Structure and Function of the Liver in Dogs
Below is information about the structure and function of the canine liver. We will tell you about the general structure of the liver, how the liver works in dogs, common diseases that affect the liver and common diagnostic tests performed in dogs to evaluate liver function. Many diseases of the liver begin with “hepatic” which means liver. For example, “Hepatopathy” means disease of the liver and “Hepatitis” means inflammation of the liver.
What Is the Liver?
A dog’s liver, a very complex and hard working structure, is the largest organ in the body. The liver filters the blood and has hundreds of other functions, most of which are necessary to life and are not done elsewhere in the body. Every part of the liver is capable of performing all of its tasks, and it is the only organ in the body with this capability. The liver has an amazing ability to recover from injury and regenerate new tissue.
Where Is the Liver Located in Dogs?
The liver is a multi-lobed organ located in the front of the abdominal cavity. It lies directly behind the diaphragm (the muscle that aids in breathing and separates the chest from the abdomen) and directly in front of the stomach. The liver is positioned horizontally across the front of the abdomen, with proportionally more of the liver on the right side.
What Is the General Structure of a Dog’s Liver?
The normal liver is a deep red color with a firm consistency. It is divided into several different lobes, and each lobe is made up of multisided units called hepatic lobules. Each lobule is composed of a curved sheet of cells, which enclose numerous blood filled cavities known as sinusoids. These sinusoids give the liver a spongy texture and enable it to hold large amounts of blood.
Two major blood vessels enter the liver: The portal vein carries most of the blood to the liver and contains nutrients from the gastrointestinal tract, as well as chemicals and drugs that have been absorbed into the body. The hepatic artery carries oxygen-rich blood to the liver from the heart and lungs.
Two major conducting structures exit the liver. The hepatic veins drain blood from the liver. The bile ducts take bile from the liver cells to the gall bladder, a pear shaped pouch located under the liver.
What Are the Functions of the Liver?
The liver regulates the levels of many different chemicals and substances in the blood, and it excretes bile, a yellowish-green digestive fluid. All the blood leaving the stomach and intestines passes through the liver. The liver processes this blood and breaks down the nutrients, chemicals, and drugs into forms that are easier for the rest of the body to use. There are more than 500 vital functions associated with the liver. Some of these functions include the following:
- The production of bile, which is released into the gastrointestinal tract to help break down fats in the small intestine during digestion. The liver makes bile continuously, even when food is not being digested, and extra bile is stored in the gallbladder.
- The production of certain proteins that circulate in the blood, such as albumin and several factors responsible for clotting.
- The production of cholesterol and special proteins (lipoproteins) that help carry fats through the body.
- The conversion of excess glucose (sugar) into a starch-like compound called glycogen and storage of this glycogen in the liver. Glycogen can later be converted back to glucose for energy whenever it is needed.
- The storage of blood that can be shunted immediately into the general circulation when needed following injury or sudden blood loss.
- The regulation of blood levels of amino acids, which form the building blocks of proteins.
- The storage of iron that is used in the processing of hemoglobin in the blood. Hemoglobin is essential for carrying oxygen in the blood.
- The conversion of ammonia in the blood to urea. The bacteria of the intestinal tract produce ammonia as they break down proteins, and ammonia can accumulate in the blood at potentially toxic levels. Urea is a safer product than ammonia and is excreted (passed out of the body) in the urine.
- Clearing the blood of foreign substances, such as medications and anesthetic agents that are administered to animals.
- Resisting infections by producing immune factors and filtering bacteria from the blood stream.
What Are Common Diseases of the Liver in Dogs?
Since the liver is involved in many biochemical processes, many different diseases can affect it. A variety of clinical signs may be seen with liver disorders; however, in many cases, one of the earliest signs of disease is jaundice. Jaundice occurs when the blood contains an excessive amount of bilirubin, causing a yellow color to the skin, gums, and sclera (whites of the eyes). Ascites (fluid in the abdominal cavity), another clinical sign seen with liver disease, occurs when there is impairment of blood flow through the portal vein. This leads to hypertension (high blood pressure) in the portal vein, which causes fluid to leak out of the portal vein into the abdomen. Low albumin protein levels in the blood from liver disease may also cause ascites.
The liver has a remarkable ability to produce new cells to replace its own diseased or damaged cells. This regenerative capability allows the liver to return to normal function in some cases.
