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 = "<img src='img/loading.gif'>";  
 }  
   
 return url;  
 }  
   
 function mudancaEstado(){  
 if (xmlRequest.readyState == 4){  
 document.getElementById("conteudo_mostrar").innerHTML = xmlRequest.responseText;  
}  
 }