Lymphatic System Study Guide

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Fluid Reabsorption
There is a small net loss of fluid because of the difference between hydrostatic pressure and osmotic pressure.  The lymphatic system picks up fluid that leaks out of the capillaries and returns it to the circulation.

Lymphatic capillaries start in capillary beds
Lymphatic capillaries snake their way through capillary beds to pick up the excess fluid and return it back to the venous circulation. As fluid pressure increases, it forces open little flaps in the lymphatic capillaries, and the excess fluid enters the lymphatic system.

Lymphatic Vessels
Lymphatic capillaries lead to lymphatic vessels, which are somewhat similar to veins. Since the pressure is so low, they also need valves to help move the flow of lymph fluid or “lymph” back to the vessels.

Two major divisions of the Lymphatic System
Lymph from the upper right part of the body drains into the right lymphatic duct, which dumps its fluid into the right subclavian vein. Lymph from the rest of the body drains into the thoracic duct, which dumps its fluid into the left subclavian vein. Just as in veins, breathing helps move lymph up to the subclavian vein. The thoracic duct starts at the junction of several lymph vessels at a sac-like structure called the cisterna chyli.  Because it collects lymph from the digestive organs, lymph from the thoracic duct is much higher in nutrients and fats.

Lymphoid Tissue
Lymphoid Tissue forms lymph nodules and lymph nodes.  A reticular connective tissue bed is loaded with lymphocytes and macrophages, which filter toxins and attack disease organisms.

Lymph nodules and lymph nodes filter lymph
Arrangement of fibers into irregular trabeculae in lymph nodes forces the fluid to “percolate” through and encounter a large SA of lymphoid tissue. Germinal centers in the outer cortex contain many lymphocytes. Lymph nodes also consolidate vessels as there are fewer (and larger efferent vessels) leaving the lymph node than afferent vessels entering it.

Groups of lymph nodes
Lymph nodules and nodes are often grouped in areas such as the cervical, inguinal (groin), and axilla (armpit).  This ensures that all of the lymph returning from a segment of the body must pass through a group of lymph nodes.

Other lymphatic organs
The tonsils, spleen, thymus, appendix, and Peyer’s patches are specialized lymphatic organs/tissues.  The thymus is a mixture of T- lymphocytes and hormone producing cells that helps to activate them. The tonsils, appendix, and Peyer’s patches will be covered more in later sections. The spleen has 2 distinct areas: White pulp: Strands of lymphocytes clustered around branches of the splenic artery; Red pulp: the remainder of spleen tissue where RBC recycling occurs. The spleen has smooth muscle which contracts when blood volume declines; thus acting as a blood reservoir in times of blood loss (or other factors that cause BP to drop).  

Non-specific defenses
The skin, mucous membranes, and digestive acids are physical and/or chemical barriers that help prevent disease organisms from entering the body, or living there for long if they do. The pH of the skin and bacteria-destroying enzymes in tears and saliva also help to kill pathogens. Urine, mucous, ciliated epithelium, and certain reflexes (e.g. emetic reflex) help rid the body of pathogens. Phagocytes and other defensive cells eat pathogens and other harmful substances. Proteins such as interferons and those in the complement system protect or destroy cells. Fever increases temperature for enhanced defense/healing.

Non-specific defenses - the Inflammatory Response
Chemicals from damaged tissues cause local changes, which increase blood flow and attract phagocytes. As a result, the area becomes red, swollen, warm, and painful. Preventing infection and cleaning the area promotes healing. Other beneficial aspects include limitation of movement, walling off the area (by clotting proteins), and stimulation of leukocyte production....
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