On the southwestern corner of the Yesod of the Outer Mizbeyach were two round depressions with small holes at the bottom that served as drains. These two drains were situated diagonally opposite each other, with one to the southeast and the other to the northwest of that corner (see illustration). After the zerikah was completed, the blood which remained in the mizrak was poured onto the Yesod. Certain korbanos (including some of the Yom Kippur korbanos) had their leftover blood poured onto the western portion of the Yesod where a small lip along this edge of the Yesod directed the blood into the northwestern drain. Other offerings had their blood poured onto the southern part of the Yesod and this blood flowed into the southeastern drain.
Beneath the southwestern corner of the Yesod was a square hollow called the Shis that was connected via an underground pipe to the nearby Amas Hamayim [stream] running through the Azarah. The term Shis denotes flowing since the blood flowed into it, and from there the blood entered the channel of water and flowed out of the Azarah. Adjacent to the Shis was a removable floor tile, one amah square, with a ring set in it that allowed it to be lifted. By climbing down a ladder into this hole the Kohanim could reach the Shis and clean out any congealed blood blocking the pipe between the Shis and the Amas Hamayim.
The Gemara also mentions, in a separate discussion, the Yam Shel Shlomo, a large copper vessel that rested upon the backs of twelve copper oxen and was used as a mikveh by the Kohanim. Although there is no conceptual connection in this context between the Amas Hamayim and the Yam Shel Shlomo, there was a physical connection: The water from the stream flowed up into the Yam through the hollow feet of the copper oxen, thereby making the Yam into a ma’ayan mikveh.