Why does the electrical impulse that simulates heart contraction first travel down to the bottom of the heart, and then stimulates contraction from the bottom of the ventricles upwards?

This happens to squeeze out blood completely, else the heart may not function properly

Explanation:

The electrical impulses are carried by bundle of His and send to the bottom of the heart via purkyne fibers. This happens in order to spread the impulse from the atrio-ventricular node to the sides of the ventricles. Now in the next round of impulse causes the ventricles to contract and squeeze out all the blood from the ventricles. If this does not happen then the heart will not be able to perform to its full capacity. Atria may also not release its entire blood into the ventricles.  

answer: grassland biomes are closer to the equator and receive more rain.

grasslands are biomes that are dominated by grasses rather shrubs and trees. savanna are biomes that exhibit grasses, shrubs and small tree as vegetation. grasslands are located near the equator of the earth. these are found in north america. savanna biomes are found in africa, south america, australia and india. grassland exhibit moderate climatic conditions, it receives   amount of rainfall higher than savanna.   therefore, savanna exhibit dry and hot climatic conditions.