Plasma is the only major fluid compartment that exists as a real fluid collection all in one location. Lymph is more easily obtained for analysis than other parts of the ISF. The lymphatic system returns protein and excess ISF to the circulation. Lymph is considered as a part of the ISF. ISF has the compositional characteristics of ECF (as mentioned above) but in addition it is distinguished by its usually low protein concentration (in comparison to plasma). Oxygen, nutrients, wastes and chemical messengers all pass through the ISF. The ISF bathes all the cells in the body and is the link between the ICF and the intravascular compartment.

This is also a ‘virtual’ fluid (ie it exists in many separate small bits but is spoken about as though it was a pool of fluid of uniform composition in the one location). Interstitial fluid (ISF) consists of all the bits of fluid which lie in the interstices of all body tissues. The ECF compositional similarity is in some ways, the opposite of that for the ICF (ie low in potassium & magnesium and high in sodium and chloride). These compartments are distinguished by different locations and different kinetic characteristics. The ECF is divided into several smaller compartments (eg plasma, Interstitial fluid, fluid of bone and dense connective tissue and transcellular fluid). 2.1.4 Extracellular FluidĪ similar argument applies to the Extracellular Fluid. The use of this convention allows predictions to be made about what will happen with various interventions and within limits these are physiologically meaningful.

Behaviour: Intracellular fluids behave similarly to tonicity changes in the ECFīecause of this physiological usefulness, it is convenient to talk of an idealised ICF as though it were a single real entity.

Composition: Intracellular fluids are high in potassium and magnesium and low in sodium and chloride ions.

Location: The distinction between ICF and ECF is clear and is easy to understand: they are separated by the cell membranes.

In particular, similarities of location, composition and behaviour: How can the term ‘intracellular fluid’ be used as though it was a single body of fluid? The reason is that though not united physically, the collections have extremely important unifying similarities which make the ICF concept of practical usefulness in physiology. The ICF compartment is really a 'virtual compartment' considered as the sum of this huge number of discontinuous small collections. The concept of a single united 'compartment' called intracellular fluid is clearly artificial. The Intracellular Fluid is composed of at least 10 14 separate tiny cellular packages. Typical values for the size of the fluid compartments are Liters) based on which side of the cell membrane the fluid lies.

The major division is into Intracellular Fluid (ICF: about 23 liters) and Extracellular Fluid (ECF: about 19 These collections are referred toĪs 'compartments''. Together into larger collections which can be discussed in a physiologically meaningful way. The water in the body is contained within the numerous organs and tissues of the body. Differences in percent body water between males and females are primarily due to differences in amounts of adipose tissue.įor any particular tissue of the body the variation is very much less but any variation that occurs is still mostly due to differences in amount of fat content. Obese adults have a lower ratio because of the greater amount of adipose tissue. Differences (between individuals) in the amount of bone and plasma are much smaller. The variation between individuals in the ratio of TBW to total body weight is quite large but the majority of the variation is due to different amounts of adipose tissue as adipose has a low water content. Plasma: 93% water (& 7% 'plasma solids').Most tissues are water-rich and contain 70-80% water. By the age of 60 years, total body water (TBW) has decreased to only 50% of total body weight in males mostly due to an increase in adipose tissue. Total body water as a percentage of total body weight decreases progressively with increasing age. By the age of 12 months, this has decreased to 60% which is the adult value.

At birth, the amount of interstitial fluid is proportionally three times larger than in an adult. Neonates contain more water then adults: 75-80% water with proportionately more extracellular fluid (ECF) then adults. 2.1.1 Variations in Water Content Variation due to Age These figures are standard values which are quoted frequently and are average values. The hypothetical adult female contains 55% of her body weight as water: this lower percent being due to a higher fat content. The 70 kg 'standard male' contains 42 liters of water - 60% of his body weight.

Body fluid compartments calculations