Life in the Fast Lane (redux)



When we run quickly why does our heart beat harder and faster?

The laws of nature demand that to be able to do anything your body must have enough energy to do it.  To get the energy they need your cells use glucose and oxygen which they get from the blood (along with other important chemicals) that is pumped to them by the heart.  At complete rest your body uses a minimum amount of energy to keep its vital organs working.  Just like a car's engine when it's idling, your heart doesn't have to work very hard when it’s sitting still so it beats slowly and with less force.  But, just as a car’s engine must work harder to provide enough horsepower for it to race up a steep hill, so too, your heart must beat harder and faster to supply enough blood to your muscles to give them what they need (oxygen and glucose) so you can run quickly. In fact, when you run as fast as you can, your heart has to pump out about five times more blood per minute than when you are at complete rest. 

Scientists think that one of the ways your body manages this control is by having sensors in your muscles to detect their level of activity and certain chemical by-products of their metabolism.  This information is relayed to your brain which analyses it and determines how fast and hard your heart should pump for a given activity level. 

Remember (from the last article) that when the body is at complete rest the brain sends out electrical signals along specific nerves that release a neurohormone called acetylcholine which attaches to specific cholinergic receptors in the pacemaker and muscle cells of the heart making it beat slowly and with less force.  However, when the brain detects that you are running quickly, it turns off the electrical signals that release acetylcholine and turns on other ones that release two related neurohormones called norepinephrine and epinephrine.

Like keys in a lock, norepinephrine and epinephrine attach to specific (adrenergic) receptors in the pacemaker and muscle cells of the heart making it pump harder and faster.  In fact, compared to acetylcholine, the combination of norpinephrine and epinephrine makes the heart beat about two and a half times faster while pumping out about twice as much blood with each contraction.   Since the amount of blood the heart pumps out per minute is determined by how fast it beats and how much blood it sends out with each beat, this means that the effects of these neurohormones is to increase the cardiac output by 2.5 x 2 = 5 times, exactly the amount your muscles need to get the job done.  That’s why when you run quickly, your heart beats harder and faster.

Three Questions for Mr. Darwin

1) Where did the information come from to tell the bodies of our earliest ancestors how to make their hearts with enough functional capacity to pump out five times more blood per minute when running quickly compared to being at complete rest?

2) Where did the information come from to tell my body how to make the sensors to detect my muscle activity level and chemical by-products from their metabolism while placing them exactly where they’re needed, and how do they work?

3) To be able to run quickly, how does my brain know how much norepinephrine and epinephrine it should send out to make my heart pump out five times more blood per minute as compared to when I’m at complete rest?

 


Also see Dr. Glicksman's Series on

"Beyond Irreducible Complexity"

"Exercise Your Wonder"


Howard Glicksman M. D. graduated from the University of Toronto in 1978. He practiced primary care medicine for almost 25 yrs in Oakville, Ontario and Spring Hill, Florida. He now practices palliative medicine for a Hospice organization in his community. He has a special interest in how the ethos of our culture has been influenced by modern science’s understanding and promotion of what it means to be a human being.

Comments and questions are welcome.

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