Circulation is defined as the to and fro movement of any substance or fluid in a closed system. This function is carried out by a specialized system known as the circulatory system with the help of fluid connective tissue called blood. It consists of the heart, three blood vessels, namely, arteries, veins, and capillaries. It forms a broad network, which carries blood to all parts of the body. Nutrients from food are also transported around the body by the circulatory system. Blood transports nutrients, gases, hormones, etc., to all the parts of the body through blood vessels. The circulatory system also helps to regulate temperature by transporting heat around the body.

The beating of the heart may be felt in the superficial arteries (at the wrist) as a regular jerk, called the pulse. This is due to the continuous pumping of blood by the beating of the heart. The normal pulse rate of an adult human being is about 72 times per minute. The pulse can be measured by using the stethoscope.

The cardiac cycle is the alternating contraction and relaxation of the walls of the heart chamber. Diastole and systole are two phases of the cardiac cycle. The contraction followed by the relaxation regulates the heartbeat. Systole is the part of the cardiac cycle during which muscles of the heart chamber contract after refilling with blood. This contraction expel the blood out of the chamber. The systolic pressure is 120 mmHg.