If the tube is filled with fluid the muscles weakens and the blood is pressed out of the tube. The heart has it own nerve center that can let the heart beat independence from the brains. There are connections with the brain that can raise the heart frequency. This can be registered if the spider become excited and its heart frequency rise. In mammals the uptake of oxygen in the blood and the release of carbon dioxide from the blood takes place in the lungs.
Spider uses other organs.
Above the spinners there is a slit that can be opened and closed. From these slit long small tubes run deep into the body. These tubes are called trachea. By diffusion the gasses are exchanged with the blood. Beside this system a lot of spiders also have book lungs.