page_title: Extracellular fluid
section_title: Interaction between the blood plasma, interstitial fluid and lymph
hierarchical_section_title: Extracellular fluid / Interaction between the blood plasma, interstitial fluid and lymph
caption_attribution_description: Deutsch: Lymphkapillaren im GewebszwischenraumEnglish: Lymph capillaries in the tissue spaces
context_page_description: Extracellular fluid denotes all body fluid outside the cells of any multicellular organism. Total body water in healthy adults is about 60% of total body weight; women and the obese typically have a lower percentage than lean men. Extracellular fluid makes up about one-third of body fluid, the remaining two-thirds is intracellular fluid within cells. The main component of the extracellular fluid is the interstitial fluid that surrounds cells.
  Extracellular fluid is the internal environment of all multicellular animals, and in those animals with a blood circulatory system, a proportion of this fluid is blood plasma. Plasma and interstitial fluid are the two components that make up at least 97% of the ECF. Lymph makes up a small percentage of the interstitial fluid. The remaining small portion of the ECF includes the transcellular fluid. The ECF can also be seen as having two components – plasma and lymph as a delivery system, and interstitial fluid for water and solute exchange with the cells.
  The extracellular fluid, in particular the interstitial fluid, constitutes the body's internal environment that bathes all of the cells in the body.
context_section_description: The arterial blood plasma, interstitial fluid and lymph interact at the level of the blood capillaries. The capillaries are permeable and water can move freely in and out. At the arteriolar end of the capillary the blood pressure is greater than the hydrostatic pressure in the tissues. Water will therefore seep out of the capillary into the interstitial fluid. The pores through which this water moves are large enough to allow all the smaller molecules (up to the size of small proteins such as insulin) to move freely through the capillary wall as well. This means that their concentrations across the capillary wall equalize, and therefore have no osmotic effect (because the osmotic pressure caused by these small molecules and ions – called the crystalloid osmotic pressure to distinguish it from the osmotic effect of the larger molecules that cannot move across the capillary membrane – is the same on both sides of capillary wall).
  The movement of water out of the capillary at the arteriolar end causes the concentration of the substances that cannot cross the capillary wall to increase as the blood moves to the venular end of the capillary. The most important substances that are confined to the capillary tube are plasma albumin, the plasma globulins and fibrinogen. They, and particularly the plasma albumin, because of its molecular abundance in the plasma, are responsible for the so-called "oncotic" or "colloid" osmotic pressure which draws water back into the capillary, especially at the venular end.
  The net effect of all of these processes is that water moves out of and back into the capillary, while the crystalloid substances in the capillary and interstitial fluids equilibrate. Since the capillary fluid is constantly and rapidly renewed by the flow of the blood, its composition dominates the equilibrium concentration that is achieved in the capillary bed. This ensures that the watery environment of the body's cells is always close to their ideal environment (set by the body's homeostats).
  A small proportion of the solution that leaks out of the capillaries is not drawn back into the capillary by the colloid osmotic forces. This amounts to between 2-4 liters per day for the body as a whole. This water is collected by the lymphatic system and is ultimately discharged into the left subclavian vein, where it mixes with the venous blood coming from the left arm, on its way to the heart. The lymph flows through lymph capillaries to lymph nodes where bacteria and tissue debris are removed from the lymph, while various types of white blood cells (mainly lymphocytes) are added to the fluid. In addition the lymph which drains the small intestine contains fat droplets called chylomicrons after the ingestion of a fatty meal. This lymph is called chyle which has a milky appearance, and imparts the name lacteals (referring to the milky appearance of their contents) to the lymph vessels of the small intestine.
  Extracellular fluid may be mechanically guided in this circulation by the vesicles between other structures.  Collectively this forms the interstitium, which may be considered a newly identified biological structure in the body. However, there is some debate over whether the interstitium is an organ.
caption: Diagram showing the formation of lymph from interstitial fluid (labeled here as "Tissue fluid"). The tissue fluid is entering the blind ends of lymph capillaries (shown as deep green arrows)
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