nginx as reverse proxy for many domains (many ips in proxypass?)

[conf at kraja.net]

Hi,
I'm trying to set up nginx as a reverse proxy for several apache2 server.
Nginx server should act as a cache / proxy for any URL that you send - so
some sort of global VirtualHost. There is no problem.
But I do not know how to configure nginx to not only one IP address as the
ProxyPass, but to automatically use the IP site (Apache2 web server - but
it is different for each domain).

It is possible to do this somehow?

Thanks so much for the help.

Michal