We continue in Zevachim chapter 6, Mishnah 3. Although the main topic of the chapter is bird offerings, the Mishnah digresses for a moment: since we have been dealing with services performed on top of the altar, we will focus here on the procedure for ascending it.
The procedure for ascending the altar:
"Kol ha'olin lamizbe'ach, olin derech yamin, umakifin, veyordin derech smol" - anyone who ascends to the top of the altar ascends on the right side of the ramp. For someone standing in the south, at the bottom of the ramp, facing the altar - his right side is the eastern side.
The basis for this is: every turn in the Beis Hamikdash is made to the right. Therefore, the kohen ascends on the eastern side, reaches the corner and turns right, circles the entire altar (counterclockwise), and descends on its other side - the western side. This constitutes a complete circuit, and this is the regular procedure.
The three exceptions:
"Chutz min ha'oleh lishloshah devarim elu, shehayu olin derech smol vechozrin al ha'akev" - except for someone ascending for these three things, who would ascend by way of the left and turn back on his heel - and they are:
The water libation.
The wine libation.
The bird olah offering.
For these three, the kohen ascends by way of the left - the western side - turns on his heel and descends immediately on that very same side, without circling the altar.
The reason for the difference:
These three require perfection and purity, and there is a lot of smoke (and sometimes even ash) on top of the altar:
The water and the wine: We do not want the smoke to enter them, as this would invalidate them.
The bird olah offering: The bird might suffocate from the smoke rising from the altar fire and die, thereby rendering it unfit for an offering.
Therefore, they do not circle the entire altar, as such a circuit would prolong exposure to the smoke; rather, they ascend by way of the left only to the required spot, and descend immediately by that very same way.
In summary: We learned that as a rule, all who ascend the altar go up on the right, circle the altar, and descend on the left. There are three exceptions - the water libation, the wine libation, and the bird burnt offering - where they ascend on the left and turn back on their heel, in order to minimize contact with the smoke that could invalidate the offering.