What makes heart muscles contract, thereby regulating heart rate?

This is the diveristy of ecosystems natural communites and habits it is the variety of ways that species interact with each other and their environment

Quick asap will give brainiest ! what best describes the same pattern of tides on earth throughout the day? neap tides spring tides semidiurnal tides nocturnal tides

Why are scientists concerned about the depletion of the ozone levels in the stratosphere?