The fluids of the body are primarily composed of water, which in turn contains a multitude of substances. One such group of substances include electrolytes such as sodium, potassium, magnesium, phosphate, chloride, and others. Another group includes metabolites, such as oxygen, carbon dioxide, glucose, urea, etc. A third important group of substances contained within the water of our body which includes proteins, most of which are vital for our existence. Examples of proteins include coagulation factors, immunoglobulins, albumin, and various hormones. As the distribution of the fluid in the body and the substances found within are critical for the maintenance of intracellular and extracellular functions pivotal to survival, the body has developed mechanisms to control compartment composition tightly. However, various clinical pathologies can alter the fluid composition and its constituents in the multiple compartments of our bodies, which can have deleterious effects on our health and often require intensive interventions to monitor and maintain normal physiological conditions. This article will primarily cover the physiologic composition of water in the human body, differentiate the various compartments in the body along with their associated volumes and compositions, depict how to measure the different volumes, and delve into the clinical relevance associated with disturbances of the normal physiological conditions.
At a cellular level, the distribution of the various fluid compartments in the body is paramount for the maintenance of health, function, and survival. For the average 70 kg man, 60% of the total body weight is comprised of water, equaling 42L. The fluid in the body separates into two main compartments: Intracellular fluid volume (ICFV), and extracellular fluid volume (ECFV).
Each space works in unison with each other and has different functions paramount for normal physiological function.
Several principles control the distribution of water between the various fluid compartments. To understand the different principles, it is essential to realize the following: ingestion and excretion of water and electrolytes are under tight regulation as to maintain consistent total body water (TBW) and total body osmolarity (TBO). To manage these two parameters, body water will redistribute itself to maintain a steady state so that the osmolarity of all bodily fluid compartments is identical to total body osmolarity.
Several different factors mediate the redistribution of water between the two ECF compartments: hydrostatic pressure, oncotic pressure and the osmotic force of the fluid. Combining these two components yields the Starling equation: Jv = Kfc [(Pc - Pi] - n (Op-Oi)]. This equation determines the rate of fluid across the capillary membrane (Jv) and takes the difference between the hydrostatic pressures of the capillary fluid (Pc) and the interstitial fluid (Pi), as well as the oncotic pressure of the capillary fluid (Op) and the interstitial fluid (Oi). It also takes into account the osmotic force between the two compartments (n).
Additionally, there is a relationship between the interstitial fluid and intracellular fluid. These two environments very closely influence each other, as the membrane of the cell separates them. Generally, nutrients diffuse into the cell with waste products coming out into the interstitial space. Ions are typically barred from crossing the membrane but can occasionally cross via active transport or under specific conditions. Water can move freely across the membrane and is directed by the osmotic gradient between the two spaces. Changes in intracellular fluid volume are a result of alterations in the osmolarity of the ECF but do not respond to isosmotic changes in extracellular volume. However, any flow of water in or out of the cell membrane will have proportional changes in the ECFV. If a disturbance causes ECF osmolarity to increase, then water will flow out of the cell and into the extracellular space to help balance the osmotic gradient; however, the total body osmolarity will remain higher than what is typical, and the cell will shrink. If a disturbance were to cause a decrease in ECF osmolarity, then water will move from the ECF into the ICF to attain an osmolar equilibrium; however, the total body osmolarity will remain lower than normal, and the cell will swell. Third, were isosmotic fluid to enter the extracellular space, then there would be no net changes in the ICF, and the ECFV will increase.
Much of this information can appear in abstract, especially when talking about compartments that are more of a theoretical space and it is, therefore, crucial to have a way to physically measure the volumes of the different compartments. The way to measure the different spaces is by using the indicator-dilution method. The theory behind this is that to measure the volume of a specific compartment, one must introduce into the body measurable substances that are distributed uniformly to a compartment of interest. Using this method, individual volumes can be measured directly, and others can be measured by subtracting the volumes of related compartments. This information can then be quantified by using the equation Volume (V) = Amount (substance injected)/Concentration (measured after equilibration). The following compartments can be measured as followed:
Aside from the significance of the study of water balance has on our physiologic understanding of the human body, the idea behind it is commonly seen in pathology and is presented clinically on a daily basis. Various conditions lead to an imbalance of water in the different compartments of the body; the specific imbalance can show in different ways and can be treated differently as well. The following presents five clinical scenarios where alterations in water balance can present, and each will have an accompanying analysis of ECF volume, ECF osmolarity, ICF volume, as well as ICF osmolarity.
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