Bovine biology series
Part - 17 Lymphatic system
Some twenty years ago during a very cold, wintry day I had the occasion to visit a dairy farmer just north of Madison, Wisconsin. I was, as a first-year graduate student, fairly confident of my abilities, especially because on this particular afternoon I had about twenty undergraduate students in my care. We were on a field trip.
Anyway, on this particular dairy farm, the owner walked around the side of three very large blue upright silos. We followed. On the ground in a partially frozen state was a fresh cow that had died soon after giving birth. The calf was fine.
A veterinarian was posting this cow. Her belly cut wide open, we could see many of the organs we learned about in class. The heart. The pancreas and spleen. The liver (much larger than I would have guessed), and of course the digestive system. Intestines many feet long. But I will not forget the watery fluid that seemed everywhere, bathing all of these organs with live-giving fluid.
Of course we know that blood supplies the nutrients, like glucose and nitrogen-containing compounds to cells so they may live and work and move. But in the belly of this cow, albeit partially frozen, was a fluid, a bathing fluid that made a lasting impression upon me. We know that this fluid, the lymph, has a role to play too. We truly live in a fluid world, and that fluid is called the interstitial fluid. We know the process of moving this fluid; collecting waste debris and filtering out the bad stuff is done by the lymph nodes.
Collectively, all of this is called the lymphatic system.
We examined the lymph fluid last month. We learned, for instance, that lymph is a clear fluid, mostly water, and that lymph is a reservoir for waste products, such as proteins and lipids -fats, but lymph contains blood too, spilled into the lymphatic system by capillaries, those very small blood vessels.
We know that the lymph fluid is moved, largely by the breathing movement that causes the diaphragm to move and other smooth muscles of the skeletal system, into the filtering tissues of lymph nodes. These are found at various points in the body; the major ones being the thoracic duct and the right lymph node. Both are located in the neck. Another one is the spleen.
If edema is present, we know that somewhere in the lymphatic system the fluid is not flowing through the lymph nodes and hence, emptied back into the circulatory - blood system.
In a normally functioning lymphatic system, the lymph enters the lymph nodes through lymphatic vessels. These vessels are like veins, in that they carry fluid (lymph) containing waste products. The vessels (called afferent vessels) empty in the nodule, and the lymph enters the filtration area, called the capsule. Percolation occurs here, as the lymph is moved through the tiny cortical sinuses, through the medullary sinuses, and then to the Hilus portion of the nodule.
At this point, the filtered lymph exits the nodule via the efferent lymphatic vessels, in turn these empty into the blood vessels. The blood vessels receiving the filtered lymph are veins. At this point the lymph fluid itself becomes a part of blood, adding to the total blood plasma volume. Remember that the blood found in veins carries the waste products of metabolism, including gasses like carbon dioxide. As such, this oxygen-depleted blood travels back to the heart, enters the superior vena cava, the right atrium, the right ventricle (muscle) and is pumped through the pulmonary artery to the lung tissue (for gaseous exchange).
Quite remarkable, then, is this function of the lymph nodule.
The filtering function of the nodule is called phagocytosis (derived from the Greek words phagein which means to eat, kytos which means cell and osis, which means intensive.
Consider this: an invading foreign bacteria breaches the primary defensive mechanisms, the skin, or perhaps the keratin layer found just inside the teat opening. The bacterium is a stranger, but a strong and healthy one. Well, since the body lives in a fluid environment, the bacteria is quickly moved about via these fluids that bath cells. The bacteria do not enter the cell but rather exists outside of it. The fluid is moved about the body until it reaches a drainage area, where entering the lymphatic system, this bacterium is emptied via the afferent lymphatic vessels into the nodule. Well, at this point the bacteria is eaten, literally, by a phagocyte, a cell found inside the lymph nodule. It is here, then, the body enforces its boundary to the blood. For if this particular bacteria would have found its way into the blood, then it could travel all over the body. At its choosing, these bacteria could wreak havoc on cells not able to defend themselves from the metabolic actions of these bacteria.
Even though the lymph drains into the venous system, the bacteria may indeed survive the ride to the heart, the lungs and then back to the heart. If it reaches the left side of the heart (arterial side), then it can go anywhere.
We have discussed three important functions of the lymphatic system: serve as a dumping fluid for cellular debris, and metabolic waste products, carry this fluid to lymph nodules and then filter out these by-products, which include the phagocytosis of harmful cells the body recognizes as foreign invaders.
There is another role the lymphatic system plays. One of immunity. We might call this the production of defensive cells called lymphocytes. We know that the lymphatic system makes antibodies too, the direct result of an antigen reaching the lymph node causing a unique antibody formation.
We shall spend several months on this topic of immunity. Immunity is a protective mechanism that permits the body to recognize and deal with foreign invaders after they have made their way past the skin and epithelial layer of cells. We live in a sea of bacteria and virus; without immunity, we do not exist in the face of the sheer numbers of these microscopic beings.
|