The heart pumps blood out through one main artery called the dorsal aorta. The main artery then divides and branches out into many smaller arteries so that each region of your body has its own system of arteries supplying it with fresh, oxygen-rich blood.
Arteries are tough on the outside and smooth on the inside. An artery actually has three layers: an outer layer of tissue, a muscular middle, and an inner layer of epithelial cells. The muscle in the middle is elastic and very strong. The inner layer is very smooth so that the blood can flow easily with no obstacles in its path.
http://www.meddean.luc.edu/lumen/Med...s/main_art.htm
The artery runs down the front of the pelvis. So your not in danger of hitting it in that area. If you dont asperate how exactly do you know if your in a VEIN? You dont unless you have exray eyes. So the moral of the story is learn about your biological structure, and the best place to inject, and always asperate to make sure you dont dump into the bloodstream causing discomfort and or possibly death. There are lots of sites available with good knowledge on biology