The electric capacitance of biological tissues serves as the basis for numerous medical technologies. For example, dielectric spectroscopy glucose reading (DSGR) is a noninvasive diagnostic technique that uses the electric capacitance of blood to estimate blood glucose levels.A DSGR device records the electric current that results from applying a voltage source to the skin and underlying blood vessels. DSGR can be modeled by the circuit shown in Figure 1, in which V represents the applied potential, CB represents the capacitance of the blood, and RE, RD, and RB represent the electrical resistances of the epidermis, the dermis, and the blood, respectively.Figure 1 DSGR circuit model including skin and superficial blood vesselThe capacitance of charged blood may be explained by red blood cell membranes acting as physical barriers that separate electrons introduced into the blood from positively charged ions in the red blood cell cytoplasm. The dielectric constant k of red blood cell membranes varies with glucose concentration (Figure 2) because glucose uptake by red blood cells alters the activity of membrane-bound proteins that regulate the flow of ions into and out of the cell.Figure 2 Blood dielectric constant vs blood glucose concentrationDSGR readings vary with blood glucose concentration because the time needed to fully charge the blood is related to total blood capacitance. However, interpreting DSGR readings may be complicated by changes in biological variables other than blood capacitance. For example, the resistivity of blood varies in accordance with osmolarity such that changes in diet or hydration status influence the current measured by DSGR devices.Livshits, L. et al. Dielectric response of biconcave erythrocyte membranes to D- and L-glucose. J. Phys. D: Appl. Phys. 40 (2007), 15–19. Question 12Assuming that the size and shape of red blood cells are uniform, by what factor would the quantity of red blood cells need to change to raise the capacitance of a blood sample to 250% of its original value?A.2/5B.(2/5)2C.5/2D.(5/2)2
Question
The electric capacitance of biological tissues serves as the basis for numerous medical technologies. For example, dielectric spectroscopy glucose reading (DSGR) is a noninvasive diagnostic technique that uses the electric capacitance of blood to estimate blood glucose levels.A DSGR device records the electric current that results from applying a voltage source to the skin and underlying blood vessels. DSGR can be modeled by the circuit shown in Figure 1, in which V represents the applied potential, CB represents the capacitance of the blood, and RE, RD, and RB represent the electrical resistances of the epidermis, the dermis, and the blood, respectively.Figure 1 DSGR circuit model including skin and superficial blood vesselThe capacitance of charged blood may be explained by red blood cell membranes acting as physical barriers that separate electrons introduced into the blood from positively charged ions in the red blood cell cytoplasm. The dielectric constant k of red blood cell membranes varies with glucose concentration (Figure 2) because glucose uptake by red blood cells alters the activity of membrane-bound proteins that regulate the flow of ions into and out of the cell.Figure 2 Blood dielectric constant vs blood glucose concentrationDSGR readings vary with blood glucose concentration because the time needed to fully charge the blood is related to total blood capacitance. However, interpreting DSGR readings may be complicated by changes in biological variables other than blood capacitance. For example, the resistivity of blood varies in accordance with osmolarity such that changes in diet or hydration status influence the current measured by DSGR devices.Livshits, L. et al. Dielectric response of biconcave erythrocyte membranes to D- and L-glucose. J. Phys. D: Appl. Phys. 40 (2007), 15–19. Question 12Assuming that the size and shape of red blood cells are uniform, by what factor would the quantity of red blood cells need to change to raise the capacitance of a blood sample to 250% of its original value?A.2/5B.(2/5)2C.5/2D.(5/2)2
Solution
The capacitance of a capacitor is given by the formula C = εA/d, where ε is the permittivity of the material, A is the area of one of the plates, and d is the distance between the plates. In the case of red blood cells, we can consider each cell as a capacitor, where the cell membrane acts as the plates and the cytoplasm acts as the dielectric material.
Assuming the size and shape of the red blood cells are uniform, the area A and the distance d are constant. Therefore, the capacitance of each cell is constant.
To increase the total capacitance of the blood to 250% of its original value, we need to increase the number of capacitors (red blood cells) to 250% of the original number.
Therefore, the quantity of red blood cells needs to change by a factor of 2.5 or 5/2. So, the correct answer is C. 5/2.
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