A closed circulatory system
The mammalian circulatory system is a mass transport system that carries substances such as oxygen and glucose around the body.
Mammals have a closed circulatory system. Blood remains within vessels as it travels around the body rather than being pumped directly into body cavities.
Two linked circuits
Mammalian circulation is also double: during one complete journey around the circulation, blood passes through the heart twice.
Pulmonary circulation
The right side of the heart pumps deoxygenated blood to the lungs.
Gas exchange occurs in the lungs and oxygenated blood then returns to the left side of the heart.
Systemic circulation
The left side of the heart pumps oxygenated blood around the body.
After supplying body tissues, deoxygenated blood returns to the right side of the heart.
Together, the pulmonary and systemic circulations form one continuous route connecting the heart, lungs and body tissues.
Exam Tip: To explain why mammalian circulation is described as double, state that blood passes through the heart twice in one complete circulation: once as part of the pulmonary circuit and once as part of the systemic circuit.