A Bug's Life

IANAD, nor do I play one on LJ, but I think that when your metabolism demands more pints per minute, your heart could respond to the demand in three ways:

• More pressure
  
• More pints per beat
  
• More beats per minute

As it happens, your heart muscle has only so much force in it to give per ounce of muscle, and it can't change its size on short notice, so it responds not by pumping the same number of pints with more pressure, or more pints per beat, but the same number of pints per beat, against the same pressure, with more beats per minute.

So a high heart rate implies you haven't been exercising, your heart muscle mass is small, you have miles of extra capillary (fatty tissue has capillaries that need feeding too) and your veins and arterioles are narrow. A low heart rate implies you've been exercising, your heart is well-muscled, your vessels are wide, and the total mileage in your system is low.

(if I hadn't missed both the weekly repeats of Dr. Alice Roberts: Don't Die Young when she did Hearts last week (it's Eyes tonight) then I probably wouldn't have to guess)

Since, I believe, blood flow is laminar in all but the largest vessels, length and diameter of your circulatory system should be related to the volume flow and pressure by the Hagen-Poiseuille equation, which I'm not going to look up and try to render in LJ's HTML, but IIRC, the flow for a given pressure goes as something like the fifth or eighth power of diameter, so a little clearing of the tubes should go a long way.