Ironically, the lymph vessels provide a path for disease to spread through the body; but, at the same time, it tries to destroy these invading organisms.
Lymph (see Note 1) is a fluid that comes from the connective tissue around blood capillaries, and is collected and transported through the lymphatic vessels to the major blood veins at the base of the neck. Lymph flows only in one direction - toward the heart. Lymphatic vessels begin near blood capillaries, where they pick up lymph and return it to the blood veins. The lymphatic vessels basically parallel the blood's veinous flow, beginning at capillary beds, and finally entering the veinous bloodstream at the base of the neck. I will mention here, in case any of you readers really get intimately involved with this body system, that lymph is called tissue fluid until it enters the lymphatic vessels.
This more-or-less circular exchange ( tissue fluids --> lymph vessels --> blood vessels --> tissue fluids - ad infinitum) is occurring constantly in the body. If, for any reason, the lymphatic vessels get blocked, tissue fluid "pools," causing a swelling in the local area (edema). This condition often occurs in the extremities, and can be relieved by elevation of affected limbs. I get this condition as a side-effect of my anti-hypertension medication - a calcium channel blocker. [And you probably know that calcium is a major player in muscle contractions, which are largely responsible for veinous flow; as opposed to arterial flow, which is facilitated by the pumping of the heart.]
In addition to transporting lymph back to the bloodstream, another function is performed by the lymphatic vessels. Blood proteins leak from capillaries into surrounding tissue fluid, and the lymphoid vessels return them to the bloodstream. These proteins are responsible for keeping water in the bloodstream. Without them, blood flow would stop, so I am guessing that this is a critical function.
The lymph vessels are a tangle of different sizes with names similar to corresponding blood vessels.
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Note 1 - Lymph is essentially a collection of bits of blood plasma (water, nutrient molecules, and respiratory gases) that leaks out of blood capillaries into the surrounding loose connective tissue.
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Lymph Capillaries Like blood capillaries, lymph capillaries are highly permeable; making it easy for them to scoop up tissue fluids from the surrounding loose connective tissues. The entry of fluid is very easy, but the porosities are really open flaps, which close after entry of fluid to prevent backflow. The flaps are, functionally, valves.
The highly porous lymph capillaries can very efficiently pick up fluid, and even large protein molecules. But there is a downside to this high degree of porosity, and that is that the enemy - unfriendly bacteria, virus, even cancer cells - can also get in. This is why the lymphatic vessels are often guilty of spreading disease within the body. This is where lymph nodes come in. They kill most invaders (pathogens, to you purists).
Lymph capillaries shadow most blood capillaries, but none are in bone or teeth, bone marrow, or the CNS (cerebrospinal fluid picks up tissue fluids here).
A specialized set of lymph capillaries, called lacteals, are present in the intestines, and they collect digested fats from the intestines, making the lymph draining from the digestive system turn a milky white color; and now, if you hear the term "lacteals," you'll know what they are. Essentially, they move fat from the small intestine to the bloodstream.
Lymphatic Collecting Vessels
Lymph moves from the capillaries into lymphatic collecting vessels, which parallel blood vessels - superficial (closest to the surface) lymphatic vessels parallel superficial blood vessels; deep lymphatic vessels, on the other hand, parallel deep arteries.
Lymphatic collecting vessels are almost delicate - thin walls. Thin because of the very low pressure in them, resulting from them not being directly connected to the heart and its strong pumping action. Lymph flow is helped by the high number of valves in lymphatic vessels. The impetus for lymph flow comes from contractions of skeletal muscle and the pulsations of nearby arteries, in addition to the valves. There is additional propulsion help from muscular contraction of the middle layer in lymphatic vessel walls. As thin as the walls are, there are still three layers, just as in blood vessel walls, and the middle, muscular layer - tunica media - provides the contractions.
The disease, elephantiasis (a tropical disease), is caused by parasitic worms, which block lymphatic vessels and result in massive swelling - usually of the lower extremities (the feet, ankles) or the testicles. The lower extremities are especially vulnerable because the parasites enter the body between the toes. [Keep your shoes on during your next trip to Africa.]
Lymph Trunks Lymph trunks are where the collecting vessels connect, and there are five of them.
