Single and double circulation
BiologyHuman & Animal PhysiologyAges 16–17
Use it with your class: students open it from a link or a QR code, with no account needed.
Next in Human & Animal Physiology
Compare the single circulatory system of fish side by side with the double circulatory system of mammals: blood flows along its real path, with blood pressure shown at each segment. See clearly why single circulation delivers blood to tissues at low pressure, while a four-chambered heart lets mammals keep oxygen-rich blood separate and pump it out at high pressure.
Lesson: Respiration and circulation in animals
What it shows
In single circulation, as in fish, blood passes through the heart once per circuit: a two-chambered heart pumps it through the gill capillaries and then on to the body, so much of its pressure is already lost before it reaches the tissues. In double circulation, as in mammals, blood returns to the heart after the lungs and is pumped out a second time. A four-chambered heart keeps oxygen-rich and oxygen-poor blood apart and sends blood to the organs at high pressure, while the pulmonary circuit stays at low pressure to protect the delicate lung capillaries.
How to use
Choose a system under Circulatory system and follow the O₂-rich and O₂-poor blood around the diagram. Turn on the Pressure graph to see where pressure falls, especially at each capillary bed. Switch between Single circulation (fish) and Double circulation (mammals) and compare the pressure with which blood reaches the organs. Then change Heart rate and watch how quickly blood moves around the circuit.
Parameters you can change
- Circulatory system Single circulation (fish), Double circulation (mammals)
- Heart rate 30–180 beats/min
- Show blood pressure graph
Questions to explore
- Why is blood pressure already low when blood reaches the body tissues of a fish?
- What advantage does a four-chambered heart give an animal with high energy demands?
- Why is the pressure in the pulmonary circuit lower than in the systemic circuit?
