Ion concentration and haematocrit as determinants of impedance in an erythrocyte suspension model of renal medullary tissue

Abstract

In order to analyse the respective roles of ion concentration and fractional volume of the interstitial compartment as determinants of the impedance, Z, of renal medullary tissue, a model was needed in which both these factors could be varied independently. An array of blood cell suspensions in saline (different haematocrit values and different NaCl concentrations) was used for this purpose. It was found that: (i) up to a measuring frequency of about 10 kHz, the complex consisting of needle electrodes and 'tissue' can be regarded as serially connected resistances, R, and capacitances; (ii) the frequency range 3-10 kHz can be regarded as optimal since it simultaneously assures low electrode polarization and a negligible role of tissue capacitance; (iii) increasing the haematocrit had two consequences-a reduced contribution of polarization impedance to the total impedance measured and a decreased sensitivity of ion concentration measurement from R^-1 (conductance); (iv) passive electrical properties of renal medullary tissue were close to those of a 75% haematocrit cell suspension.