Canine Liver and Anatomy
Canine liver disease is among the top five leading causes of non-accidental death in dogs and as such should be taken seriously. The liver is responsible for a number of essential bodily functions, and if it is compromised in any way your dog’s overall health is in jeopardy.
Among the liver’s many responsibilities are blood detoxification, waste removal and bile production to aid digestion. This large gland is involved in just about every process in the body. As such, it is very resilient, with the ability to work even during the onset of liver disease in dogs.
It is involved with almost all of the biochemical pathways that allow growth, fight disease, supply nutrients, provide energy, and aid reproduction. Liver cells, which are called hepatocytes, go through thousands of chemical reactions every second in order to perform these myriad functions. Since the liver is involved with almost all biochemical processes it is no wonder that there are many different diseases that will affect it.
The liver processes raw materials, manufactures the building blocks of the body, recycles the old to make new, and detoxifies the industrial waste of the body. In short the liver is involved in just about every biochemical process required to run the body. As a result of this relationship, liver disease can affect just about any other part of the body and thus the symptoms of liver disease are typically unpredictable and non- specific. Furthermore, because the liver acts as a “biochemical cross roads” for the body, it is affected by a wide range of diseases, including viral and bacterial infections, degenerative and neoplastic disease, and toxic insults. It is estimated that three per cent of all disease seen by veterinarians is liver based.
The liver has a double edged nature which, while being life preserving, makes diagnoses and treatment of liver disease extremely difficult. The liver has a tremendous reserve capacity, which means that it can easily perform its duties with up to 70 to 80 per cent of the liver mass affected by disease. While it certainly is a benefit that our liver can keep us alive despite an overwhelming infection or a massive tumor, it also means that the disease is well advanced and possibly untreatable before any symptoms are noted. We all know that disease is most easily conquered early, but the very nature of the liver makes this an impossible task. One thing about livers though: they are the only organ in the body which is capable of complete regeneration and thus is we do manage to successfully treat the disease, there is a chance of complete recovery.
SPECIFIC LIVER FUNCTIONS
The liver is the organ that orchestrates the metabolism of fats, carbohydrates, and protein. It does this in conjunction with the circulatory system, the lymphatic system, and the endocrine (hormone) system. A healthy liver is critical to proper protein, carbohydrate, and fat metabolism.
The liver produces all of the proteins except for the proteins synthesized by the immune system. It does this by reassembling amino acids into protein. The main protein produced by the liver is called albumin.
Normal albumin in the bloodstream is important for many physiologic functions. One of these functions involves the normal maintenance of fluid pressure in the arteries and veins. When the protein level falls below a certain point the fluid in these vessels can leak out and pool in the abdominal or thoracic cavities. This fluid is called ascites when it occurs in the abdominal cavity, pleural effusion when it occurs in the thoracic cavity. Albumin also functions to “carry” other compounds through the bloodstream. These compounds include calcium, vitamins, hormones, fatty acids, many drugs, and bilirubin.
A consistent finding with liver disease is a low protein level (hypoproteinemia). This low level usually occurs only when the liver has been severely diseased for a prolonged period of time, because of the great reserve capacity of the liver to produce more albumin.
With the aid of the hormones insulin and glucagon, the liver maintains a normal blood glucose level. Abnormalities in blood glucose level can result from an insulinoma or diabetes mellitus (sugar diabetes).
Glucose that is stored in hepatocytes is called glycogen. It is used as a reservoir during times when carbohydrate intake is low (fasting or starvation). The liver can also manufacture glucose from proteins or fats.
In liver disease the body can have a difficult time regulating the blood glucose level, usually leading to hypoglycemia (low blood glucose). This is one of the reasons why caloric intake is an important aspect of treatment.
The liver regulates fats (called fatty acids) in the bloodstream. It does this by converting excess amounts of carbohydrates and proteins into fatty acids. The liver also manufactures cholesterol from this fat. Cholesterol is necessary for many functions, particularly the sex hormones and steroids like cortisone.
Drug detoxification is an important liver function. It is a complex process that occurs in the endoplasmic reticulum of the hepatocyte. Several phases are involved with this detoxification:
The offending drug is inactivated. This inactive drug is eliminated by the body, usually through the kidneys, or secreted into bile and passed out in the feces.
The drug is converted from an inactive drug to an active metabolite. This active metabolite circulates in the bloodstream and has an effect on the body.
