function abrirPag(valor){  
   var url = valor;  
      
   xmlRequest.open("GET",url,true);  
   xmlRequest.onreadystatechange = mudancaEstado;  
   xmlRequest.send(null);  
      
   if (xmlRequest.readyState == 1) {  
   document.getElementById("principal").innerHTML = "<img src='../Scripts/loader.gif'>";  
   }  
     
   return url;  
   }  
    
   function mudancaEstado(){  
   if (xmlRequest.readyState == 4){  
   document.getElementById("principal").innerHTML = xmlRequest.responseText;  
}  
}