Water is the main component of body fluids and can be found mostly in two major compartments: intracellular and extracellular.

Intracellular fluid is found within the cells of the body and makes up two-thirds. Extracellular fluid is found outside of the cells and is one-third of our water. Blood plasma and interstitial fluid are the major compartments of extracellular fluid.

Plasma is the extracellular fluid found within the capillaries of the cardiovascular system. The interstitial fluid surrounds tissue cells and is a filtrate of plasma. It transports nutrients from the blood to the cells, and carbon dioxide and other wastes back to the blood from the cells. Due to the difficulty of separation, interstitial fluid and lymph are considered a single component.

The capillaries of the cardiovascular system carry plasma which contains many substances, like oxygen, salts, amino acids, fatty acids, and hormones, to cells and then removes waste products that accumulate; this exchange of substances between blood and tissue fluid occurs in capillary beds. Many additional substances which cannot enter or return through the capillary walls, including excess fluid and protein molecules, are returned to the blood as lymph. Lymph is plasma in the interstitial spaces and is transported by way of lymphatic capillaries and vessels to eventually reenter the blood stream.

Lymph forms when blood plasma filters out of capillary membranes into the interstitial spaces between cells because of the hydrostatic pressure generated by the pumping action of the heart. Much of this interstitial fluid goes back into the blood, back through capillaries. The remainder of the interstitial fluid enters the lymphatic system before it returns to the blood. That fluid, lymph, enters a network of tiny tubes which are distributed in the tissue spaces. These lymphatic capillaries permit excess fluid and other substances, such as protein molecules, to leave the tissue spaces. Lymphatic and blood capillaries are similar; however, lymphatic capillaries are more porous and allow larger molecules, including proteins and other substances, to enter and eventually return to the general circulation.

Lymph capillaries are bigger than blood capillaries and remove unwanted fat, waste, toxins, excess water and proteins. Laying directly underneath the skin and covering your whole body, they mingle between all the spaces between your cells. The movement of lymph is one way. Lymphatic vessels often have a “beaded” appearance resulting from the presence of valves that assist in maintaining a one-way flow of lymph. These valves, similar to those in veins, sometimes cause lymph to back up behind them and cause swelling. Lymph flowing through the lymphatic capillaries moves into successively larger and larger vessels sometimes called lymphatic venules and lymphatic veins. These vessels eventually empty into one of two terminal vessels, called the right lymphatic duct and the thoracic duct, which empty their lymph into large veins in the neck region. Lymph from about three-fourths of the body drains into the thoracic duct, which is the largest lymphatic vessel in the body. Lymph from the right upper extremity and from the right side of the head, neck, and upper torso flows into the right lymphatic duct. The thoracic duct has an enlarged pouch-like structure, the cisterna chili, which serves as a temporary holding area for lymph moving toward its point of entry into the veins.

The skin loses its role as an abutment for the skeletal muscle venous pump and the increased compliance of subcutaneous adipose tissue results in an increase in capillary compliance. The permeable capillaries release excess protein-rich fluid into the interstitium along with blood. The veno-arteriolar reflex in lipedema is also absent so that under orthostatic conditions (standing), there is limited vasoconstriction and increased net filtration driving edema. Early on, lymphatic transport increases to accommodate the increased fluid flux from the capillaries. During this time, visualization of lymphatic vessels on a gross level by lymphoscintigraphy is normal. As lipedema progresses, microaneurysms appear in the lymphatics in the skin which eventually leak. It is during this time that hypertrophy and hyperplasia of fat cells accelerates, further altering the microlymphatic architecture and increasing venous congestion. The resultant edema increases hydrostatic pressure in the tissue and pain (Herbst, 2012).

Painful SAT is a chronic problem in lipedema.

arteriolar end                              venule end
net force of 13 mmHg                 net force of 7 mmHg

difference of 6 mmHg                could lead to gradual loss
thus less fluid returns                  of blood volume


What prevents this? LYMPHATICS



structure > capillaries, nodes, thymus, spleen, bone marrow

functions > returns leaked capillary fluid to blood
> transports digested lipids and vitamins
> produces + distributes lymphocytes

hydrostatic pressure

muscle contraction (skeletal/respiratory muscle pump)


blind ended

larger than vascular capillaries

remove unwanted fat, toxins, excess
water & proteins

DIRECTLY underneath the skin

COVER the entire body

“beaded” appearance

maintain one-way flow of lymph

lymphatic venules

lymphatic veins

058 Net Hydrostatic Pressure and Filtration Pressure

Capillary Exchange

Thank you to Servier Medical Art for the use of this photo. This work is licensed under a Creative Commons Attribution 3.0 Unported License.
Skip to toolbar