function abrirPag(valor){ 
var url = valor;
xmlRequest.open("GET", url, true);  
xmlRequest.onreadystatechange = mudancaEstado; 

xmlRequest.send(null); 
if (xmlRequest.readyState == 1) { 
document.getElementById("pagina").innerHTML = "<img src='ajax_loader.gif'>"; 
} 
return url; 
} 
function mudancaEstado(){ 
if (xmlRequest.readyState == 4){ 
document.getElementById("pagina").innerHTML = xmlRequest.responseText; 
} 
}