Extracellular fluid (ECF) compartment volume and electrolyte concentration, majorly sodium, must be tightly regulated to avoid osmosis and cell damage. The intracellular space is surrounded by extracellular fluid, separated by the water permeable cell membrane.
There are three fluid compartments discussed in relation to human fluid balance. Critically unwell patients may suffer morbidity secondary to high-volume fluid losses or oedema. A disturbance in normal fluid balance induces a physiological ‘stress’ response via metabolic, neuroendocrine and immune-mediated systems. Total body water is distributed between two compartments - rd is intracellular fluid or ICF and rd is extracellular fluid or ECF. Therefore, homoeostatic regulation of fluid balance is impaired. Facilitating an understanding of integrative physiology: Emphasis. Anaesthesia and critical care patients are often fasted and under physiological stress. Fluid balance, when managed accurately and safely, can prevent significant morbidity and mortality. To calculate the intracellular fluid volume subtract the extracellular fluid volume from the total fluid volume.The physiology of fluid balance in humans should be understood and applied in clinical practice. To calculate the interstitial (fluid not in the cells and not in the blood) fluid volume, subtract the plasma volume from the extracellular volume. To measure the extracellular fluid volume, use a cell inpermeant marker substance such as inulin or mannitol that will equilibrate everywhere except in the cells (it is possible to make inulin and mannitol radioactive). One such marker is Evan's Blue, a dye which binds to plasma proteins. Exchange of gases, nutrients, water, and wastes between the three fluid compartments of the body. The ECF is the body’s internal environment and the cells external environment. The remaining one-third of body water is outside cells, in the extracellular fluid compartment (ECF). Therefore, to measure the volume of the blood plasma fluid compartment, you need a marker which equilibrates throughout the blood supply and nowhere else. The intracellular fluid is the fluid within the cells of the body.
The ECF is sub-divided into two other fluid compartments known as. M U is usually calculated from C U, the concentration of marker lost in the urine and V U, the volume of the urine thus: M U = C U. The ECF contains roughly one-third of the total body water or about 20 of total body weight. Where V is the volume of the body fluid compartment, M is the mass of marker injected, M U is the mass of marker lost in the urine during equilibration and C is the measured concentration of the marker. As this is not possible (the kidney will excrete everything dissolved in the bloodstream) the calculation must correct for excretion. To be an absolutely perfect marker, the substance should also not be excreted. Therefore: if you know the mass of marker injected into the body and are able to measure the marker concentration once equilibration is complete, you can calculate the volume of the compartment occupied by the marker. Given that concentration (C) = mass (M) / volume (V) it should be obvious that:
Furthermore, it must be possible to measure the concentration of the marker once equilibration is complete.Tritiated ( 3H) water is a good marker for the whole body fluid compartment because it diffuses throughout the body, it is chemically identical to normal water and it is easy to measure the equilibrium concentration because 3H water is radioactive. To be a perfect marker a substance must also not be metabolised. To measure the volume of any fluid compartment within the body you must inject or infuse a marker substance that will equilibrate (diffuse freely to a uniform concentration) throughout this compartment. Welcome to Module 2 of Introductory Human Physiology We begin our study of the human body with an overview of the basic concepts that underlie the. Body fluid compartments calculations Body Fluid compartments Measuring Body Fluid Compartments
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