exert them enough to quickly fatigue them.
[+] [-] 01100011|6 years ago|reply
[+] [-] aaaaarghZombies|6 years ago|reply
It also has a pretty good blood supply.
[+] [-] blurbleblurble|6 years ago|reply
[+] [-] blurbleblurble|6 years ago|reply