Fluid compartments
The human body may be conceptually divided into two major fluid compartments: the intracellular compartment and the extracellular compartment. The intracellular compartment is the space within the organism's cells; it is separated from the extracellular compartment by cell membrane.[1]
About two thirds of the human body's water is held in its cells and the remainder is found in the extracellular compartment. The extracellular fluids may be divided into three types: interstitial fluid in the "interstitial compartment" (surrounding tissue cells and bathing them in a solution of nutrients and other chemicals), blood plasma in the "intravascular compartment" (the blood vessels), and small amounts of transcellular fluid such as ocular and cerebrospinal fluids in the "transcellular compartment". The interstitial and intravascular compartments readily exchange water and solutes but the third extracellular compartment, the transcellular, is thought of as separate from the other two and not in dynamic equilibrium with them.[2]
Contents
Intracellular compartment[edit]
Intracellular fluid is contained by the cell's plasma membrane, and is the matrix in which cellular organelles are suspended, and chemical reactions take place.[3] In humans, the intracellular compartment contains on average about 28 litres of fluid, and under ordinary circumstances remains in osmotic equilibrium. It contains moderate quantities of magnesium and sulphate ions.
Extracellular compartment[edit]
The interstitial, intravascular and transcellular compartments comprise the extracellular compartment.
Interstitial compartment[edit]
The interstitial compartment (also called "tissue space") surrounds tissue cells. It is filled with interstitial fluid. Interstitial fluid provides the immediate microenvironment that allows for movement of ions, proteins and nutrients across the cell barrier. This fluid is not static, but is continually being refreshed and recollected by lymphatic channels. In the average male (70 kg) human body, the interstitial space has approximately 10.5 litres of fluid.
Intravascular compartment[edit]
The main intravascular fluid in mammals is blood, a complex fluid with elements of a suspension (blood cells), colloid (globulins) and solutes (glucose and ions). The average volume of plasma in the average (70 kg) male is approximately 3.5 liters. The volume of the intravascular compartment is regulated in part by hydrostatic pressure gradients, and by reabsorption by the kidneys.
Transcellular compartment[edit]
The third extracellular compartment, the transcellular, consists of those spaces in the body where fluid does not normally collect in larger amounts,[4][5] or where any significant fluid collection is physiologically nonfunctional.[6] Examples of transcellular spaces include the eye, the central nervous system, and the peritoneal and pleural cavities. A small amount of fluid does exist normally in such spaces.
Fluid shift[edit]
Fluid shifts occur when the body's fluids move between the fluid compartments. Physiologically, this occurs by a combination of hydrostatic pressure gradients and osmotic pressure gradients. Water will move from one space into the next passively across a semi permeable membrane until the hydrostatic and osmotic pressure gradients balance each other. Many medical conditions can cause fluid shifts. When fluid moves out of the intravascular compartment (the blood vessels), blood pressure can drop to dangerously low levels, endangering critical organs such as the brain, heart and kidneys; when it shifts out of the cells (the intracellular compartment), cellular processes slow down or cease from intracellular dehydration; when excessive fluid accumulates in the an interstitial space, edema develops; and fluid shifts into the brain cells can cause increased cranial pressure. Fluid shifts may be compensated by fluid replacement or diuretics.
Third spacing[edit]
Third spacing is the unusual accumulation of fluid in a transcellular space. In medicine, the term is most commonly used with regard to burns, but also can refer to ascites and pleural effusions. With regard to severe burns, fluids may pool on the burn site (i.e. fluid lying outside of the interstitial tissue, exposed to evaporation) and cause depletion of the fluids. With pancreatitis or ileus, fluids may "leak out" into the peritoneal cavity, also causing depletion of the intracellular, interstitial or vascular compartments.
Patients who undergo long, difficult operations in large surgical fields can collect third-space fluids and become intravascularly depleted despite large volumes of intravenous fluid and blood replacement.
The actual volume of fluid in a patient's third space is difficult to accurately quantify.
Third spacing conditions may include peritonitis, pyometritis, and pleural effusions.[7]
See also[edit]
References[edit]
- ^ Rodney A. Rhoades; David R. Bell (18 January 2012). Medical Phisiology: Principles for Clinical Medicine. Lippincott Williams & Wilkins. pp. 5–6. ISBN 978-1-60913-427-3.
- ^ Jacob M, Chappell D, Rehm M (2009). "The 'third space'--fact or fiction?". Best Pract Res Clin Anaesthesiol 23 (2): 145–57. PMID 19653435.
- ^ W. Kapit, R. Macey, E. Meisami, The Physiology Coloring Book 2nd ed pg 1-7. Addison/Wesley/Longman, Inc. San Francisco, 2000.
- ^ Barbara Kuhn Timby (1 January 2008). Fundamental Nursing Skills and Concepts. Lippincott Williams & Wilkins. pp. 319–. ISBN 978-0-7817-7909-8. Retrieved 9 June 2010.
- ^ Redden M, Wotton K (June 2002). "Third-space fluid shift in elderly patients undergoing gastrointestinal surgery: Part 1: Pathophysiological mechanisms". Contemp Nurse 12 (3): 275–83. PMID 12219956.
- ^ Drain, Cecil B. (2003). Perianesthesia nursing: a critical care approach. Philadelphia: W.B. Saunders Co. ISBN 0-7216-9257-5. [1]
- ^ "FLUID AND ELECTROLYTE THERAPY". Retrieved 2010-06-08.
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||
