The circulatory system is most advanced in spiders utilizing book lungs as their main source of respiration rather than those who breath by having bundles of tracheae going to different parts of the body. Despite this, all spiders posses a tubular heart in their abdomen which has a number of openings (ostia). The black widow's heart also has one artery to carry blood toward the heart when it contracts, and one that goes away from it. The outgoing blood being pumped by the heart becomes branched off in the spider by it's book lungs. Eventually, the black widow's blood flows into empty spaces, where it is then sucked into the sinuses of the book lungs. After this, the blood goes into the pericardial cavity where it then enters the heart through one of two holes. The blood of the black widow caries different blood cells and a respiratory pigment called hemocyanin. The blood pressure of the black widow changes in order for it to be able to maneuver its legs, or fragment its skin when shedding.