The first and foremost duty of Aztec Priests was to nourish Tonalli and keep the world moving, the Gods fed and the Sun lit. Priests also sought to win favor with the gods by fasts and sacrifice. One of the most important functions of the priests was to determine which days would be lucky for engaging in activities such as war and baptism. A religious calendar of 260 days provided this information. The dates of ceremonies to honor the gods were determined by a solar calendar of 365 days. Variants of both calendars were developed by earlier Mesoamerican civilizations such as the Olmec, Maya, and Zapotec. The meshing of the two calendars produced a 52-year cycle, at the end of which the Aztec would let their hearth fires go out. To begin the next cycle, they would hold the important “new fire ceremony,” in which priests lit a sacred fire in the chest cavity of a sacrificial victim, and the people rekindled their hearth fires and began feasting.

Tonalli (much paraphrased from this site)

The important element in relation to human sacrifice, very briefly, is that the universe was thought to run on an energy called tonalli, "animating spirit". This word comes from tona, "to make heat or sun". It nourished the gods, and kept the sun moving. Throughout Aztec religion there is a great emphasis on motion, and motion is driven by tonalli. In humans it is located in the blood stream. When a man is frightened, it concentrates in the heart. The gods' thirst for it is perpetual. It drives the universe. Without it, everything stops.

As well, religious motivation is inadequate to explain the degree to which human sacrifice was practiced. Quetzalcoatl didn't think it was necessary at all. Religious necessity can be served symbolically, or else a single human sacrifice can be imbued with very great significance, as in the case of Roman Catholicism. Even one sacrifice to the bloodiest goddess in the Hindu pantheon, Kali, would keep the goddess happy for a thousand years (Kalika Purana, in Campbell, 1962; pg. 6). This would not prevent her worshippers from attempting to make her very happy, but even at their most enthusiastic, Kali looked like a tea granny compared to Huitzilopochtli.

There's very compelling evidence that the religious sacrifices, and the attendant ritual cannibalism, were actually economically motivated. Tenochtitlan was not self sufficient. Its population could not be supported by even the most copious yields of its technologically advanced intensive agriculture. Tribute from neighbouring states was required to make up the difference. When Aztec demands for tribute were greatest, there is evidence that reconquest was necessary for states that had had enough.

For my own practices involving Tonalli, I certainly wouldn't be involved with human sacrifice or even that of animals. However, the concept of an animating force that drives the world and requires nourishment is very central to Aztec religious thought but there are many ways this could be accomplished including menstrual blood, flower buds, seeds, and others. My thanks to the Pagan Forum for several of these ideas.