The lymph ducts are the final collection points for the lymph vessels before they enter the bloodstream. There are, interestingly, commonly two of these ducts, but some people have only one. [These internal eccentricities are not uncommon; bones, organs - various body parts - seem to have some latitude as to position and number.] The two ducts are: 1) the thoracic duct - everyone has this duct, and 2) the right lymphatic duct, which empties into the bloodstream at the right jugular and subclavian veins (neck).
Generally speaking, the thoracic duct may collect lymph from as much as 3/4 of the body. For those who have a right lymphatic duct, it collects lymph from the upper right quarter of the chest.
This collection of "stuff" comprises the fundamental elements of the immune system. The organs are the lymph nodes, spleen, thymus, tonsils, Peyer's patches, and the appendix. These organs are composed mainly of lymph tissue, which is the home to lymphocytes - the cellular components of the immune system. These cells are the body's primary defense mechanisms against microbial invaders and cancer cells; as well as being the primary component of long-term immunity against disease.
Lymphocytes
These immune cells are the strongest weapon in the body's arsenal against foreign invaders (pathogens (see Note 2)).
Special types of lymphocytes are: 1) B cells, which multiply to become plasma cells that secrete antibodies (proteins that, in essence, "mark" antigens for later destruction; and 2) T cells, which penetrate antigens and destroy them. These types of cells attack specific antigens. B cells fight bacteria; and T cells concentrate on fungi, cancer cells, and human cells with virus infections. As kind of an aside, T cells are the cause of organ transplant rejections.
The B and T cells travel throughout the body's loose connective tissue, in the lymph and blood vessels. The constant travel of B and T cells between the vessels and connective tissue is called recirculation, and guarantees rapid deployment of lymphocytes to infected areas.
Lymphocytes are born in bone marrow. Some of these cells (still immature - teenagers, functionally - get into the bloodstream and travel to the thymus for a short stay. During this time they are rapidly dividing and learning to recognize (without ever actually seeing) specific pathogens ( a process called immunocompetence). These cells are the T (thymus) cells. Other cells, that remained in the bone marrow, reached immunocompetence [I love that word.] there, and are identified as B (bone) cells.
B and T cells are activated after entering the bloodstream, and arriving at an infected, loose connective tissue site, and finally meeting their designated antigen (see Note 3) (called the antigen challenge). Additionally, helper T cells stimulate the rush to a specific area by hordes of B and T cells. Sort of a mobilization of forces.
HIV, which causes AIDS, is a virus which destroys helper T cells, thus reducing the normal mobilization of B and T cells, making the victim susceptible to a variety of diseases from which death is the likely (inevitable?) result.
An interesting part of the immune puzzle is the presence of memory lymphocytes, a variety that encounter a disease, but remain inactive - maybe for years. But at the next encounter, these memory cells activate, and vigorously attack the remembered target. That is why we typically only get certain diseases once - like measles, mumps, and chickenpox.
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Note 2 - Pathogens are any virus, microorganism, or other substance that causes disease.
Note 3 - An antigen is a substance that,when it touches animal tissues, induces sensitivity (and/or resistance) to infection or toxic substances after a latent period.
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Lymphoid Tissue
This tissue is a reticular (very porous - think of a miniature volleyball net) connective tissue. There are five salient characteristics of this tissue: 1) it is the site of most infections, 2) the site where most invading microorganisms are destroyed, 3) the site of most recirculating lymphocytes, 4) the site of most antigen challenges, and 5) the location where most effector and memory lymphocytes are produced.
Lymphoid tissue makes up most of the lymphoid organs - thymus excepted, and it is present in localized patches in the walls of digestive and respiratory tubes. The reticular fiber structure of this tissue contains a myriad of B and T cells, allowing them to ambush invaders.
Lymphoid tissue contains many spherical clusters of lymphocytes (lymphoid follicles, or nodules). Each of these follicles can be activated by just one memory B cell, resulting in hoards of lymphocytes being produced in the follicle.
Lymphoid Organs
Lymph nodes, of which there are about 500, are located around collecting vessels. Their sole purpose: to kill infectious microorganisms and cancer cells in lymph. Within lymph nodes are areas of reticular (remember the volleyball nets) fibers where many macrophages live. As lymph flows by, disease organisms are attacked. By virtue of the large number of these areas are pretty thoroughly purged of invaders by the time lymph reaches the bloodstream. An important feature of lymph nodes is that they add to the body's supply of memory lymphocytes.
