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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. For a complete diagram of body fluid compartments, see body fluid compartments of a 70-kg man and body fluid compartments of a 55-kg woman.When thinking about fluid within the body we are essentially thinking about water. Note that this diagram places focus only on these three major fluid compartments. Plasma is the smallest fluid compartment (~8% of total body water). Interstitial fluid contains ~25% of the total body water. The intracellular fluid compartment contains most of the water in the body (~67% of total). The right diagram shows the three major fluid compartments drawn to scale. The left diagram allows for a better demonstration of the relationship between the intracellular fluid, interstitial fluid, and plasma, however, the relative size of each of the compartment is not drawn to scale. Waste products produced by cells follow the reverse path from the cytoplasmic compartment to plasma. They then must cross the plasma membrane to enter the cytoplasmic compartment of cells. Nutrient molecules traveling in the blood must first cross the capillary endothelium to enter the interstitial fluid. The capillary endothelium is the physical barrier that separates the interstitial fluid from plasma. The physical barrier separating the intracellular fluid compartment (i.e., cytoplasm) and the interstitial fluid is the cell plasma membrane. Fluid, molecules, and ions flow across physical barriers between the fluid compartments. These are the (1) intracellular fluid compartment, (2) interstitial fluid, and (3) plasma. In the human body plan, there are three major fluid compartments that are functionally interconnected.
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