There is no tracer available so ICF is measured indirectly as the difference between concurrently measured totalīody water and ECF. Two individual measurements and can be significant. Measurement error is the sum of the errors of the
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ISF is determined indirectly as theĭifference between concurrently measured ECF & plasma volumes. There is no tracer which are distributed only throughout this compartment. 2.2.6 Other Major Compartments Interstitial Fluid Large vein haematocrit is higher then in arteries because the various reactions in the red cell due to carbonĭioxide transport lead to an increase in the number of particles intracellularly and an osmotic increase in waterĪccounting for these effects, the whole body haematocrit can be estimated as about 91% of large vein haematocritĪnd this value should be substituted in the equations.(Haematocrit in muscle capillaries is typically only 0.20 !) Blood from capillaries has a lower haematocrit then in larger vessels because of axial streaming of redĬells.Remains trapped with the red cells in the tube Haematocrit measured in the laboratory overestimates true haemotocrit because about 4 to 8% of the plasma.Blood Volume = Red cell vol x (100/Hct)Īs mentioned previously, there are several problems in estimating an average or 'whole body' haematocrit:.Blood Volume = Plasma volume x (100/100-Hct).Plasma volume or red cell volume can be determined indirectly if the blood volume and haematocrit (Hct) are Usual therefore to measure the amount of the label in a red cell sample and therefore to directly measure the Of the label is not uniform because the haematocrit is different in different parts of the circulation. As the radioactive label distributes throughout the whole intravascularĬompartment, the measured VD is the blood volume (rather than the red cell volume). The volume of distribution (VD) isĭetermined after about 30 minutes. Red cells are centrifuged, resuspended in saline and infused into the patient. With a sodium chromate solution, the label is taken up by the red cells and any excess in the solution is removed by dilution and centrifuging with removal of fluid. Typically a 10 ml sample of the subject's blood is incubated The tracer is the patient's own red cells which are tagged with radio-chromium (Cr 51-red cells). As the concentration of the tracer is determined in a plasma sample, the measured volume of distribution is The volume is determined via the dilution principle using this concentration at zero Extrapolation back to zero time allows estimation of the virtual concentrationĪt this time. Line when plotted on a logarithmic scale.
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This is a first order process (ie exponential decline) which gives a straight
#Body fluid compartments calculations serial
This problem is overcome by using serial measurements and plotting theĭisappearance curve of the label. Distribution is rapid but no equilibrium is reached because of continuousĭisappearance of albumin from the vascular space. The tracers used are the azo dye known as Evan’s blue (or T1824) which binds avidly to albumin, or radio-iodine This compartment and this is achieved by using a tracer which binds to albumin. Measurement of plasma volume requires a tracer which is mostly limited to The water of dense connective tissue and bone and some of the transcellular fluid) a slowly equilibrating pool (24 hours) which makes up 15% of total body water.a rapidly equilibrating pool ("functional ECF") which makes up about 27 to 30% of total body water (This rapid pool represents.Measurements indicate that the ECF can be modelled as consisting of: What is measured is not the ‘true’ ECF so it is conventional to refer to the compartment measured not as ECF but as a spaceĭefined by the tracer used and the equilibration time (eg ‘20 hour bromide space’). They do not enter cells but the lack of fullĮCF distribution results in a low estimate of ECF. The crystalloids are larger and less diffusable throughout the ECF. The ionic tracers are small and distribute throughout the ECF but there is some entry into cells. Ionics (eg 82Br, 35SO4, chloride isotopes).Throughout all compartments occurs during a 3 to 4 hour equilibration period. It is a weak beta emitter making it easy to measure in a liquid scintillation counter. This is estimated by measuring the volume of distribution of isotopes of water. The volume of distribution can be determined by extrapolation back to zero time.
#Body fluid compartments calculations series
The tracer is metabolised, a series of measurements can be made and assuming exponential decline (first order kinetics),
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If the tracer is excreted in the urine, then the loss can be determined and corrections made in the calculation.