Once apon a time...

There were a group of kids that lived in bodyville. Their names were red blood cell and white blood cell. Red carried oxygen in her backpack with her and white always beat up any infection bullies that came their way. They traveled through arteries to capillary park all the time. Gas exchange happened there which just meant the carbon dioxide which is produced by respiration in the cells diffuses into the blood plasma. They would leave the park through veins to their heart home because veins collect/carry blood from capillaries to the heart. Capillary park was known as the smallest blood vessel, arteries were the largest(so that there wouldn’t be traffic), and veins were the size in between both. 

Sometimes there would be traffic . People called traffic clots but the traffic was caused by platelets. Red and White blood cell took their plasma bikes to travel along with them. Their plasma bikes carried lipids, nutrients, and waste.