A form of cancer that originates in the endothelium, which is the lining of blood vessels and spleen. These tumours are highly malignant and can be found almost anywhere in the body. The spleen, pericardium and heart are prone to be affected. Quite often there is little warning of the presence of these tumours before they cause severe clinical signs of the disease. An estimate of the average time from discovery of the tumour until death occurs in affected dogs is six to eight weeks.
The most common initial symptoms include visible bleeding, usually in the form of nosebleeds, and signs associated with blood loss, such as weakness, tiring easily, paleness to the mucous membranes of the mouth and eyes, increased respiratory rates, and abdominal swelling. In some cases, dogs just suddenly die with no clinical signs.
Full information can be found at this link Pet Education - Hemangiosarcoma.