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