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I am sorry if this is not appropriate for this site, but I think it fits so I am asking it here:

I went for a run yesterday, and it was about -8⁰C. I was wearing a couple pairs of shorts, a t-shirt and a long sleeved shirt, which is not that much, so I went with the idea that I would warm up a lot while running. After running for a while, I found that I was tired and out of breath much earlier on then I would normally be. It raised the question:

How does decreasing the ambient temperature affect the cardiovascular system?

There were two things I could think of that might directly affect it. (1) The fact that my body's metabolism has to work to heat itself, and this takes energy. However I am not sure how much of a role this plays since running warms you up anyway. (2) The air being so cold, and entering the lungs, I imagine my body has to work much harder in some way to deal with it. What exactly happens here, and how is the process different from breathing at 25⁰C?

Also, how much faster would we expect a runner to be able to finish a 10km run when it is 25⁰C outside versus -5⁰C outside?

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This question is somewhere between Fitness and Biology. If you're interested in a biological answer maybe you could focus the question more on that. If you want a more fitness-oriented answer I can migrate the question to our Physical Fitness site. –  Mad Scientist Feb 3 '12 at 14:19
    
@MadScientist: A more biological answer would be better –  user443 Feb 3 '12 at 16:24
    
Your question mismatches your description: as long as you weren't jogging naked outside your body wasn't actually exposed to cold (and provided good warmth insulation by your clothes there wasn't need for excessive heat production). So, are you interested in considering your personal case or an abstract case of people running naked in cold environment? :) –  Alexander Galkin Feb 5 '12 at 21:11
    
@AlexanderGalkin: I guess I would be most interested in how breathing changes, and how much work it takes for your body to breath in the cold air. –  user443 Feb 5 '12 at 22:00

2 Answers 2

I think you are correct to discount your first point as the cellular respiration proceeding to supply your muscles with ATP when running will produce heat anyway, as you say.

As to the lungs having to work harder in cold conditions, its an interesting proposition. I would hypothesise that this may indeed be the case:

Gas exchange across the lungs occurs by diffusion. Fick's law (simplified) states that the rate of diffusion is proportional to the difference in concentrations multiplied by the surface area along which diffusion is occurring, all divided by the length of the diffusion pathway:

$V_d=k\frac{\large a.([C_2]-[C_1])}{\large l}$

In the situation of the diffusion of O2 and CO2 across the alveolar membrane, k is dependent solely on the temperature of the diffusing particles. Therefore the rate of diffusion will almost certainly be reduced in lower temperatures.

The slower rate of diffusion will mean that the body will indeed have to 'work harder' to expel CO2 and take up O2. This would be achieved by increasing both the hearts stroke rate and the ventilation rate. This holds true in theory, however I can not say for certain if it would occur in reality as I do not know how greatly Vd (diffusion rate) would be affected by the change in temperature and also crucially how much of a decrease in Vd would be sufficient to initiate the homeostatic responses mentioned above.

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In relation to the cardio vascular system a combination of adrenaline from exercise and the bodies natural response to the cold causes constriction of blood vessels in the skin and extremities. This helps to reduce heat loss.

The movement from running increases the amount of cold air that runs across your body and into your lungs, which would offset the effect of heat generated from running. According to the article about Thermoregulation in cold conditions on Wikipedia, the offset is great enough to make shivering while standing still a better way to keep warm.

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