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A-Level Biology OCR A

Blood Single Circulatory Heart Closed Systems Fish Passes

Single closed circulatory systems involve blood passing through the heart only once per complete circulation. Blood travels through two sets of capillaries before returning to the heart, with the second set facilitating exchange between blood and cells. Fish have an efficient single circulatory system due to countercurrent gas exchange in gills and lower metabolic demands.

Single closed circulatory systems involve blood passing through the heart only once per complete circulation. Blood travels through two sets of capillaries before returning to the heart, with the second set facilitating exchange between blood and cells. Fish have an efficient single circulatory system due to countercurrent gas exchange in gills and lower metabolic demands.

Front Describe the features of single closed circulatory systems using fish as an example.
Back Single closed circulatory system:
  • Blood travels only once through the heart for each complete circulation
  • Blood passes thru 2 sets of capillaries before returning to heart
    • First set - exchanges oxygen and carbon dioxide
    • Second set - substances exchanged between blood and cells in diff organ systems
  • Blood returns to the heart slowly bc it passes thru 2 sets of very narrow vessels the pressure drops considerably - efficiency of exhange processes relatively low
  • Fish have efficient single circulatory system and can be very active bc:
    • Countercurrent gas exchange system in gills maintains steep conc gradient for diffusion of oxygen
    • Body weight supported by water & they do not maintain their own body temp - lower metabolic demands so no need for double circ

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