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.  

1: most common garden plants prefer loam — soils with a balance of different-sized mineral particles (approximately 40% sand, 40% silt, and 20% clay) with ample organic matter and pore space. however, some plants grow better in sandy soils, while others are well-adapted to clay soils.

2: available water capacity is the maximum amount of plant available water a soil can provide. it is an indicator of a soil's ability to retain water and make it sufficiently available for plant use. available water capacity is the water held in soil between its field capacity and permanent wilting point.

3: it is the same for all living things (plants and animals). it also includes very tiny organisms that our eyes cannot see. they are known as microorganisms, and include viruses, bacteria and fungi.