Bovine biology series
Part 2 - The Spleen
The spleen is the largest lymphoid organ in the animal body. A lymphoidal organ is defined as tissue producing and secreting the various white blood cells used by the body's immune system in fighting bacterial invasion.
The spleen has one significant difference from the liver, which we examined last month. The spleen is not required in the adult body, because all of the functions of the spleen can be accomplished by other body organs. However, in adult animals, the spleen does have some specific functions, largely associated with blood cell formation, iron metabolism, blood metabolism and the production of white blood cells.
In the fetus the spleen produces and secretes red blood cells. Thus in the fetal animal the spleen is a required body organ. Later on in adult life, it is the bone tissue, primarily the bone marrow that takes over the responsibility of red blood cell production.
In the adult, the spleen is a reservoir of blood volume. In the cow, the spleen is located next to the rumen. The blood is responsible for transporting many molecules to target cells and organs, and one of the functions of the spleen is to contribute blood volume into the system when called upon. This can be initiated by the obligatory demand of the digestive system to increase the blood volume so that after a major intake of food, the digestive metabolites can be transported to the liver. And during exercise, the spleen once again can contract, at the response of the smooth muscles cells surrounding the gland, squeezing this blood volume into the blood vessels, helping the transport of oxygen from the lungs to the muscles that are working hard.
Another function of the spleen is serving as a filter for old, worn out blood cells no longer needed by the body and no longer able to perform. Other organs of the body do this cleansing function as well, such as the liver, lungs and bone marrow. But it is the spleen that does this cleansing the best.
When an animal is excited, there is a rapid release of a hormone called epinephrine. This hormone helps the body prepare for fight or flight, and that usually involves an elevated awareness of immediate action. This means the animal must move quickly, and that means mobilizing muscle cells for rapid oxidation of glucose and stored glucose in the form of glycogen.
The spleen serves the role in not only providing additional blood cell volume, but also releases into the blood volume a higher concentration of blood cells themselves. Why? Because blood cells, which we also call erythrocytes, include a specific blood cell responsible for transporting oxygen: hemoglobin. Thus when the body needs to convert chemical energy, found in the form of digested feed or food, into chemical energy that cells use to move the connective fibers that move tissue that move body parts like legs and rumen motility, oxygen is required. Thus hemoglobin becomes the transport system for oxygen.
Another function of the spleen is the production and secretion of white blood cells that may be called lymphocytes. We may also call them somatic cells. Of significance here is the fact that the immune system, which is very much a required system of an animal, is based upon a collection of very specific cells which serve as policeman in the body. They circulate around, doing what policeman do.....protect the rest of the body cells from the invasion of bacteria or viruses that may be harmful in some way.
Lymphocytes produce antibodies, which are directly produced from an antigen that is introduced into the body. The body responds to potential disease by having a number of circulating antibodies in the bloodstream, ready to suppress or kill an invading bacteria or virus carrying a specific antigen. Yes, this is what a vaccine is, an injection of antigens that produces a specific lymphocyte or antibody to that antigen. The disease is caused by bacteria that contains cells with the antigen present. When the bacteria invade the body and reach the blood, an antigen, or policeman is there with exactly the right weapon, an antibody, to suppress or kill the invader. The spleen, then, serves the very important role of making sure the immune response, the production of specific lymphocytes and therefore specific antibodies, is underway.
Finally, the spleen helps in the formation of bile pigment, which aids the liver in one of its roles as the organ responsible for detoxifying certain substances that as metabolites represent a threat to the well being of the body.
The spleen is an organ serving many functions, from the preparedness
when confronted with a life threatening situation, or when an invading bacteria breaches the bodies primary defense system, the skin or a sphincter muscle or the lining of the entire digestive tract and lung system. The spleen contributes hemoglobin to carry oxygen and lymphocytes that can recognize the antigens, those pesky little part of invading bacteria or viruses that without an antibody would certainly get the best of an animal.
A cow and we ourselves may live without this yellow-green organ, but I suspect that in our own bodies, as well as the cows, the spleen is making a significant contribution.
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