The Spleen is the largest organ in the lymphatic system: it is more-or-less fist-sized, and is located next to the stomach.
Two large vessels, the splenic vessels, are responsible for the heavy blood flow through this organ. Two blood-cleaning jobs are performed here: 1) antigens are removed, and 2) old or defective blood cells are removed. An additional function performed is to store blood platelets.
The thymus is located in the chest cavity behind the sternum. Its job is to control the maturation of lymphocytes into T cells, which it does by secreting hormones that effect immunocompetence. (there it is again)
Interestingly enough, the thymus is most active from infancy, and begins to shrink after adolescence. In old age, the thymus is difficult to find, but it is still alive and functioning, albeit at a much reduced rate
The thymus is unique among the lymphoid organs by virtue of: 1) it doesn't fight antigens, but, as previously mentioned, works toward lymphocyte maturation; and 2) its cell structure is different - no reticular framework. These cells are called thymocytes, rather than lymphocytes, but they secrete hormones that cause T cell immunocompetence.
Tonsils The tonsils are primitive organs that are, in essence, swellings of the lining of the pharynx. There are four groups of tonsils: 1) Paletine tonsils, which are the largest, and are the ones that often become infectious and are removed during childhood; 2) the lingual tonsils; 3) pharyngeal tonsils (adenoids), and 4) the tubal tonsils. All are in the throat , and are designed to expunge pathogens that enter the throat through breathed air or ingested food.
Structurally, tonsils contain deep crypts, which trap bacteria and particles of foreign "stuff," and ultimately shuffle it off into the land of the lymphocytes for elimination. Ironically, this trapping of bacteria often leads to tonsillitis in children; but, at the same time, memory lymphocytes are produced to help with later protection against infections.
Peyer's Patches and the Appendix
Peyer's patches, also called aggregated lymphoid follicles, are clusters of lymphoid follicles in the intestines. There is also a substantial amount of lymphoid tissue in the appendix (a tubular offshoot of the cecum (the upper portion of the large intestine)).
The location of Peyer's patches and the appendix puts them in an ideal spot for destroying the many bacteria that inhabit the intestine. Many memory lymphocytes are generated in these organs for long-term immunity.
Gut-associated lymphoid tissue (GALT) is the name for a collection of lymphoid tissue (including Peyer's patches, the appendix, and the tonsils) that is spread throughout the wall of the digestive tract. Similarly, lymphoid tissue congregates in the walls of the respiratory tubes - called bronchus-associated lymphoid tissue (BALT). GALT and BALT work to protect the two most vulnerable (to invading micro-organisms) areas in the human body. Happily, the production of these tissues in never-ending.
And so ends our journey through the lymphatic system.
Water is composed of two gins, oxygin, and hydrogin.
Oxygin is pure gin. Hydrogin is gin and water.
- Anonymous student -
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(Q) I am getting sore shoulders from doing back exercises, like pulldowns and rows. Am I doing something wrong?
J.A., Woodside. CA
(A) Maybe. One possible cause is over-extending your shoulders. We are often told to fully extend our arms - to work through a full range of motion; but we are working the back (latisimus dorsi muscle), not the shoulders. What you should feel is full extension of the lat muscles, not the shoulder joints. Try this, and see if it helps.
(Q) I recently saw, at the gym I go to, a trio of young men doing upright rows. They were pulling the bar up in front of their faces, while the presumed leader of the group was admiring his rear delts (invisible, to me), and extolling the virtues of the exercise they were doing. To get to the point, I thought this exercise was properly done by pulling the bar up under your chin. Am I mistaken?
E.J., Santa Clara, CA
(A) No. To properly execute this exercise, start with a pronated grip, hands just inside of shoulder width, and holding a barbell - arms extended - in front of your thighs; pull the bar (or dumbbells) up toward your head. Pull with your elbows - up and back. The pull should be close to your body, and it should end with your elbows high and the bar under your chin. Using this form puts emphasis on the rear deltoid, trapezius, and rhomboid muscles. The exercise, as you described it, with the pull ending in front of the face, shifts emphasis to the front, not rear, deltoids. The upright row is designed to work the rear, not front, deltoids - the front delts are worked in other exercises, but the rear delts need specific attention, so follow this form carefully.
When you smell an odorless gas,
it is probably carbon monoxide.
-Anonymous student-
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