Pressure is the physical quantity of the continuous environment equal quantitatively to force pressing per unit area perpendicular to a surface, and the surface can be located in any plane of space. Pressure is atmospheric and blood.
The concept of atmospheric pressure is applicable to the weight of air which it presses on the adjoining surface. The lower layers of air which are at the earth with a huge force press on people, animals and other living organisms. But this pressure imperceptibly because it is compensated by the internal pressure of air. At the height more than 3 thousand meters air is less saturated with oxygen, becomes rarefied, and pressure in an upper atmosphere (envelope of air around the Earth) becomes weaker. The person at this height can have a rupture of blood vessels as the internal pressure of air of the person never changes. Normal atmospheric pressure is 760 millimeters of mercury. Depending on temperature and humidity of air atmospheric pressure can change. Damp warm air mass (cyclone) lowers pressure, and dry, perhaps cold – raises (anti-cyclone). Force with which presses blood on walls of blood vessels in all human body is called blood pressure. It best of all characterizes work of a blood system. Arterial blood pressure is the easiest measured. In different arteries pressure is different. It depends on an artery location concerning heart: the closer to heart, the pressure is higher. Normal arterial blood pressure at measurement by a tonometer has two limits: systolic pressure (top value) and diastolic (lower value). Systolic arterial blood pressure depends at most reductions of heart at the moment when it contracts and pushes out blood in an artery. And diastolic arterial blood pressure shows pressure in arteries at the moment when the cardiac muscle is relaxed. Normal value of arterial blood pressure for the healthy person of 120/80 millimeters of mercury. The increased arterial blood pressure shows as far as liquid pressure in vessels exceeds atmospheric pressure.