It is the letter "e". The word day has no "e" in it, the word week has two, the word year has one "e" in it, and the word decade has two in it.
Blood types are codes which determine what type of blood you have, such as A, AB, O,etc.
Carbon Dioxide
No, arteries have high pressure as they carry blood from the heart to the rest of the body.
nostrils
Because the arteries carry blood away from the heart.
Particle Pollution
the ball-and-socket joint
medulla
There are many good books on blood types. The book "Blood Types" is a simple one to start with. It lists all blood types and what the different types mean.