The drug is converted from an active drug to an active metabolite.
In this phase biochemical processes occur that make these drugs more water soluble and thus easier to eliminate through the kidneys. They are excreted out of the body in the urine.
Another example of this detoxification process occurs with the compound ammonia. Ammonia is one of the by-products of the digestion and metabolism of protein into amino acids. Ammonia is detoxified by the liver and excreted by the kidneys. A diseased liver will not metabolize this ammonia to urea properly, leading to the disease called hepatic encephalopathy.
Bile is made up of electrolytes, cholesterol, bile acids, bilirubin, and globulins. It is produced by hepatocytes, secreted into channels in the liver called and stored in the gall bladder. Drugs are eliminated in the bile, red blood cells are re-circulated through the bile system, and fats are absorbed from the intestines into the bloodstream only in the presence of bile.
When red blood cells break down and are recycled they release bilirubin from their hemoglobin. The liver, along with spleen and bone marrow, recycle this bilirubin, salvaging some of the compounds and excreting the rest in the bile. Bilirubin, which is toxic, binds to albumin and is detoxified and excreted. This is eventually excreted into the intestines and broken down by intestinal bacteria, where it imparts the dark color to stool. If this bilirubin cannot be excreted from the gallbladder (when there is an obstruction in the bile duct) there will be very light colored stool. The excess amounts of bilirubin that build up in the bloodstream will cause jaundice, the yellow discoloration of the skin and mucous membranes that can occur with liver disease.
The fat soluble vitamins, A, D, E, and K, require bile for proper absorption from the intestines. These vitamins are stored in the liver, and are converted to active compounds as the liver maintains normal physiology.
The proteins that initiate and maintain clotting of blood are synthesized by the liver. These proteins go through very complex biochemical processes to achieve this vital function. A diseased liver is unable to synthesize these proteins, leading to a potential bleeding problem. Vitamin K is also an essential component of these clotting mechanisms.
Red Blood Cell System
The liver removes old or damaged red blood cells from the circulation, and is involved with the storage of iron and the breakdown of hemoglobin. Because of this, chronic liver disease could cause anemia. The liver (along with the spleen), is a storage organ for blood. If there is a severe blood loss the liver expels this blood into the bloodstream to help make up for the loss.
Specific cells called Kupffer cells line the inside of the liver. These cells are part of the immune system. They eliminate and degrade the substances that are brought into the liver by the portal vein. Some of these substances are bacteria, toxins, nutrients, and chemicals. A diseased liver will not filter these compounds normally, resulting in toxic accumulations of drugs, chemicals, or bacteria. Excess accumulation of bacteria in the bloodstream is called septicemia, and is one of the reasons that antibiotics are commonly used in liver disease.
Many vitamins are stored in the liver, and perform their functions only when activated by the liver, and are degraded by the liver. These include some of the B vitamins and Vitamin C, along with A, D, E, and K previously described.
The pictures in this section are reprinted with permission by the copyright owner, Hill’s Pet Nutrition, from the Atlas of Veterinary Clinical Anatomy. These illustrations should not be downloaded, printed or copied except for personal, non-commercial use
The liver is a multi-lobed organ that is located at the most forward part of the abdomen. It is so far forward that it lays up against the diaphragm, the muscle that aids in breathing in mammals. The liver is the largest organ that is located in the body, a testament to its importance. It has 6 distinct lobes organized into 3 regions. Like the kidneys, 25% of the blood ejected with each beat of the heart goes to the liver.
The liver is supplied with nutrients from the hepatic artery and the portal vein, which is different from other internal organs. Blood from the heart goes through the aorta and into the hepatic artery. The hepatic artery supplies a large amount of the oxygen and nutrients the hepatocytes use during metabolism. Approximately 1/3 of the blood that flows into the liver comes from this artery.
The other source of blood for the liver is the portal vein, supplying 2/3 of the blood that flows into the liver. The nutrients in the portal vein originate from the digestive tract, this time, not the heart, like the hepatic artery does. In essence, raw food that is absorbed from the intestines flows directly to the liver. This exposes the liver to toxins and bacteria, which are metabolized and detoxified by a normal liver before they leave the liver and enter the general circulation via the heart. This detoxification process protects other organs, particularly the brain, from bacteria and toxins that could injure brain cells. When this system fails, which happens in liver shunts, excess ammonia will build up in the bloodstream and affect the brain.
**Information from this section was compiled from the following sources: