AQA AS Biology

Revision Notes

3.5.1 Mammalian Circulation

Blood Circulation in Mammals

The need for a circulatory system

  • All organisms need to exchange materials with their environments
  • Small animals with large surface area to volume ratios (or relatively inactive animals like jellyfish) can rely on diffusion alone to exchange oxygen, carbon dioxide and nutrients with their environment
  • Larger animals have smaller surface area to volume ratios, so diffusion alone is insufficient for exchange of materials between cells further from the surface of the organism with the environment
  • Circulatory systems are systems which carry around fluids containing materials needed by the organism, as well as waste materials that need to be removed

Open & closed systems

  • Circulatory systems are either described as being open or closed
  • In a closed circulatory system, blood is pumped around the body and is always contained within a network of blood vessels
    • All vertebrates and many invertebrates have closed circulatory systems
  • In an open circulatory system, blood is not contained within blood vessels but is pumped directly into body cavities
    • Organisms such as arthropods and molluscs have open circulatory systems.
  • Humans have a closed double circulatory system: in one complete circuit of the body, blood passes through the heart (the pump) twice
  • The right side of the heart pumps blood deoxygenated blood to the lungs for gas exchange; this is the pulmonary circulatory system
  • Blood then returns to the left side of the heart, so that oxygenated blood can be pumped efficiently (at high pressure) around the body; this is the systemic circulatory system

The double circulatory system in mammals, IGCSE & GCSE Biology revision notes

The double circulatory system in mammals

Main Circulatory System Structures Table

1. Closed Double Circulatory System Table, downloadable AS & A Level Biology revision notes

The main blood vessels

  • Pulmonary artery – carries deoxygenated blood away from the heart, towards the lungs
  • Pulmonary vein – carries oxygenated blood away from the lungs, towards the heart
  • Coronary arteries – supply the heart with oxygenated blood
  • Aorta – carries oxygenated blood out of the heart and to the rest of the body
  • Vena cava – carries deoxygenated blood into the heart
  • Renal artery – supplies the kidneys with oxygenated blood
  • Renal vein – carries deoxygenated blood away from the kidneys, towards the heart

Exam Tip

The oxygenation of blood in the pulmonary artery and vein is unique as the pulmonary vein is the only vein in the circulatory system that contains oxygenated blood!

Author: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.

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