# 1.2.5c Circulation of Blood in Humans

1. The circulation of blood in human is due to the pumping of the heart.
2. The cardiac muscles contract to produce heartbeat which pumps the blood to the whole body.

### The Cardiac Cycle

1. The heartbeat is initiated by the sinoatrial node (also called the SAN or pacemaker) .
2. The SAN consists of specialised cardiac muscle fibres that generates nerve impulses.
3. From the SAN, the electrical impulses spread to the atrioventricular node (AVN) located at the base of the right atrium (See image above).
4. Special muscle fibres called His Bundle and the Purkinje fibres then sends the impulses to the ventricles and then stimulates both ventricles to contract simultaneously and pumps the blood out of the heart.

### Contraction of the Skeletal Muscles Around Veins

1. The blood pressure in the vein is low and hence make it hard for the blood to flow back to the heart.
2. Therefore, there are skeletal muscles presence around the veins to help blood circulation.
3. When the skeletal muscles contract, blood is forced to move along the veins.
4. The valves in the veins prevent the back flow of the blood and hence direct the blood back to the heart.