Describe the relationship between altitude and density and pressure. also describe the relationship between altitude and temperature

The greater the altitude the less pressure there is on gas, when there is less pressure the gas is of lower density. As you descend towards the surface of the earth, the column of air above you grows more massive and as such the pressure this column exerts on you becomes greater and of course anytime pressure increases you should expect density to do so as well.

Blood will come back to the heart through the veins(smaller veins coming together turn into the inferior vena cava then go to the right atrium).