The abdominal cavity contains vital organs such as the stomach and intestines, liver, spleen, pancreas, kidneys and urinary bladder. It also contains numerous blood vessels, lymph vessels and lymph nodes are also present in the abdominal cavity and is lined with a thin, specialized membrane (the peritoneum) that contains the contents within a sterile environment.

Abdominal distension can be caused by fat accumulation, fluid accumulation in the peritoneal space, enlargement of abdominal organs or weakness of the abdominal muscles. The fluids that cause abdominal distension can be blood, urine, exudate, transudate or any combination of these. Causes of these different fluid types are listed below:

Blood

Blood can fill the abdomen because of trauma, erosion of blood vessels, failure to form blood clots normally, or tumors causing organs to rupture.

Urine

Urine can fill the abdomen and cause distension. Rupture of the urinary tract is generally the result of trauma (such as being hit by a car).

Exudates

Exudates are thick, cellular fluids. These fluids often result from infection within the abdominal cavity. Dogs may develop exudate in response to bacterial infection as a result of a penetrating injury or a tear in the gastrointestinal tract. This can occur in dogs with a string-type foreign body that "saws" its way through the intestine. Exudates may also accompany cancers of the abdomen (neoplastic effusion) or result from obstruction to drainage of lymphatic fluid (chylous effusion). Lymphatic fluids are fluids that surround cells and are collected and transported by lymph vessels into the bloodstream.

Transudates

Transudates are clear fluids, without many cells or much protein, resulting from pressure blocking normal blood flow or from decreases in the protein (albumin), which holds water in the blood. Examples of processes likely to produce transudates include:

Right-sided heart failure, in which blood "backs up" in the vessels because it cannot enter the heart easily, and some liquid from the blood is forced out of the vessels. Congenital heart disease, heart muscle disease (cardiomyopathy), arrhythmia (abnormal electrical activity of the heart), and diseases of the pericardium are also potential causes of right-sided heart failure in dogs. However, heart failure is a very uncommon cause of abdominal fluid accumulation in dogs.

Cirrhosis, or fibrosis of the liver, also causes changes in pressure to the blood vessels within the abdomen. In addition, cirrhosis results in liver failure. When the liver fails, it does not produce normal amounts of albumin (blood protein).

Loss of albumin through the kidneys (or the gastrointestinal tract (lymphangectasia or protein loosing enteropathy) can lead to very low albumin levels. When albumin levels are too low, liquid is not held within the blood and can leak out into the abdomen. This situation is very uncommon in dogs.

Organ Enlargement

Enlargement of any abdominal organ can cause distension. Enlargement of the liver, kidneys or spleen may be due to obstructions of fluid flow (either blood flow or urine flow) or infiltration with cells (cancer or leukemia cells or inflammatory blood cells).

Other causes of abdominal distension that are not caused by fluid accumulation include:

Distension of the stomach caused by air (bloat) or distension of the uterus (womb) during pregnancy

Tumors within the abdomen, which can be malignant (invasive cancer) or benign, and may involve any of the abdominal organs, including intestines or glands

Loss of abdominal muscle tone with or without significant weight gain