A hemoabdomen is when blood fills up the abdominal cavity due to an internal bleed. Typically the abdominal cavity is filled with all the internal organs, such the intestines, the stomach, spleen, liver, pancreas, bladder, and so on. These organs all nestle together in the space below the diaphragm and above the pelvis. If an organ or blood vessel gets injured, and blood enters the abdominal cavity (abdomen), it fills up that space between organs. Unfortunately, there is no way for the blood to drain out of the abdomen once it enters, so it just keeps filling up all the available space. As the blood builds up, it puts pressure on the organs and the outer wall of the abdomen, pushing out as if it were a balloon blown up beyond capacity.
Meanwhile, the blood supply cannot circulate in the body because it’s leaking into the abdomen. This is an emergency for your dog: internal organs aren’t getting blood and oxygen (which means anemia has set in), and your dog is at risk of bleeding out. If the bleeding (hemorrhaging) is severe, dogs can go into shock with a fast heart rate, low blood pressure, labored breathing, pale gums, and even pale eyelids. Your dog might collapse on even die. With less severe bleeding, the blood might clot temporarily, sealing off the leak, and your dog might even seem to feel better … only to feel worse when the bleeding starts again.
If your dog has a swollen belly, especially a quickly expanding one, and seems distressed, weak, pale, get to the vet for an emergency ultrasound to find the source of the bleed.