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