12 5 The structure of a frog heart is different from those of mammals and fish. The diagram shows the structure of a frog heart. The arrows show the direction of blood flow. A B (a) (i) Name the parts of the heart labelled A and B. (2) A................................................................................................................................................................................................................................................................................ B................................................................................................................................................................................................................................................................................. (ii) Compare and contrast the structure of the frog heart with the structure of the mammalian heart. 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Turn over 13 (b) Amphibians, such as the frog: • absorb oxygen through their lungs and through their moist skin • have a lower metabolic rate than mammals • have a double circulation system. (i) State one difference between the double circulation system of the frog and that of the mammal. (1) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (ii) Explain how the circulatory system of the frog is adapted for the metabolism and gas exchange of the frog. (4) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... 14 (c) Describe the sequence of stimulation that occurs in a mammalian heart in one cardiac cycle. 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(Total for Question 5 = 14 marks)