A spiracles is an opening in front of the spinnerets in most true spiders. Book lungs are in cavities in the spider's abdomen. Air enters the cavities through a tiny slit on each side and near the front of the abdomen. Each lung consist of 15 or more thin, flat folds of tissue arranged like the blood vessels. As air circulates between the sheets, oxygen passes into the blood.
Tarantulas have two pairs of book lungs.
Most true spiders have 1 pair. The blood of spiders contains many pale blood cells and is slightly bluish in color. The heart, a long, slender tube in the abdomen, pumps the blood to all parts of the body. The blood returns to the heart through open passsages instead of closed tubes, such as those of the human body. If the spider's skin is broken, the blood quickly drains from its body.