As the arterial blood flow through the capillaries, to deliver nutrient and oxygen to cells of the body, due to pressure gradient between the capillaries and interstitial space, blood plasma, small protein, and nutrients move out into the surrounding tissue, where it becomes interstitial fluid. After nutrients are delivered to the cell, waste products are then push back into the venous circulation.
Two types of pressure are exerted both in interstitial fluid & inside the blood vessels.
Oncotic or colloidal osmotic pressure, exerted by the proteins present in blood plasma or interstitial space.
Hydrostatic pressure, exerted by the pressure of fluid present within or outside of blood vessels.
Remember, hydrostatic pressure always pushes fluid to its opposite direction.
Blood hydrostatic pressure → pushes the fluid toward the interstitium.
Interstitial fluid hydrostatic pressure → pushes the interstitial fluid inside the blood vessels.
Similarly, colloidal osmotic pressure always tends to full the fluid towards them.
Interstitial fluid colloidal osmotic pressure → tends to pull the fluid toward the interstitium.
Blood colloidal osmotic pressure → tends to pull the fluid of the interstitium inside the blood vessel.
Albumin & Globulin are the major plasma proteins to maintain this pressure.
Due to pressure dynamic, at the arterial end, net filtration pressure is plus 10mm of mercury which denotes fluid from the vessel is seeping out into interstitial space. Similarly, at the venous end, net filtration pressure is minus 9mm of mercury, which means, interstitial fluid is being reabsorbed into the vessel.
Around 20 liters per day of blood plasma, move out of the arterial end of the capillary. But only 17 liters per day of filtered plasma is reabsorbed at the venous end. While the remaining 3 liters per day of fluid is left as interstitial fluid. If these fluids get accumulated in excess, then it leads to edema.
Thus, the lymphatic system comes into play, where this remaining interstitial fluid is drained into porous, blunt-ended lymphatic capillaries, where it is now referred to as lymph.
Unlike the blood circulation system, which is a closed-loop, the lymphatic system is a one-direction open-ended network of vessels. Lymphatic capillaries are the blind sac tube, made up of overlapping endothelial cells, which form flap-like valves that only open in one direction.
When the interstitial pressure increases, pushes the flaps inward, opening the gaps between the cells, allowing fluid to flow in. As the pressure inside the capillary increases, the endothelial cells are pushed outward, closing the gaps, thus preventing the backflow.
Factors responsible for lymph flow are same as that of in the vein.
Terminal lymphatic ducts (Right lymphatic duct and thoracic duct)
2. Lymphoid organs are comprised of
Primary lymphoid organs and secondary lymphoid organs. Bone marrow and the Thymus are the primary lymphoid organs as the formation and maturation of T and B lymphocytes occurs here.
Secondary lymphoid organs are spleen, lymph nodes, Appendix, Tonsil, Solitary lymphoid follicle, and Payer's patches of the ileum.
3. Lymphoid cells
Natural killer cells
Interstitial fluid is first drained into the lymph capillaries, lymph vessels, and then from the afferent lymph vessels, it passes to the sub-capsular sinus and trabecular sinus of the lymph node. The lymph node filters the lymph. Then lymph flow from the hilum to efferent lymph vessels which ultimately drained into either the lymphatic trunk or directly into Terminal ducts.
The thoracic duct is the largest lymphatic vessel allowing the return of lymph from most parts of the body, except the right upper quadrant of the body, which is drained by the right lymphatic duct.
The thoracic duct originates from the cisterna chyli, at the level of the 12th Thoracic vertebra, to the junction of the left internal jugular and left subclavian vein.
The gaps between overlapping endothelial cells are larger in lymph capillaries than in blood capillaries. Thus, it will allow passing bacteria, immune cells like macrophages, cellular waste products & other larger particles like fat.
The lymphatic system is the route of the transport of fat from the gastrointestinal tract.
Fat is breakdown into fatty acid, which is absorbed by the intestinal epithelium. Within the epithelium, it gets packaged with chylomicron & transported to lymph capillaries called a lacteal. The lymph in the lacteal has a milky appearance due to its high-fat content and is called chyle.
Clinical correlation of the lymphatic system
If hydrostatic pressure increased drastically in the lymphatic, then lymph will leaks from the lymph vessels back to the interstitium, leading to edema called lymphedema.
Lymph nodes that drain infected region, become enlarged & swollen. This condition is called lymphadenitis (which is usually tender).
Lymphoma is a cancer of the lymphatic system. (which is non-tender) Major subtypes of lymphoma are Hodgkin’s lymphoma and Non-Hodgkin’s lymphoma.
Cancer cells break away from where they begin, (the primary tumor) and travel through the lymphatic system or blood to other parts of the body is called metastasis.
Breast cancer is the best example to demonstrate this.
If you have breast cancer, it will spread to its surrounding tissues. Then Cells may be metastasized from that primary tumor area, gets into the lymph vessels, and will end up in lymph nodes, causing lymphoma. In breast cancer, mainly axillary lymph nodes are involved.
In the final stage, if breast cancer is not diagnosed well and treated, then from axillary lymph nodes, breast cancer may spread to other parts of the body, mostly bones, lungs, liver, or brain.