The anatomy of the penis is complex. It is comprised primarily of two cylinders of sponge-like vascular tissue that fills with blood to create an erection. Blood is pumped into the penis under great pressure and a series of valves keep it in the penis to maintain the erection. A third cylinder is the urethra, a tube that carries the urine and the ejaculate. The knobby head of the penis is called the Glans.
Blood flows to the penis by two very small arteries that come from the Aorta.
These arteries are the same size as the arteries to your finger. The main problem that causes impotence is that the blood vessels become blocked and the blood can not get to the penis. The other major problem is leakage of blood from the penis into the veins around the penis called a venous leak. This is very common, similar to a hole in a tire. The enlargement of the penis by surgery is twofold.