Bovine biology series
Part 1 - The Liver
Having just completed a twelve part series on accounting and financial analysis, I feel it is time to shift directions. There is much interest in approaching animal health and the longevity factor from a "holistic approach". That is, from the standpoint of looking at the entire biological range of components making up the animal. However, each component, each organ or system or function has to be understood before putting all of this together. I am imagining this series could take two years before we look at the whole of an animal. So we begin.
In this first article of the series, we look at the liver.
The liver is the single largest gland in the body. In a mature cow, the liver can weigh 10 to 15 pounds. It is located on the right side of the cow.
In many essential ways, it is the liver that directs the fate of digested food nutrients. Undigested carbohydrates, usually fiber, and protein, usually nitrogen-containing compounds not converted to ammonia in the rumen or absorbed in the small intestine, and fat are excreted as feces - manure. The digested portion of the feed, however, enters the portal vein, a large capacity blood vessel connecting the stomach, spleen, pancreas and intestines. This blood enters the liver, coming into contact with hepatic cells, which take up, store, convert or alter the food molecules.
Let's examine the primary functions of the liver.
In the cow, the primary end product of rumen fermentation are volatile fatty acids - acetate, propionate and to a lessor extent butyrate. These metabolites are fed, via the portal vein, to the liver. The liver, through a process of gluconeogenesis, converts these VFA's into glucose. The mature cow needs a tremendous volume of glucose for milk and fat synthesis and other body maintenances. A high producing cow requires 1800 - 2000 grams (4-4.5 pounds) of glucose daily. If this level of glucose is decreased, ketosis results in which the lack of glucose impairs body maintenance and there is an increase of ketone bodies produced by the liver. These ketone bodies result from the liver's efforts in trying to make glucose from fat or lipid compounds. This process is slower than gluconeogenesis from VFA's or protein compounds. Thus given the glucose requirement for body functions, milk and fat secretion and maintenance, the liver needs a continuous supply of glucose precursors - volatile fatty acids.
Once the liver has performed its function of making the simple glucose sugar molecule, these molecules are carried in the blood tot he heart for distribution throughout the body.
The liver can also store glucose in the form of glycogen, a molecule that is quickly converted into glucose during conditions of immediate glucose requirement. Glycogen may be called upon to satisfy the need for a stress reaction to a fight or flight situation. Once used up, however, the liver attempts to maintain blood glucose level (normal in a cow is 60-80 milligrams per 100 milliliters) by metabolizing fat if the supply of VFA's is limiting.
Another function of the liver is secreting bile salts and acids. Bile is secreted by the hepatic cells of the liver, is stored and concentrated in the gall bladder, and is carried through the bloodstream (in the bile duct) to the duodenum, a part of the small intestine. Bile is also made up of cholesterol, lecithin and bile pigments. One function is to enter the small intestine, increasing the pH of the digesta leaving the acid conditions of the stomach.
Another function is in fat absorption. Bile breaks apart the large fat molecules called triglycerides (a process called emulsification) into smaller droplets called micelles. These smaller units of fat can be absorbed from the digesta into the bloodstream of the small intestine.
Bile actually makes a loop in the body. Formed mainly from cholesterol in the liver, bile is stored and concentrated in the gallbladder, carried to the small intestine where the pH is increased, the fat is emulsified and put into solution along with fat soluble vitamins and other lipids, and all of this dumped into the portal vein back to the liver.
Another important component of bile is bilirubin. This is a bile pigment, and serves the role of carrying waste products that are excreted by the liver into the small intestine and exit the body as feces or urine. The liver, therefore, not only serves the role of providing glucose and enabling the body to absorb fat and lipid molecules, but in detoxification.
This is a significant role for this organ. Substances that are fed, or chemicals that enter the body either through feed or injection, may be toxic to the animal, especially if given at megadose levels. It is the function of the bile system to transform these substances into compounds or molecules that are carried back to the digestive system and eliminated from the body. This detoxification role is accomplished by reducing the biological activity through the processes of conjugation, oxidation or chelation.
Another function of the liver is deamination. This process involves the alteration of amino acids, the building blocks of peptides and protein. The end result of deamination is a carbohydrate molecule and an ammonium ion. The ammonium ion is converted into urea in the liver, transported directly to the kidney and excreted in the urine.
Thus the liver is the organ largely responsible for converting excess digested protein into two substances: carbohydrate that can converted to glucose for immediate use or glycogen as stored glucose, and ammonium into urea for excretion in the urine volume.
Protein can become a source of energy because of deamination. And the liver can synthesis new amino acids by taking carbohydrate compounds, adding a nitrogen-containing molecule and forming a non-essential amino acid. The characterization of amino acids into the group called essential amino acids are the ones the liver cannot synthesize and must be a component of the ration itself.
One final note about this organ. As you might have guessed by now, the liver is accomplishing many things requiring energy. The fuel driving these energy-requiring reactions is oxygen. The hepatic artery supplies this oxygen, as well as other metabolites from the heart muscle that receives oxygen from the lungs. Thus the liver has a dual blood supply: the portal vein, rich in raw food molecules and compounds from the digestive system and the hepatic artery, rich in oxygen from the lungs and heart.
The liver can be abused by overloading the obligations of it, such as high levels of protein that must be deaminated or a chemical compound that overwhelms the detoxifying abilities of the organ. But the liver can be regenerated and brought back to some degree of function if the abuse is not prolonged. Chronic abuse will alter its function and given time, liver cirrhosis will occur. This is a common condition of fatty livers, in which the liver is not able to transport fat or lipid compounds out of the hepatic tissue, or the diet is too high in fat and too much fat or lipid compounds come into the liver.
In summary, the liver is an essential organ. It is involved with carbohydrate, protein and fat metabolism. The liver is also responsible for detoxifying substances that may compromise the maintenance of the body. It is a storage site for lipids, vitamins and glycogen. And the liver is an organ capable of synthesizing sugars, proteins, lipids, urea and ketone bodies.
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