Placenta. Illustration of blood flow through a hu- man placenta, showing a foetus next to a diagram of red blood cells flowing through part of the placenta. The irregular structure at centre is a chorionic villus - a projection of foetal tissue into the maternal part of the placenta. Deoxygena- ted blood (purple cells) from the foetus flows through this, and picks up oxygen from the nearby maternal blood. Arrows show direction of flow. The red cells are oxygenated. Some of the foetal blood cells (blue outline) have passed through the mem- brane into the maternal bloodstream. This can cause haemolytic disease if the mother is rhesus negative when the baby is rhesus positive.