The liver
by Linda Kennard of NAF
We are all aware that the equine digestive system is complex. Much is written about the gut, but the three accessory glands of the digestive system - the liver, the pancreas and the salivary glands - often go unnoticed. They are all vitally important to good general health and this month I have chosen to focus on the liver, without which life cannot be sustained.
The liver is of great importance to the horse because of the complex part it plays in the horse's body and in particular it's role in the digestive system. It is the largest gland in the body and weighs about 1.5% of the horses body weight, on average about 5kg. The liver is red/brown in colour and is found in the abdominal cavity between the diaphragm, the stomach and the intestines, under the rib cage with most of the bulk lying right to the midline.
The liver has right, middle and left lobes, is attached by ligaments to the diaphragm and the abdominal wall and is supplied by arteries, veins, nerves and lymph channels. The horse's liver is peculiar in that it does not have a gall bladder.
The liver is a centre for a great deal of the digestive processes as well as being a major organ for detoxification. The liver cleanses the blood of substances such as drugs and poisons and forms part of the defence system against microbes. The liver is capable of over a hundred different functions.
Approximately 75% of the blood entering the liver is carried through the portal vein, this carries blood containing products absorbed from the digestive processes in the intestine directly to the liver. After passing through the liver blood flows out into large veins leading to the heart. The liver is therefore strategically placed to act as a filter for the digestive processes that start in the gut. The blood leaving the liver is cleansed of toxins and foreign matter which may have entered through the bowel.
Major functions of the liver.
• Metabolism of protein, carbohydrates and fats.
The liver plays a key role in the formation of glycogen. This is then stored and converted to glucose when required for metabolism. It also plays a role in the formation of amino acids from protein breakdown which can then be stored or utilised in various part of the body for growth etc.
Fatty acids - propionic, acetic and butyric - which are produced in the caecum and the colon are metabolized here. The liver controls the storage depots of fats both in the liver and elsewhere.
• Detoxification and removal of harmful substances.
Many of the harmful waste products and other absorbed materials are detoxified by the liver, rendering them harmless before removal from the body. For example the liver prevents the build up of ammonia, a toxic substance formed by the breakdown of amino acids, by converting it to urea.
The liver also detoxicates drugs. For example phenylbutazone (bute) is converted by the liver into oxyphenbutazone before it can be excreted in the urine.
• Secretion of bile
An adult horse's liver will produce about 10 litres of bile a day. Bile contains pigment (bilrubin), salts, protein, cholesterol and substances formed from the breakdown of haemoglobin.
Because the horse is a herbivore and in the natural state eats continuously the bile is continuously formed in large quantities so there is no reason for the horse to have a gall bladder.
• Excretion of waste products
Many of the harmful waste products and other absorbed materials are detoxified by the liver rendering them harmless before removal from the body.
• Regulating blood content
The liver helps to regulate the concentrations of fat, sugar and amino acids in the bloodstream. The cells in the liver will either take up these substances or deliver them back into the bloodstream as required.
The liver also forms blood proteins albumin and globulin, which gives the blood its viscosity, and fibrinogen as well as other substances needed for clotting.
• Storage of vitamins
The liver stores vitamin B and vitamin A as well as copper and iron. The liver controls storage for fats both in the organ itself and elsewhere. Transport of fats from the liver relies on good provisions of vitamin B.
From the few functions of the liver mentioned above it can be appreciated that diseases involving the liver produce symptoms involving a wide range of systems within the horse, reflecting the importance of this organ.
Liver disease
At least 75% of the livers functional capacity must be lost before clinical signs of disease become apparent. Because of this the onset of acute or chronic states are sudden.
Liver disease is mostly caused by the indirect result of infections, toxins or poisons which may arise from within or from outside the horses body. Although the degree of compromise is substantial before the disease becomes evident, any degree of liver damage is likely to influence the athletic horse and may reduce the capacity to perform in disciplines such as eventing, racing etc. Liver disease can also influence secondary symptoms such as tying-up and affect resistance to infectious disease.
The chronic (longer lasting) disease is more common than the acute form and may be a low grade response to the chemical or plant toxins which produce the acute form. More frequently it results from eating plants of the genus Senecio, namely ragwort. This plant is bitter and the horse does not usually eat ragwort when green. However, remember ragwort is palatable in hay. Ragwort causes cirrhosis of the liver cells a process in which the cells become fibrous. Liver damage can also be caused by tumours or abscesses, although these are less common. Liver fluke (a parasitic flatworm) infestation will also cause liver disease. Liver fluke may invade and block the bile duct.
Confirmation of liver disease is by biochemical examination of blood samples or examination of a small sample of liver (a biopsy).
For further advice please call the NAF Freephone Advice Line 0800 373 106
or email info@naf-uk.com
