What effect does ambient temperature have on the cardiovascular system?

Question

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 -8 degrees Celsius. 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 cardio vascular 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 25C?

Also, how much faster would we expect a runner to be able to finish a 10km run when it is 25 C outside versus -5 C outside?

Answer 1

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.

Answer 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:

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 V_d (diffusion rate) would be affected by the change in temperature and also crucially how much of a decrease in V_d would be sufficient to initiate the homeostatic responses mentioned above.