The constant flow of blood around the body is called circulation. The heart is the pumping organ of the human body which shows rhythmic contraction and relaxation of muscles. These are known as heartbeats. The heart is made up of cardiac muscles. The contraction of heart chambers is known as systole and relaxation is known as diastole. When all the heart muscles relax, deoxygenated blood from the head and body enters the right auricle through two veins namely - the superior vena cava from the head and the inferior vena cava from the body. At the same time, oxygenated blood from the lungs enters the left auricle through the pulmonary vein. This phase is known as diastole.

One complete rhythmic contraction of the heart followed by its relaxation is called a heartbeat. The throbbing sensation due to flow of blood under pressure in the superficial arteries (at the wrist) is called pulse. The number of beats per minute is called the pulse rate. The normal pulse rate of a resting person is between 72 and 80 heartbeats per minute. The pulse rate increases upto 100 to 110 beats per minute whenever we do a physical activity such as running, jogging, cycling, etc. The pulse can be measured by using the stethoscope.