Fluid compartments

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The human body and even its individual body fluids may be conceptually divided into various fluid compartments, which, although not literally anatomic compartments, do represent a real division in terms of how portions of the body's water, solutes, and suspended elements are segregated. The two main fluid compartments are the intracellular and extracellular compartments. The intracellular compartment is the space within the organism's cells; it is separated from the extracellular compartment by cell membranes. [1]

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About two-thirds of the total body water of humans is held in the cells, mostly in the cytosol, 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 and lymph in the "intravascular compartment" (inside the blood vessels and lymphatic vessels), and small amounts of transcellular fluid such as ocular and cerebrospinal fluids in the "transcellular compartment".

The normal processes by which life self-regulates its biochemistry (homeostasis) produce fluid balance across the fluid compartments. Water and electrolytes are continuously moving across barriers (eg, cell membranes, vessel walls), albeit often in small amounts, to maintain this healthy balance. The movement of these molecules is controlled and restricted by various mechanisms. When illnesses upset the balance, electrolyte imbalances can result.

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]

The science of fluid balance across fluid compartments has practical application in intravenous therapy, where doctors and nurses must predict fluid shifts and decide which IV fluids to give (for example, isotonic versus hypotonic), how much to give, and how fast (volume or mass per minute or hour).

Intracellular compartment

The intracellular fluid (ICF) is all fluids contained inside the cells, which consists of cytosol and fluid in the cell nucleus. [3] The cytosol is the matrix in which cellular organelles are suspended. The cytosol and organelles together compose the cytoplasm. The cell membranes are the outer barrier. In humans, the intracellular compartment contains on average about 28 liters (6.2 imp gal; 7.4 U.S. gal) of fluid, and under ordinary circumstances remains in osmotic equilibrium. It contains moderate quantities of magnesium and sulfate ions.

In the cell nucleus, the fluid component of the nucleoplasm is called the nucleosol. [4]

Extracellular compartment

The interstitial, intravascular and transcellular compartments comprise the extracellular compartment. Its extracellular fluid (ECF) contains about one-third of total body water.

Intravascular compartment

The main intravascular fluid in mammals is blood, a complex mixture with elements of a suspension (blood cells), colloid (globulins), and solutes (glucose and ions). The blood represents both the intracellular compartment (the fluid inside the blood cells) and the extracellular compartment (the blood plasma). The average volume of plasma in the average (70-kilogram or 150-pound) male is approximately 3.5 liters (0.77 imp gal; 0.92 U.S. gal). The volume of the intravascular compartment is regulated in part by hydrostatic pressure gradients, and by reabsorption by the kidneys.

Interstitial compartment

The interstitial compartment (also called "tissue space") surrounds tissue cells. It is filled with interstitial fluid, including lymph. [5] 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 by the blood capillaries and recollected by lymphatic capillaries. In the average male (70-kilogram or 150-pound) human body, the interstitial space has approximately 10.5 liters (2.3 imp gal; 2.8 U.S. gal) of fluid.

Transcellular compartment

The transcellular fluid is the portion of total body fluid that is formed by the secretory activity of epithelial cells and is contained within specialized epithelial-lined compartments. Fluid does not normally collect in larger amounts in these spaces, [6] [7] and any significant fluid collection in these spaces is physiologically nonfunctional. [8] Examples of transcellular spaces include the eye, the central nervous system, the peritoneal and pleural cavities, and the joint capsules. A small amount of fluid, called transcellular fluid, does exist normally in such spaces. For example, the aqueous humor, the vitreous humor, the cerebrospinal fluid, the serous fluid produced by the serous membranes, and the synovial fluid produced by the synovial membranes are all transcellular fluids. They are all very important, yet there is not much of each. For example, there is only about 150 milliliters (5.3 imp fl oz; 5.1 U.S. fl oz) of cerebrospinal fluid in the entire central nervous system at any moment. All of the above-mentioned fluids are produced by active cellular processes working with blood plasma as the raw material, and they are all more or less similar to blood plasma except for certain modifications tailored to their function. For example, the cerebrospinal fluid is made by various cells of the CNS, mostly the ependymal cells, from blood plasma.

Fluid shift

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 interstitial space, oedema 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

"Third spacing" is the abnormal accumulation of fluid into an extracellular and extravascular space. In medicine, the term is often used with regard to loss of fluid into interstitial spaces, such as with burns or edema, but it can also refer to fluid shifts into a body cavity (transcellular space), such as 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 precise volume of fluid in a patient's third spaces changes over time and is difficult to accurately quantify.

Third spacing conditions may include peritonitis, pyometritis, and pleural effusions. [9] Hydrocephalus and glaucoma are theoretically forms of third spacing, but the volumes are too small to induce significant shifts in blood volumes, or overall body volumes, and thus are generally not referred to as third spacing.

See also

Related Research Articles

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<span class="mw-page-title-main">Endomembrane system</span> Membranes in the cytoplasm of a eukaryotic cell

The endomembrane system is composed of the different membranes (endomembranes) that are suspended in the cytoplasm within a eukaryotic cell. These membranes divide the cell into functional and structural compartments, or organelles. In eukaryotes the organelles of the endomembrane system include: the nuclear membrane, the endoplasmic reticulum, the Golgi apparatus, lysosomes, vesicles, endosomes, and plasma (cell) membrane among others. The system is defined more accurately as the set of membranes that forms a single functional and developmental unit, either being connected directly, or exchanging material through vesicle transport. Importantly, the endomembrane system does not include the membranes of plastids or mitochondria, but might have evolved partially from the actions of the latter.

