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='images/carregando.gif'>";
                }

        return url;
}

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