Chapter 1 Physical Properties of the Body Fluids1.1 Body FluidsThe proportion of body weight composed of water decreases from birth to old age, with thegreatest decrease occurring the first 10 years of life. As shown in Table 1.1-1, adult femaleshave less water content in their body since they have more subcutaneous (under the skin)adipose (fat) tissue. Because the water content of adipose tissue is relatively low, the fractionof the body’s weight composed of water decreases as the amount of adipose tissue increases. Table 1.1-1 Approximate water content in humanType of person % of body weightInfantsAdult malesAdult females756050Many of the transport phenomena we study will either involve or affect the fluids within thehuman body. Therefore, we will focus our attention on the characteristics of the two majorfluid types: intracellular and extracellular fluids. The third fluid type is the transcellular fluidsuch as the cerebrospinal, sweat, and digestive fluids that are found only within specializedcompartments. The intracellular fluid includes all the fluid in the several trillion cells of thebody. The intracellular fluid from all cells has a similar composition, and it is the largestfraction of the fluid volume or about 36% of the body weight. The extracellular fluid includesall of the fluid outside the cells except the transcellular fluid. The extracellular fluid consistsof the interstitial fluid that occupies the extracelllular spaces outside the fluid vessel, and theplasma that occupies the extracellular space within the blood vessel. Table 1.1-2 shows thefluid content of an average 70-kg male.Table 1.1-2 Body fluidsTotal Intracellular ExtracellularInterstitial PlasmaFluid volume 40 liters 25 liters 12 liters 3 liters% of Body weight 57 wt % 36 wt % 17 wt % 4 wt %Interstitial fluid circulates within the spaces between cells. It is formed as a filtrate from theplasma within the capillaries as shown graphically in Figure 1.1-1. Water diffuses from onefluid to another, and small molecules and ions are either transported or diffuse freely betweenthem. Large molecules like proteins are much more restricted in their movement because ofthe permeability characteristics of the membranes that separate the different fluids.The blood volume is about 5 liters, with 3 liters of plasma and 2 liters representing thevolume of the cells that are in the blood, primarily the red blood cells. The fraction of theblood volume due to the red blood cells is called the hematocrit. 1E x t r a c e l l u l a r f l u i dp l a s m ai n t e r s t i t i a l f l u i dI n t r a c e l l u l a rf l u i dC a p i l l a r yC e l lFigure 1.1-1 The extracellular and the intracellular fluids.In the blood circulation, contraction of the left ventricle ejects blood through the aortic valveinto the aorta, the largest artery in the body, whereupon the blood divides as it moves throughmany generations of junctions passing through progressively smaller arteries and arteriolesthat have diameters in the range of 5 to 50 µm and a length of a few centimeters. Eacharteriole empties into 10 to 100 capillaries, which have porous walls and are the sites of theexchange between the blood and interstitial tissue fluid. These are shown in Figure 1.1-2.There are about 1010 capillaries in peripheral tissue. This cascading of blood vessels results ina large increase in the total flow cross section Au, as listed in Table 1.1-3 along with thecross-sectional area, the time-area averaged blood velocity <uf>, and the average Reynoldsnumber.Table 1.1-3 Cross-sectional area (total) and time-area averaged blood velocity throughvarious segments of blood pathways.Au, cm2<uf>, cm/s Reynolds numberaortasmall arteriesarteriolescapillariesvenulessmall veinsvena cava2.520402,500250808334.12.10.0330.331.01030005000.70.0020.011501000The return circuit is through the venules, the veins, the vena cava, and finally into the rightatrium. The deoxygenated blood is then ejected through the tricuspid valve into the rightventricle, then pumped into the pulmonary arteries, ultimately reaching the pulmonarycapillaries surrounding the aveoli in the lung where it is reoxygenated, returning then to theleft atrium. Ejection through the mitral valve into the left ventricle completes one cycle of theblood circulatory system (systemic circulation).Even though it is more convenient to discuss blood flow through the heart one side at a time,it is important to understand that both atria contract at about the same time and bothventricles contract at about the same time.2Figure 1.1-2 Blood supply to tissue by arterioles feeding the capillaries.While blood has about the same density as water, it is about three times more viscous,primarily the result of the large fraction (40 volume %) of suspended red blood cells. Exceptthrough the aorta, the average Reynolds numbers are sufficiently low for the flow to belaminar throughout the circulation. The flow in the arteriole and capillaries has very lowReynolds number. The mean hydrostatic pressure falls from a relatively high value of 100mmHg in the largest arteries to values of the order of 20 mmHg in the capillaries and evenlower in the return venous circulation as shown in Figure 1.1-3. The arteries and arteriolesmust have relatively thick-walled, whereas the capillaries, venules, and veins can be, and are,thin-walled.Figure 1.1-3 Pressure variation in the systemic circulation.3Table 1.1-4 lists the compositions of the body fluids in concentration units of m illiosmolar(mOsmole/liter of solution). One osmole is defined as one mole of a nondiffusing andnondissociating substance. Therefore one mole of a dissociating substance such as NaCl isequivalent to two osmoles. About 80% of the total osmolarity of the interstitial fluid andplasma is produced by sodium and chloride ions. The composition of these two fluids is verysimilar since the interstitial fluid arises from the filtration of plasma through the capillaries.The extracellular and intracellular fluids have almost the same osmotic pressure. Changes inwater volume in body fluids alter the osmolality of body fluids, blood pressure, andinterstitial fluid pressure. Table 1.1-4 Composition of the extracellular and intracellular fluidsSolute Plasma Interstitial IntracellularNa+K+Ca++Mg++ClHCO3HPO4, H2PO4SO4PhosphocreatineCarnosineAmino