<span class="mw-page-title-main">Oncotic pressure</span> Measure of pressure exerted by large dissolved molecules in biological fluids

Oncotic pressure, or colloid osmotic-pressure, is a type of osmotic pressure induced by the plasma proteins, notably albumin, in a blood vessel's plasma that causes a pull on fluid back into the capillary. Participating colloids displace water molecules, thus creating a relative water molecule deficit with water molecules moving back into the circulatory system within the lower venous pressure end of capillaries.

<span class="mw-page-title-main">Pleural cavity</span> Thin fluid-filled space between the two pulmonary pleurae (visceral and parietal) of each lung

The pleural cavity, pleural space, or interpleural space is the potential space between the pleurae of the pleural sac that surrounds each lung. A small amount of serous pleural fluid is maintained in the pleural cavity to enable lubrication between the membranes, and also to create a pressure gradient.

In physiology, body water is the water content of an animal body that is contained in the tissues, the blood, the bones and elsewhere. The percentages of body water contained in various fluid compartments add up to total body water (TBW). This water makes up a significant fraction of the human body, both by weight and by volume. Ensuring the right amount of body water is part of fluid balance, an aspect of homeostasis.

<span class="mw-page-title-main">Body fluid</span> Liquids inside of the body, sometimes excreted or secreted

Body fluids, bodily fluids, or biofluids, sometimes body liquids, are liquids within the body of an organism. In lean healthy adult men, the total body water is about 60% (60–67%) of the total body weight; it is usually slightly lower in women (52–55%). The exact percentage of fluid relative to body weight is inversely proportional to the percentage of body fat. A lean 70 kg (150 lb) man, for example, has about 42 (42–47) liters of water in his body.

<span class="mw-page-title-main">Extracellular fluid</span> Body fluid outside the cells of a multicellular organism

In cell biology, extracellular fluid (ECF) denotes all body fluid outside the cells of any multicellular organism. Total body water in healthy adults is about 50–60% 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.

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Thirst is the craving for potable fluids, resulting in the basic instinct of animals to drink. It is an essential mechanism involved in fluid balance. It arises from a lack of fluids or an increase in the concentration of certain osmolites, such as sodium. If the water volume of the body falls below a certain threshold or the osmolite concentration becomes too high, structures in the brain detect changes in blood constituents and signal thirst.

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<span class="mw-page-title-main">Intracellular pH</span>

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<span class="mw-page-title-main">Gibbs–Donnan effect</span> Behaviour of charged particles near a semi-permeable membrane

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<span class="mw-page-title-main">Glymphatic system</span> System for waste clearance in the central nervous system of vertebrates

The glymphatic system is a system for waste clearance in the central nervous system (CNS) of vertebrates. According to this model, cerebrospinal fluid (CSF) flows into the paravascular space around cerebral arteries, combining with interstitial fluid (ISF) and parenchymal solutes, and exiting down venous paravascular spaces. The pathway consists of a para-arterial influx route for CSF to enter the brain parenchyma, coupled to a clearance mechanism for the removal of interstitial fluid (ISF) and extracellular solutes from the interstitial compartments of the brain and spinal cord. Exchange of solutes between CSF and ISF is driven primarily by arterial pulsation and regulated during sleep by the expansion and contraction of brain extracellular space. Clearance of soluble proteins, waste products, and excess extracellular fluid is accomplished through convective bulk flow of ISF, facilitated by astrocytic aquaporin 4 (AQP4) water channels.

References

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  2. ↑ Jacob M, Chappell D, Rehm M (2009). "The 'third space'--fact or fiction?". Best Pract Res Clin Anaesthesiol. 23 (2): 145–57. doi:10.1016/j.bpa.2009.05.001. PMID   19653435.
  3. ↑ Liachovitzky, Carlos (2015). "Human Anatomy and Physiology Preparatory Course" (pdf). Open Educational Resources. CUNY Academic Works: 69. Archived from the original on 2017-08-23. Retrieved 2021-06-22.
  4. ↑ Usage example: Schweiger, A; Mazur, G (1974-09-15) [1974]. "Mammalian proteins with affinity to polynucleotides: Isolation by affinity chromatography from rat liver cytosol and nucleosol". FEBS Letters. 46 (1–2). p255, right column, line 11. doi: 10.1016/0014-5793(74)80381-9 . PMID   4417675. A soluble fraction of rat liver nuclei (nucleosol) was...
  5. ↑ "Fluid Physiology: 2.1 Fluid Compartments".
  6. ↑ 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.
  7. ↑ 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. doi:10.5172/conu.12.3.275. PMID   12219956. S2CID   19554202.
  8. ↑ Drain, Cecil B. (2003). Perianesthesia nursing: a critical care approach. Philadelphia: W.B. Saunders Co. ISBN   0-7216-9257-5.
  9. ↑ "FLUID AND ELECTROLYTE THERAPY". Archived from the original on 2010-07-07. Retrieved 2010-06-08.
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