It depends. In the early stages, hemangiosarcoma is a disease that does not really cause pain. In fact, the rate of growth during this stage is somewhat slow. There have been multiple cases where canines with large hemangiosarcomas did not show symptoms or clinical signs that they have this dangerous disease. The blood vessels with tumor cells get twisted and become abnormally formed so when the blood cells flow in, they stay there and clot. Tumor cells need blood and nutrients to keep growing, but malformed vessels and clots prevent this from happening. As a result, the tumor cells die and when it does happen, it ruptures the tumor and vessels and the blood escapes out into the chest, heart sac, abdomen, and subcutaneous space.Dogs may suffer from weakness and lethargy depending on how much blood they lost. If you notice any of these signs, make sure to bring your pooch to the vet right away. The symptoms of this disease may seem subtle, which is why many owners may not think twice about it until this life-threatening disease has gone too far. By the time some dog owners notice abnormal behavior, the hemangiosarcoma has metastasized to the mesentery, intestines, liver, and lungs. Consult with your vet because when a large tumor ruptures, it may lead to collapse, severe hemorrhage, and can be fatal.