Pressure is the force exerted on the walls of a vessel. Blood pressure is the force exerted on the walls of blood vessels. It is measured in mm Hg, where atmospheric pressure is 760 mm Hg at sea level. Blood pressure measures how hard it is to cause blood to flow through vessels.
Pulse is felt as a pressure change where an artery is near the surface, such as at the wrist. Systolic pressure is the higher pressure due to contraction of the ventricles. Diastolic pressure is the pressure when the ventricles relax. Pulse pressure is the difference between the two. Systemic blood pressure is ordinarily measured in the brachial artery of the upper arm with a sphygmomanometer. It is written as (systolic pressure)/(diastolic pressure). Normal blood pressure is 120/80.
Two major factors contribute to blood pressure: the volume of blood pumped by the heart and the diameter of the arterioles. Blood pressure increases if the heart pumps more blood or resistance to blood flow (peripheral resistance) increases. Peripheral resistance increases with systemic vasoconstriction (narrowing of the arterioles). On the other hand, vasodilation increases blood flow and decreases blood pressure.
In response to activation of the sympathetic nervous system (“fight-or-flight”), blood pressure increases and blood flow is diverted from most tissues to the heart, liver, and skeletal muscles in order to prepare to defend ourselves or flee.
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