Some examples of liver diseases include:
- Congenital absence of enzymes that convert ammonia to urea. This condition is rare in the dog, but may occur in Dalmatians and other breeds.
- Congenital portosystemic shunts are defects in the portal vein that leads to the liver. In animals with these defects, the portal vein bypasses the liver and materials that are normally carried from the intestines do not reach the liver. Certain materials (e.g. ammonia) continue to circulate in the blood until they reach toxic levels.
- Viral, parasitic, protozoal and bacterial infections may affect the liver and gall bladder. Bacterial infections may result in abscessation within the liver.
- Hepatic lipidosis is a syndrome seen in dogs with sugar diabetes. Abnormalities in the metabolism of glucose and fat in diabetic dogs cause an accumulation of fat in the liver that may eventually result in liver dysfunction.
- Steroid hepatopathy is a condition in which there is excessive glycogen deposition in liver cells due to high circulating levels of steroids in the blood. This condition may develop when the animal’s own body produces too many steroids due to diseases of the adrenal glands, or when steroid medications are administered for a long period of time.
- Hepatotoxins (agents or drugs harmful to the liver) can cause severe, sometimes irreversible liver disease. Examples include heavy metals (e.g. lead, arsenic, thallium, copper), anti-inflammatory drugs, certain antibiotics and anesthetics, anticonvulsant medications, and certain anti-parasite drugs and dips.
- Chronic active hepatitis is an ongoing inflammatory disorder of the liver. It is one of the most common liver diseases of dogs and has many different causes.
- Cirrhosis occurs when scar tissue replaces healthy liver cells. Cirrhosis may develop from any chronic, long-standing liver disease.
- Cholangiohepatitis is an inflammation of the bile carrying structures and the surrounding liver tissue. It is seen most often in cats but may rarely occur in the dog.
- Both benign and malignant tumors may develop within the liver. These tumors may develop only in the liver, or may spread to the liver from other organs. Common liver tumors in the dog include hemangiosarcoma, lymphosarcoma, and metastatic tumors.
What Types of Diagnostic Tests Are Used to Evaluate the Liver?
There are many diagnostic tests that are helpful when evaluating the liver.
- Initial tests usually include a complete blood count, biochemistry profile and urinalysis. A blood count may reveal evidence of infection, anemia or low protein. A biochemical profile may reveal elevations in liver enzymes and bilirubin (cause of jaundice), and/or decreases in glucose, protein, blood urea nitrogen and cholesterol. It may also show electrolyte abnormalities. A urinalysis may show increased excretion of protein by-products.
- Other laboratory tests may be considered, depending upon results of the initial blood tests. Bile acids are blood tests that assess the function of the liver and the amount of liver that is diseased. Blood ammonia, blood steroid and amino acid levels may be measured. Serology tests for certain viruses, protozoa and fungal diseases may be considered.
- Abdominal radiographs (x-rays) may show changes in liver size and shape. They may also reveal the present of ascites (abdominal fluid), gallstones, and abnormalities in other abdominal organs.
- Chest x-rays may be performed to look for signs of metastatic tumors, fluid in the chest, and problems with the diaphragm.
- Abdominal ultrasonography is very helpful in evaluating the internal structures of the liver. It provides valuable information about the consistency of the liver and can often identify blood vessel shunts, cysts, abscesses and tumors. It is a noninvasive procedure that often necessitates the expertise of a specialist and/or referral hospital. Your veterinarian may refer your dog to a veterinary internal medicine specialist to perform an ultrasound.
- A clotting profile is often performed when there is evidence of chronic or severe liver disease. If the liver cannot manufacture normal amounts of clotting factors, then the animal will be very prone to bleeding disorders. Evaluation of clotting function is particularly important before any attempt is made to biopsy the liver.
- Advanced imaging tests that may be helpful in diagnosing liver disease or shunts include radioisotope studies, CT scans, MRIs and dye contrast studies of the portal vein (portogram).
- A liver biopsy is often necessary to determine the specific type of liver disease present. A liver biopsy may be performed under the guidance of an ultrasound, through laparoscopy (insertion of a small rigid scope into the abdomen), or by surgical opening of the abdomen (exploratory laparotomy). Material retrieved for biopsy can be submitted for culture and for microscopic examination.