Illustration of the kidneys (dark grey, right and left). The kidneys filter waste products from the blood and excrete them as urine. Across centre is part of the duodenum, the start of the small intestine, where digestion begins and nutrients are absorbed into the blood. At centre are the aorta (red), the bodys main artery, and the inferior vena cava (green), the bodys largest vein. From Traite dAnatomie Humaine, Vol. 4 (1905) by French anatomist Leo Testut (1849-1925).