if sodium and potassium ions form the cations in our body what forms the anions?

sodium and potassium are not present in elemental form . both are present in ionic form in body . for complete chemistry of these ions click on http://charles_w.tripod.com/arthritis4.html.

There are lots of counterions. Chloride and bicarbonate probably top the list but the amino acids in poroteins will also figure.

IIRC, you have a lot of Ca++ cations too.

 

For anions, Cl-, PO3--, and RCOO- are probably the most abundant.