I answered a very similiar question to this a miniute ago, and this is an expansion on Marku2019s comment about blood pressure:For something to stay in its shape, it seem obvious that is must have equal outside and inside pressure.The atmospheric pressure is around 760 mm of HG, and the pressure of blood is around 120/80 mm of HG.But the measurement device of blood pressure is also open to the atmosphere so the pressure is actually 760+120 which is 880 mm of HG.Still the blood pressure is higher inside the body than outside.Although if you think of fizzy drinks or hosepipes, the pressure on the inside is far greater than that of the outside.So the pressuure of the human body is slightly greater than the pressure of the atmosphere but not enough to have any major impact.This is one of my first posts, and I am only 14 so please be kind.