The internal iliac veins are major blood vessels located in the pelvis. They are responsible for draining deoxygenated blood from the pelvic organs, including the bladder, rectum, and reproductive organs. These veins run alongside the internal iliac arteries, which supply blood to the same regions. The internal iliac veins merge with the external iliac veins to form the common iliac veins. These common iliac veins then converge to create the inferior vena cava, which carries blood back to the heart. Proper functioning of the internal iliac veins is essential for maintaining healthy blood circulation in the pelvic area.