There are, however, a number of popular theories which may cause, increase the chances of, or otherwise exacerbate a stitch. A leading theory is that the pain may be caused by an increase in blood flow to the liver or spleen. Increases in the heart rate during exercise will force extra red blood cells into the liver which can cause temporary hepatomegaly and portal hypertension.[4] Temporary hepatomegaly and portal hypertension can restrict blood flow through the portal vein of the liver thus slowing blood flow to the rest of the body; this is why most runner's cramps are felt on the right side near the liver.[4][5] A plausible mechanism for the pain is that high internal pressure in the liver or spleen restricts blood flow, causing hypoxia.