MadSci Network: Anatomy

Re: What causes that pain I get in my side when I run, jog, etc. for to long.

Area: Anatomy
Posted By: Lynn Bry, MadSci Admin
Date: Mon Mar 31 02:29:22 1997

Hi Ben -

The 'pain' in your side probably reflects "referred visceral pain" from your gut. I'll describe what this means below.

You can think of your gut - stomach, small intestine and colon/large intestine - as a long muscular tube with a special lining on the inside. This tube has its own "nervous system" (the enteric nervous system) which includes pain fibers. These fibers primarily respond to two kinds of stimuli:

  1. Excessive strecting of the tube (as happens with gas pains!)
  2. Lack of oxygen.
In your case the lack of oxygen causes the pain. When you're running around your muscles have an increased demand for blood and the oxygen and nutrients it carries. More oxygen to these tissues can mean less going to your gut. If you have just eaten a full meal, your stomach and intestines really need the blood to give these tissues oxygen to make cellular energy for digestion, and to transport the nutrients you have eaten to the liver so they can be processed appropriately. If you start running around, you derpive them of what they need, and they let you know about it by sending pain signals!

Why does your side hurt instead of a site more related to the internal location of your stomach?

The pain fibers from your gut enter the spinal cord and travel to the brain. You may know that your "backbone" consists of many bones called vertebrae. Nerves from the spinal cord exit between two vertebrae. Furthermore, these nerves also supply sensation (touch, holt/cold, pain) and innervation (control over muscles) to specific regions of the body. It just so happens that the pain fibers coming from your stomach enter the spinal cord in the same areas as the fibers that detect pain over the left side of your abdomen. For reasons that we don't fully understand, the brain's perception of visceral pain can be referred to the location of the "peripheral" pain fibers that go to areas of skin supplied by nerves coming from the same region of the spinal cord. Weird huh?

You may also find it interesting to know that people suffering a heart attack often feel pain in places as far away as their arm or shoulder. The pain fibers of the heart enter the spinal cord at the same level as fibers supplying sensation to the arms, thus the "referred pain" to these areas.

-L. Bry, MadSci Admin

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