What is the most important vein in the human body?

All veins are important because they carry blood throughout the body and all veins have a specific purpose/job to do to help the body function. Many veins take the deoxygenated blood...blood that doesn't have oxygen in them any more because it was used up...and transport it back to the heart where the heart pumps  the  deoxygenated blood  to the lungs to become oxygenated...oxygen is put back into the blood.

here are two of the many veins in the body:

- pulmonary veins and other veins involved with the heart system/the functioning of the heart

-systemic veins: I believe this also deals with the heart but it can also deal with veins in the upper part of our body (includes: head, neck/throat area and chest area I believe).  We also have veins in the lower part of the body (abdomen (area that contains organs/stomach area), legs, and whatever else falls under the lower part of the body.

I tried to keep this in simple terms from what I learned in biology. For example, within the veins in the abdomen there are more specific names to the veins and their function.

a. as the temperature increases, the kinetic energy of the gas molecules increases & the molecules move faster & further away from each other.

b. if you decrease the temperature of a fixed amount of gas at constant pressure, the combined gas law reduces to charles's law. in this case, volume will also decrease.

organisms that lack symmetry are called asymmetrical and belong to phylum porifera/sponges.

organisms with specialized stinging cells belong to phylum cnidaria. stinging cells are called cnidoblast cells.

cnidarians show two morphological forms : polyp (sessile) and medusa (mobile)

filter feeding is observed in porifera members.  

alternation of generation is observed in coelenterata or cnidaria. there is alternation of asexual polyp form and sexual medusoid form. it is also referred as metagenesis.