Rav Nahman quotes Rabbah bar Avuha as teaching that there are seven different cleansing agents that must be used to remove the stain properly. These seven cleansing agents are those enumerated in a Mishnah in Masechet Niddah (61b), which teaches that only by means of these specific cleansers could blood be removed from clothing, so that it could be rendered ritually pure.
The challenge presented by the Gemara to this ruling is that one of the seven cleansing agents is mei raglayim, and according to a baraita, ein makhnisin mei raglayim la-mikdash – mei raglayim are not permitted within the precincts of the Temple. Ultimately the Gemara concludes that the mei raglayim must be mixed with ordinary saliva so that it could be used.
What is this mei raglayim and why was it not permitted in the Temple?
The literal translation of mei raglayim is “water of the feet” and a simple explanation would be that this is a polite reference to urine. The high acid content of urine makes it a powerful cleansing agent, but we can easily understand why it would be forbidden to use in the Temple. The Shittah Mekubetzet (in Masechet Keritut 6a) suggests that this term refers to a kind of plant, which was forbidden to use in the Temple because of the associations that the name mei raglayim held for people. According to the Kol Bo the reference to mei raglayim is actually to water from one of the springs that flows near the Temple Mount – Ein Rogel. Again, according to this opinion, the water itself is fine; the problem is the association that people made with this name.