<Table> Figure 1: Intra - and extracellular ion concentrations (mmol / L) <Tr> <Th> Element </Th> <Th> Ion </Th> <Th> Extracellular </Th> <Th> Intracellular </Th> <Th> Ratio </Th> </Tr> <Tr> <Td> Sodium </Td> <Td> Na </Td> <Td> 135 - 145 </Td> <Td> 10 </Td> <Td> 14: 1 </Td> </Tr> <Tr> <Td> Potassium </Td> <Td> </Td> <Td> 3.5 - 5.0 </Td> <Td> 155 </Td> <Td> 1: 30 </Td> </Tr> <Tr> <Td> Chloride </Td> <Td> Cl </Td> <Td> 95 - 110 </Td> <Td> 10 - 20 </Td> <Td> 4: 1 </Td> </Tr> <Tr> <Td> Calcium </Td> <Td> Ca </Td> <Td> </Td> <Td> 10 </Td> <Td> 2 x 10: 1 </Td> </Tr> <Tr> <Td_colspan="5"> Although intracellular Ca content is about 2 mM, most of this is bound or sequestered in intracellular organelles (mitochondria and sarcoplasmic reticulum). </Td> </Tr> </Table> <Tr> <Th> Element </Th> <Th> Ion </Th> <Th> Extracellular </Th> <Th> Intracellular </Th> <Th> Ratio </Th> </Tr> <Tr> <Td> Sodium </Td> <Td> Na </Td> <Td> 135 - 145 </Td> <Td> 10 </Td> <Td> 14: 1 </Td> </Tr> <Tr> <Td> Potassium </Td> <Td> </Td> <Td> 3.5 - 5.0 </Td> <Td> 155 </Td> <Td> 1: 30 </Td> </Tr>

The importance of the plateau phase of the action potential of myocardial cells is in