He cells of the act as the heart's pacemaker, which establishes the pace for cardiac activity.

He cells of the sinoatrial (SA) node act as the heart's pacemaker, which establishes the pace for cardiac activity.
The SA node is one of the major elements in the cardiac conduction system. It consists of a cluster of cells that are situated in the upper part of the wall of the right atrium (the right upper chamber of the heart) and is responsible for generation of electrical impulses.

Well, both latitude and intensity of light affect temperature because, the higher the light intensity, the higher the temperature and since latitude is the measure of how close you are to the equator, then the closer to the equator you are, the higher the temperature is.