Syllabus Edition

First teaching 2023

First exams 2025

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Circulatory Systems (HL) (HL IB Biology)

Revision Note

Marlene

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Marlene

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Comparing Single & Double Circulation

Differences between single circulation in bony fish and double circulation in mammals

  • Both fish and mammals have a closed circulatory system
    • This means blood is contained within a system of blood vessels
  • Bony fish have single circulation
    • Blood moves through the heart once during each complete circuit
    • The heart has two chambers
    • It enters the heart from the body before being pumped to the gills
    • Here blood is oxygenated before moving to the rest of the body

Single circulation diagram

Fish circulatory system

Bony fish have single circulation

  • Mammals have double circulation
    • This means blood flows through the heart twice for each complete circuit of the body
    • The heart has four chambers; two on the right and two on the left
    • The right side of the heart pumps deoxygenated blood to the lungs for gas exchange; this is pulmonary circulation
    • Blood then returns to the left side of the heart, so that oxygenated blood can be pumped at high pressure around the body; this is systemic circulation

Double circulation diagram

The double circulatory system in mammals

Mammals have double circulation

  • The main advantages of the mammalian double circulation system include:
    • Keeping oxygenated and deoxygenated blood separate, so that cells can receive blood with high oxygen levels for aerobic respiration
    • Maintaining a high pressure for the transport of oxygenated blood to every part of the body
    • Pumping blood at a lower pressure in the lungs so that delicate blood vessels are not damaged

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Marlene

Author: Marlene

Expertise: Biology

Marlene graduated from Stellenbosch University, South Africa, in 2002 with a degree in Biodiversity and Ecology. After completing a PGCE (Postgraduate certificate in education) in 2003 she taught high school Biology for over 10 years at various schools across South Africa before returning to Stellenbosch University in 2014 to obtain an Honours degree in Biological Sciences. With over 16 years of teaching experience, of which the past 3 years were spent teaching IGCSE and A level Biology, Marlene is passionate about Biology and making it more approachable to her students.