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.  

americans began to notice growing differences between the northern and southern sections of the country. there were more people living in cities in the north, which made it more urban. more people lived on farms in the south, which made it more rural. while many people were farmers in the northern states, there were now other ways to make a living, such as working in mechanical cotton gins factories manufacturing cloth, tools and other goods. people in the north supported high tariffs on goods imported from europe because the extra cost encouraged people to “buy american” instead. in the southern states, planters did not like tariffs because they sold most of their production to europe and bought their tools from there as well. southerners believed the protective tariff was something that only northern factory owners keep their profits high. so the southern politicians came up with an idea. they would opt out. they would simply ignore the protective tariff. it was ‘null and void’ in south carolina. the state would not collect the taxes on goods imported into south carolina, no matter what the president said they had to do. this decision led to the nullification crisis.

a. the production of carbon dioxide through the process of photosynthesis