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The human heart pumps oxygen to the body, and the heart itself requires oxygen.

Both the body and the heart use energy, usually expressed in calories or ergs.

If we look at the energy consumption (in Watts) of the heart as a fraction $F$ of the energy consumption of the body as a whole it is about

$F \approx \frac{1.3}{91.3}=0.014.$

Humans consume about 550L of oxygen per day per this website and the heart itself consumes about 8 ml of $O_2$ per 100g per minute at rest according to this site. Assuming an average global human heart mass of about 310g (Google) the daily total heart oxygen consumption is about (8ml x 3.1 x 1440 min) = 35712 ml or 35.7L of oxygen. The ratio $R$ of heart to body oxygen consumption is thus about

$R \approx \frac{36}{550-36} = \frac{36}{514} =0.07$

These figures are rough at best but not rough enough to account for a discrepancy of a factor of 5 in the ratios. Can someone suggest what might account for the difference?

The calculation seems to show that the heart's proportional consumption of oxygen is a lot higher than its proportional consumption of energy. My understanding is that the respective oxygen consumption of the heart and body are a good reflections of their respective energy consumption. Is that a sound assumption?

Thanks for any insights.

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  • $\begingroup$ This is a really stupid question from me to you. Do you mean to say 7% percent when you say factor? But then you say factor in the ratios. Do you mean the consumption of heart versus body ratio amounts to 7? If so, I am not sure the word ratio is mathematically applied, but I might just be confused. $\endgroup$
    – Srihari Yamanoor
    Oct 1, 2016 at 16:24
  • $\begingroup$ Also, I think oxygen consumption most likely varies by cell/tissue type. Cardiac muscles may consume more per sq.m or cu.m compared to skeletal tissue and smooth tissue. You are also probably right in that resting heart rate might lead to a more steady, lower consumption rate. Other tissue, might consume even lesser than muscle tissue, etc. So maybe heart to body rate consumption is not all too meaningful. You could look at cardiac to overall muscle energy consumption, etc. $\endgroup$
    – Srihari Yamanoor
    Oct 1, 2016 at 16:29
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    $\begingroup$ @SrihariYamanoor: I may have worded it badly. What I mean is that 0.07 is very roughly 7 times 0.014. It should be 5 and I will edit. Yes, I mean that the heart's proportional consumption of oxygen seems to be a lot bigger than its proportional consumption of energy. Does this help? $\endgroup$
    – daniel
    Oct 1, 2016 at 16:30
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    $\begingroup$ In the case of the heart, aerobic metabolism is necessary for it to work. But that's not the case for all tissues (some have significant anaerobic metabolism). The difference being only 6% that would not strike me as too much of a stretch to explain it this way, but I have nothing reliable on the subject so that's just speculation. $\endgroup$
    – Eliane B.
    Oct 1, 2016 at 19:49
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    $\begingroup$ 0.07-0.014, considering the total energy expenditure/total oxygen consumption. That's ~7 times but also absolute value 6% of total expenditure. $\endgroup$
    – Eliane B.
    Oct 1, 2016 at 19:57

2 Answers 2

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At first glance there are a few major issues with your calculation and assumptions.

  1. You're comparing the energy output in work (pumping action) with the energy consumption of the rest of the body in input (oxygen consumption). I don't know how efficient the heart is, but I'd guess only somewhere from 20-40% of input energy is actually converted to pumping action.

  2. Oxygen consumption is correlated with energy consumption, and would be a good way to measure energy consumption if you know exactly what the subject is burning. However, if you have a big difference between the preferred fuels between the heart and the rest of the body, this doesn't really apply anymore. A quick google leads me here: https://www.ncbi.nlm.nih.gov/pubmed/17081788, showing that the heart prefers to burn fat over glucose, and this creates a significant difference:

    Carbohydrate oxidation typically generates approximately 120 kcal per mole of respired oxygen, whereas fatty acid oxidation typically generates only approximately 100 kcal per mole of oxygen.

This means that the heart needs more oxygen than the rest of the body to generate the same amount of energy.

I think combined this could explain the 5-fold difference.

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  • $\begingroup$ You say you compare consumption, but the link you use actually has a list of outputs, based on the mechanical work of the heart. One of the lines actually reads: "The mechanical power of the human heart is ~1.3 watts. It takes a much higher rate of energy turnover (~13 watts) to provide this mechanical power, since the mechanical efficiency of the heart is very low (less than 10%)" $\endgroup$
    – VonBeche
    Oct 8, 2016 at 13:05
  • $\begingroup$ And the 6% of Eliane B is relating to something completely different. The difference between one calculation and the other is 5x, that this number is also only 6% of some larger number doesn't really change that. $\endgroup$
    – VonBeche
    Oct 8, 2016 at 13:11
  • $\begingroup$ The only thing is that 1.3 watt is not the input, it's the estimated output. You can infer this by looking at the numbers they use for their estimates: all numbers related to the output power, not input. $\endgroup$
    – VonBeche
    Oct 8, 2016 at 13:23
  • $\begingroup$ Come on! In the second line you cite they even write Pmech = 1.71 (Nm)/s and Pchem = 8.6 (Nm)/s. There's your 5-fold difference. $\endgroup$
    – VonBeche
    Oct 8, 2016 at 13:33
  • $\begingroup$ Let us continue this discussion in chat. $\endgroup$
    – VonBeche
    Oct 8, 2016 at 13:44
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You are comparing the theoretical mechanical work performed by the heart (your 1.33 W number) with the chemical energy consumed in accomplishing that in a biological system. One is theoretical and one is empirical. The heart in fact consumes about 10% of the energy of the body even though it’s only 1% of the weight, owing to the special nature of cardiac muscle tissue (which is unique to the heart organ). It’s a “design” that can go nonstop for one billion contractions or more, and which would have been ludicrously inefficient to use for skeletal musculature. Your 5x factor is in line with typical efficiency measurements of muscle work, around 20-25%.

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  • $\begingroup$ Cites would help. Your 10% is not in line with the sources I checked (doesn't mean it's wrong). $\endgroup$
    – daniel
    Jul 19, 2018 at 15:09

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