Each time a blood vessel splits into smaller vessels, red blood cells (RBCs) are presented with the same decision: Take the left capillary or the right. While one might think RBCs would divide evenly at every fork in the road, it is known that at some junctures, RBCs seem to prefer one vessel over the other. One new computer model looks to determine why RBCs behave this way, untangling one of the biggest mysteries in our vascular system