Blood circulates within the body with the help of blood vessels. Blood vessels form a closed transport system called the vascular system.

As the heart beats, it propels blood into the larger arteries. As the large arteries branch, blood moves into successively smaller and smaller arteries and then into the arterioles, which feed the capillary beds that supply blood to the various cells in different parts of the tissues. Capillary beds are drained by venules, which in turn empty into veins that merge and finally empty into the great veins entering the heart.

The main difference that you should understand between arteries and veins is that:  arteries carry blood away from the heart, and veins, drain the tissues and return the blood to the heart

Blood pressure (BP) is defined as the pressure the blood exerts against any unit area of the blood vessel walls, and it is generally measured in the arteries. Because the heart alternately contracts and relaxes, the resulting rhythmic flow of blood into the arteries causes the blood pressure to rise and fall during each beat.

Thus you must take two blood pressure readings: the systolic pressure, which is the pressure in the arteries at the peak of ventricular contraction, and the diastolic pressure, which reflects the pressure during ventricular relaxation.

Blood pressures are reported in millimeters of mercury (mm Hg), with the systolic pressure appearing first; 120/80 translates to 120 over 80, or systolic pressure of 120 mm Hg and a diastolic pressure of 80 mm Hg. Normal blood pressure varies considerably from one person to another.

The sphygmomanometer, commonly called a blood pressure cuff, is an instrument used to obtain blood pressure readings by the auscultatory method. It consists of an inflatable cuff with an attached pressure gauge. The cuff is placed around the arm and inflated to a pressure higher than systolic pressure to occlude circulation to the forearm. As cuff pressure is gradually released, the examiner listens with a stethoscope for characteristic sounds called the sounds of Korotkoff, which indicate the resumption of blood flow into the forearm.

As the pressure in the cuff is the same as the pressure produced by the heart, some blood will be able to pass through the upper arm when the pressure in the artery rises during systole. This blood flows in spurts as the pressure in the artery rises above the pressure in the cuff, resulting in turbulence that results in an audible sound. As the pressure in the cuff is allowed to fall further, thumping sounds continue to be heard as long as the pressure in the cuff is between the systolic and diastolic pressure. 

Eventually, as the pressure in the cuff drops further, the sounds change in quality, then become muted, then disappear altogether when the pressure in the cuff drops below the diastolic pressure.