A mixture is where one substance is added to another but no physical or chemical reaction takes place and the two remain separate. In this way the solute becomes completely incorporated with the solvent. To form a solution the solvent and solute undergo a reaction with breaking and forming of physical bonds. A good example would be dissolving salt in water the result still looks and behaves like water! This results in a solution which is in the same physical state as the solvent. A solvent is a liquid, solid, or gas into which another solid, liquid, or gaseous substance can dissolve (the solute). In order to fully understand the distribution and movement of water within the body there are some definitions, principles, and concepts that must first be understood: Solvents, solutes, and solutions Relations of the body fluid compartments. The final 1.5L comprises the transcellular fluid which is made up of intraocular fluid, cerebrospinal fluid (CSF), urine in the bladder, and fluid within the lumen of the bowelįigure 1. 3.5L exists as blood plasma in blood vessels and is termed intravascular fluid. 10L (approximately 2/3) exists in the spaces between cells and is termed interstitial fluid (ISF). The extracellular fluid is divided into several other compartments (Figure 1) The remaining one-third (15L) of TBW is outside the cells or “extracellular fluid” (ECF). The largest fluid ‘compartment’ which accounts for two-thirds (30L) of TBW is within the cells of the body and is called intracellular fluid (ICF). The compartments are separated from each other by membranes that regulate flow of water between each compartment and thereby control the amount of water which can exist in each of the compartments. Total body water is distributed throughout the body, and can be thought of as being in different ‘compartments’ or ‘spaces’. To simplify matters, all subsequent calculations will be based on this ‘ideal’ person with a total body water of 45 litres. 
Total body water (TBW) accounts for 60% of an adult male’s total body weight a normally hydrated 75 kilogram man will consist of approximately 45 litres of water. The amount of water in the body varies with a patient’s age, weight, and sex (Table 1). HOW MUCH FLUID IS THERE IN THE BODY, AND HOW IS IT DISTRIBUTED?
If a normally hydrated 75 Kg male patient is kept ‘Nil By Mouth’ for 24 hours, what volume of water would have to be given by alternative routes to maintain hydration, and what electrolytes should the water also contain to replace ongoing losses?.What are the main sources of fluid loss from the body? What volume is lost via these routes in each 24 hour period?.What is the normal osmolality of plasma? How can you estimate plasma osmolality?.What is the volume of extracellular fluid in a healthy 75 Kg male? Where/how is this fluid distributed?.The answers can all be found in the following text. These are for you to test your current knowledge and understanding of body fluid physiology prior to reading this tutorial. Therefore it is important to have a good understanding of the physiology of normal fluid homeostasis and what happens when these mechanisms fail or are overcome. In a wide variety of illnesses and during surgery, disturbances to this fine balance occur which must be identified and corrected to prevent deterioration, complications and to promote recovery.Ĭare of unwell patients with body water abnormalities and patients undergoing surgery, are encountered daily by medical practitioners. Tight regulation of the balance between water intake and output, and its distribution, is vital to the optimal function of every organ system within the body. When thinking about fluid within the body we are essentially thinking about water.
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