Double circulation separates pulmonary and systemic blood flow, allowing mammals to oxygenate blood at low lung pressure and deliver it systemically at high pressure.
Coverage 2016–2025 · Updated 15 Jul 2026
Bony fish have a single circulation, whereas mammals have a double circulation with separate pulmonary and systemic circuits.
Fish route: heart → gills → body tissues → heart. Blood passes through the heart once per complete circuit and pressure falls as it passes the gill capillaries before reaching the body.
Mammal route: right heart → lungs → left heart → body → right heart. Passing through the heart twice allows low pulmonary pressure that protects lung capillaries and high systemic pressure for rapid delivery, while the septum prevents mixing.
A red blood cell in a mammal is re-pressurized by the left ventricle after leaving lung capillaries; in a fish it travels directly from gills to body tissues without returning to the heart first.
Single and double refer to how many times blood passes through the heart per full circuit, not to the number of chambers or whether the animal has one or two hearts.
This objective is assessed through structured response, commonly using Identify / Label / Explain.
Identify / Label / Explain
Build the answer around this relationship: Fish have single circulation, while mammals have pulmonary and systemic circuits.
Describing double circulation without explaining why blood must be pumped twice.
Representative question
Explain how circulation of the blood to the lungs and to other systems is separated in humans and what the advantages of this separation are.
HL transport adds pressure and route systems. Tissue fluid forms by capillary pressure and returns by osmotic pull or lymph. Double circulation separates pulmonary and systemic routes. The heart creates directional pressure with chambers, septum, valves, and cycle timing. Plants add root pressure and phloem pressure-flow translocation.
