What it is
Hemangiosarcoma grows from the cells that line blood vessels, so the tumour itself is made of fragile, blood-filled channels. In internal organs such as the spleen or heart, it can enlarge without causing pain, and the first sign may be a bleed when part of it tears. A small bleed can stop on its own and the body slowly reabsorbs the blood, which is why a dog may seem to recover and then weaken again.
It is linked to age and to larger body size. The skin form, on pale, thinly haired areas such as the belly, has been linked to sun exposure.
For you, this can mean living with uncertainty: a dog that seems fine can suddenly become weak. Knowing the normal colour of your dog's gums and its usual energy makes it easier to notice when something changes.
Signs you may notice
A dog may suddenly become weak or collapse, with pale gums, fast breathing and a swollen belly; such episodes can pass and then return. In the skin the tumour may look like a dark red or purple lump.
When to see a vet
A dog that suddenly collapses or becomes very weak, or has pale gums or a swelling belly, needs a vet immediately. Book a visit for any new dark red or purple lump in the skin.
This is general information, not veterinary advice. For your own dog's health, tests and treatment, see a vet.