ON ELECTRICAL CONDUCTANCE; EEG AND EKG 219 



trates this point, showing the pressure changes and the relative total cross- 

 sectional areas. 



Two quantities can be measured, the flow rate (cc/sec), and the speed of 

 flow (cm/sec). Measurements in the aorta show that enough blood flows 

 past a flow-meter detector per second for one complete cycle to require 45 

 min. Insertion into the aorta of a bit of radioactive argon as an inert tracer, 

 and measurement of how long it takes for the tracer to complete the ciruit, 

 confirms this. 



Speed is less easily measured. One method is by tracer. The ultrasonic 

 method (see Chapter 3) introduces no pathological changes, but needs 

 calibration. 



ON ELECTRICAL CONDUCTANCE; EEG AND EKG 



The next rate process to be considered in this chapter is the movement of 

 ions under the influence of an electrical field — in other words, the con- 

 ductance of solutions of salts in water. This subject is basic to an under- 

 standing of the gross current paths through the human body upon which are 

 based the techniques of electrocardiography (EKG) and electroencepalog- 

 raphy (EEG), and also basic to some of the transport processes driven by 

 membrane potentials which are of importance in nerve conduction and elec- 

 trical shock treatment. 



Towards the latter part of the last century the big-three "solution" pio- 

 neers, Kohlrausch, Arrhenius, and Van't Hoff, showed that salts dissolve in 

 water as ions. These are electrically charged and free to move about at 

 random because of thermal energy, but subject to movement in a preferred 

 direction under the force of an electrical voltage gradient. Positive ions are 

 forced to the negative electrode, and negative ions to the positive electrode 

 by the electrical field. The speed of movement, or mobility (centimeters per 

 second under a voltage gradient of 1 v per cm) was understood quantitatively 

 by 1923 (the work of Debye and Hiickel, Onsager, and later others) as being 

 determined by the ease with which a charged ion, complete with "hangers- 

 on" such as electrically charged ions and water molecules, can slip from 

 hole to hole in the liquid. The process is very similar to diffusion, which 

 was described earlier. The difference is that ions are charged and move under a 

 voltage gradient, whereas the diffusing particle may or may not be charged and moves 

 under a concentration gradient . If a potential difference exists for any reason be- 

 tween two parts of an electrolyte, or is applied from the outside, ions move 

 and current flows — in other words, charge is transferred. Hence this is just 

 another transport process. 



Ohm's Law Concerning Current 



\{ n is the number of charge carriers per cc, w their average velocity under 

 the impressed voltage, and q the electrical charge carried by each, then the 



