Fluid compartments in the mammalian body broadly comprise two compartments, each with several subdivisions: intracellular fluid (ICF), which makes up approximately 60-65% of body water, and extracellular fluid (ECF), which makes up the other 35-40% of body water (for all practical purposes, the only solvent in the body is water).
Intracellular fluid is found inside the two-layered plasma membrane of the body's cells, and is the matrix in which cellular organelles are suspended, and chemical reactions take place. In humans, the intracellular compartment contains on average about 28 liters of fluid, and under ordinary circumstances remains in osmotic equilibrium with the ECF.
The interstitial compartment (also called extravascular compartment or tissue space) is the space that surrounds the cells of a given tissue. It is filled with interstitial fluid. Interstitial refers to a "small opening or space between objects". Together with the vascular space, the interstitial space comprises the extracellular space. When excessive fluid accumulates in the interstitial space, edema develops. 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 liters of fluid.
In the lungs there is an interstitial space between capillaries (tiny blood vessels) and the alveoli (the microscopic air-filled sacs in the lungs responsible for absorbing oxygen from the atmosphere). For gas exchange to occur, carbon dioxide must diffuse across the endothelium of the capillaries across the interstitial space, and across the alveolar epithelium; oxygen must diffuse the other direction, into the blood in the capillaries.
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 lumen is regulated in part by hydrostatic pressure gradients, and by reabsorption by the kidneys.
The third space is space in the body where fluid does not normally collect in larger amounts, or where any significant fluid collection is physiologically nonfunctional. Major examples of third spaces include the peritoneal cavity and pleural cavity. Still, small amount of fluid does exist normally in such spaces, and function for example as lubricant in the case of pleural fluid. Also, the lumen of the gastrointestinal tract is often classified as belonging to the third space, although it has substantial fluid content physiologically. In this classification system, the first and second space generally refer to the intravascular space (within vessels) and the extravascular space (the interstitial and intracellular spaces), respectively.
Fluid which collects in the second space (i.e. the combined interstitial and intracellular space) is more readily available for the body to use (such as for the correction of ionic imbalances in other compartments) than fluid in the third space, because fluid in the second space is physiologically more active with the intravascular ("first") space than third space fluid is.
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 chamber 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 space (the blood vessels), blood pressure can drop to dangerously low levels, endangering critical organs such as the brain, heart and kidneys. When fluid shifts out of the cells (the intracellular space), cellular processes slow down or cease from intracellular dehydration. Fluid shifts into the brain cells can cause increased cranial pressure. Fluid shifts may be compensated by fluid replacement or diuretics.
Third spacing is the physiological phenomenon in which body fluids accumulate in the third space (see above). 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 in the first and second compartments. With pancreatitis or ileus, fluids may "leak out" into the peritoneal cavity, also causing depletion of the first and second 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. Extensive tissue swelling occurs when the third space fills with excess fluid, known as edema.
Clinically, it is common that the actual volume of fluid in a particular patient's third space is difficult to accurately quantify.
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