Placental circulation, illustration. The umbilical arteries (red) and veins (blue) terminate in the placenta as tree-like villi. Between the villi are the blood-filled intervillous spaces lined with trophoblast tissue (green). In these spaces blood is in continuity with maternal/uterine vessels (red, blue) arising from the uterine tissues in which the placenta is embedded. From Gray, H, (1918) Anatomy descriptive and applied. 20th ed. Longmans Green, London.