Blood is called a connective tissue As it helps in the trasport of essential substances to the cells and carries waste substances fro there. It connects the body systems, transports oxygen and nutrients to all the parts of the body, and removes the waste products. Blood also has an extra-cellular matrix called plasma, with red blood cells, white blood cells, and platelets floating in it.

In the human heart, the contraction is initiated by a special modified heart muscle known as sinoatrial node or the SA-node. It is located in the right atrium. The SA node has the inherent power of generating a wave of contraction and controlling the heart beat. Hence, it is known as the pacemaker. Since the heart beat is initiated by the SA node and the impulse of contraction originates in the heart itself, the human heart is termed myogenic. The hearts of vertebrates and molluscs are also myogenic.

Sino-auricular node is called pacemaker, because nerve impulse or wave of contraction starts from it and goes to both auricles. Since the SA node initiates and maintains the rhythmicity of the heart and maintains the pace of the heart beat, it is known as the natural pacemaker of the human body.