Bear and bigcats anatomy

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Circulatory system of tiger,

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Heart rate of tiger ranges from 56-97 beats per minute. Their cardio vascular system is similar to humans, they contain heart and blood vessels. 
Bear,

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The increased amount of valves in heart allow more fluid flow to the body of bear as they are on four legs.Heart rate:
• The normal heart rate of bears is 60 - 90 beats per minute (the higher rates are found in cubs). (B64.26.w5)
• In summer 40-50 bpm. (D243)
• The heart rate is 40-50 bpm (resting rate), reduced to 8-12 bpm during hibernation. (B490.26.w26, D284.w5)
  

During hibernation

• During winter dormancy, 8-10 bpm. (D243)
• The heart rate decreases markedly during hibernation. (B147)
• During hibernation the heart rate may be as low as 10 beats per minute. (B406.36.w36)
  Such a slow beat during hibernation causes blood to pool in the heart's chambers. In a human, the increased pressure would cause the chambers to stretch out. The dilated muscle would be weaker and less efficient, leading ultimately to congestive heart failure.https://www.eurekalert.org/pub_releases/2011-02/uocp-aco020711.php"Bears are able to avoid this," Rourke saidNelson and Robbins had previously shown that, during hibernation, the muscle of a bear's left ventricle stiffens to prevent it from stretching as blood accumulates. But the stiffening of the ventricle presents another problem. The left atrium, which pushes blood into the left ventricle, must then work against greater resistance.The atrium is pushing against a brick wall," Rourke said. "We thought there must be some kind of mechanism to keep the atrial muscle from wearing itself out."Muscle contractions in the heart are controlled by a protein called myosin heavy chain. The protein comes in two varieties, alpha and beta. The alpha version produces a quicker but slightly weaker contraction compared to the beta. "We found that the muscle in the left atrium produces more alpha protein during hibernation, which results in a slightly weaker beat," Rourke said. "The lower force keeps the atrium from being damaged as it pushes against the stiffer ventricle." When the bears emerge from hibernation, the protein ratio switches back and the atrial contraction returns to its original strength.
  

Source: http://www.press.uchicago.edu/pressRelea...earts.html
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