The Human Circulatory System (WJEC GCSE Science (Double Award): Biology): Revision Note

The Flow of Blood around the Body

• The circulatory system consists of a closed network of blood vessels connected to the heart

  • Oxygenated blood is carried away from the heart and towards organs in arteries

  • These narrow to capillaries as they pass through the organ

  • In the organs, respiring cells use oxygen from the blood

  • The capillaries widen into veins as they move away from the organs

  • Veins carry deoxygenated blood back towards the heart

• Details of the four main arteries and veins that enter and leave the heart are as follows:

Main blood vessels of the circulatory system table

Double Circulatory Systems

• The circulatory system is a double circulatory system

  • This means that the blood passes through the heart twice on each full circuit of the body

• The circulatory system has two 'loops' or systems of blood vessels

• One system carries deoxygenated blood from the heart to the lungs, where it becomes oxygenated and then carries the blood back to the heart

  • This system is called pulmonary circulation

• The second system carries oxygenated blood from the heart to the respiring body tissues, where it delivers the oxygen to the cells for respiration, and then carries the deoxygenated blood back to the heart

  • This system is called systemic circulation 

Circulatory system diagram

The circulatory system showing the pulmonary and systemic systems