Water body, physical volume

This section provides a general overview of the properties of lake systems and presents tlie basic tools needed for modeling of lake water quality. The priiiciptil physical features of a lake are length, depth (i.e., water level), area (both of the water surface and of tire drainage area), and volume. The relationship betw een the flow of a lake or reserv oir and the volume is also an important characteristic. The ratio of the volume to the (volumetric) flow represents tlie hydraulic retention time (i.e., the time it would take to empty out the lake or reservoir if all inputs of water to the lake ceased). This retention time is given by the ratio of the water body volume and tire volumetric flow rate. 

In summary, the extent of passive distribution depends on macromolecule binding in plasma and tissue, lipophilicity, and pKa. The extent of distribution can be evaluated by comparing the apparent volume to the physical volume of body water. Distribution is a kinetic process as reflected by the different terms of volume of distribution V., Vdss, and Vp and their ratios. Vdss is a time-averaged parameter that should be used to compare the extent of distribution among lead optimization candidates. It takes a longer time for a compound with large Vdss to be cleared from the body. 

Water and water-soluble compounds are excreted with the urine. The volume and composition of urine are subject to wide variation and depend on food intake, body weight, age, sex, and living conditions such as temperature, humidity, physical activity, and health status. As there is a marked circadian rhythm in urine excretion, the amount of urine and its composition are usually given relative to a 24-hour period. 

Volume of distribution (V ) The apparent volume into which the drug must have been distributed to reach a specific concentration. Many psychotropic drugs have much larger apparent volumes of distribution than would be expected based on physical size of the body, because the drugs dissolve disproportionately more in lipid and protein compartments (i.e., tissue) than in the body s water compartment. 

The volume of distribution (Vd) is a hypothetical volume of fluid into which the drug is disseminated. Although the volume of distribution has no physiological or physical basis, it is sometimes useful to compare the distribution of a drug with the volumes of the water compartments in the body (Figure 1.9). 

If we wish to study the laws of the flow of heat, we must introduce exact measures for the merely psychological conceptions of heat and temperature, which we have been considering hitherto. Our sensation of temperature cannot help us in this, as sensations cannot be compared with one another quantitatively. We cannot perform a measurement of heat or of temperature until we have found some physical property of a body which varies uniquely and continuously with its thermal condition, and which can be measured in time and space by one of the ordinary scientific methods. Fortunately experiment has shown us that there are a great many physical properties of bodies which vary uniquely in this way with their thermal condition, and which may all, therefore, be used for the measurement of heat or of temperature. Nearly all the physical properties of a body are altered when it is heated. One of the easiest properties to measure is the volume. A body occupies more space when it is hot than when it is cold. We say, therefore, that heat causes expansion, and cold causes contraction. The property of expansion on heating is common to all bodies with very few exceptions (one exception is water between 0° and 4°C.), and is generally used as the basis of temperature measurements. 

The body s only other mechanism for restoring Na /H20 homeostasis is ingestion of H2O. Thirst is stimulated by either decreased blood volume or a hyperosmotic condition. It is important to remember that the receptors that influence renal handhng of Na" and H20, and thirst, sense changes only in the intravascular blood volume and not the total ECF. Furthermore, laboratory assessment of water and electrolyte disorders is primarily made from the blood volume (plasma). As discussed in subsequent sections, the clinician must assess the status of TBW and blood volume before interpretation of laboratory values in the diagnosis of water electrolyte disorders. The physical findings and clinical manifestations of these disorders are every bit as important as laboratory values (see Table 46-1). 

In order that the composition of the other body fluids may be kept constant, that of the urine must be able to vary widely. For this reason, even in a normal individual, the composition of the urine may be quite different according to the times and circumstances and, since the composition of samples collected over short periods may differ significantly from the average, a complete 24 hour sample should be analysed in order to obtain reliable values for the output of its various constituents. In the normal adult the volume excreted is of the order of 1000-2000ml per day and 1500 ml is usually taken as the average figure. The quantity depends on the water intake, the external temperature and the diet as well as on the mental and physical state. 

Numbers are a fundamental component of measurements and of the physical properties of materials. However, numbers without units are meaningless. Few quantities do not have units, e.g., specific gravity of a substance is the ratio of the mass of a substance to the mass of an equal volume of water at 4°C. Another unitless quantity is the Reynolds Number, Re = pvllr where p is the density v is the velocity rj is the viscosity of the fluid, and / is the length or diameter of a body or internal breadth of a pipe. The ratio r]lp = /i the kinematic viscosity with units of fit. R = vHp and has no units if the units of v, I, and p are consistent. 

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