Home to connective tissues!
Some connective tissues of the body are fluids. Although it can be difficult to visualize fluids as "connective" components, an examination of their roles in body homeostasis provides insight into why they are considered tissues of this type. It might be better to think of fluid connective tissues as providing "physiological" rather than physical connections between tissues.
Blood and lymph are two extremely important fluids involved in transport and body defense. Most of us are familiar with blood and its transport roles in the body. We recognize that blood carries nutrients and oxygen to tissue cells and carries wastes from these cells to the lungs or kidneys for elimination. On the other hand, few of us recognize critical links between blood and lymph fluids in tissue spaces and the specific role of lymph in body homeostasis. Review the events of inflammation induced by histamine and other agents. This is central to understanding lymph and its role in body homeostasis.
Now, envision blood sweeping through vessels into tissues. As this blood enters the tiny, branching, thin-walled capillaries, fluid pressures cause plasma to leak across capillary walls into tissue spaces. The analogy here is a hose with tiny holes! If you run fluid through the hose, some fluid leaks out. If you increase flow through the hose, more fluid leaks out!
The fluid components of plasma that leak into interstitial spaces across capillary walls form interstitial fluids. Interstitial fluids are important in a physiological sense in that solutes and respiratory gases(i.e. oxygen, glucose) of the blood diffuse across these fluids to body cells. The waste products that accumulate within active cells also diffuse in the opposite direction across these fluids, from cells to capillaries. Some of this formed interstitial fluid returns to the blood, leaking back across capillary walls from interstitial spaces. Remaining fluid drains off through nearby lymphatic vessels. Subsequently, lymphatic capillaries and vessels are best described as a drainage/filtration system for tissue spaces. Under normal conditions, a balance exists with no excess fluid accumulating in tissues. Lymph draining from interstitial spaces eventually returns to the venous vascular flow.
Inflammation disrupts the existing balance between these fluids, resulting in events that lead to abnormal plasma loss and its accummulation in interstitial spaces, a condition called edema. Although uncomfortable, one has to think of inflammation and edema from an immunological perspective to better understand why these events are supportive of body homeostasis.
Increased blood flow brings more defensive leukocytes to an inflamed tissue. Enhanced capillary permeability allows the arriving defensive cells to move more easily from the blood into the interstitial spaces where they engage in phagocytosis and other activities. Phagocytosis is an important "first-step" in body defense.
Phagocytic cells attracted to inflammatory events detect and attempt to destroy invading organisms or non-cellular agents that cause inflammation. Some of these phagocytic "veterans" then enter the lymphatic flow where they meet with lymphocytes in lymph nodes or other lymphatic tissues to "serve-up" or present antigens. These cells are called antigen presenting cells or APC's. These cells help stimulate immunological reactions involving B and T-lymphocytes. Highly specific T-lymphocytes, antibodies, or both are then produced and released into lymph returning to the venous blood flow. Eventually, these new "high tech" weapons find their way to sites of inflammation. The histamine-induced vascular effects induced there enable these "highly specific" defenses to arrive in larger numbers and move more efficiently into tissues to participate in the battle with the disease or foreign agent.
In short, inflammation is important as part of a complex, positive-feedback mechanism of body defense that operates through blood and lymph fluids. Both initial and longer-term defenses involve cells of blood and lymph working together. Typically these defensive responses get progressively stronger until the initiating condition is removed and homeostasis returns.
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