function abrirPag(valor){
var url = valor;

xmlRequest.onreadystatechange = mudancaEstado;
xmlRequest.open("GET",url,true);
xmlRequest.send(null);

xmlRequest.open("GET",url,true);
xmlRequest.onreadystatechange = mudancaEstado;
xmlRequest.send(null);

if (xmlRequest.readyState == 1) {
document.getElementById("conteudo_mostrar").innerHTML = "<center><img src='images/loading.gif'>";
}

return url;
}

function mudancaEstado(){
if (xmlRequest.readyState == 4){
document.getElementById("conteudo_mostrar").innerHTML = xmlRequest.responseText;
}
}