Heart rate and exercise

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We could say that heart rate is determined by the number of times the heart beats in one minute (60sec). However, the respiratory frequency (breathing rate) has to do with the rhythm that occurs in the process of inspiration and expiration (entry and exit of air in the lungs). During a physical activity, the human body increases its energy requirement depending on the intensity of the activity that is being performed. In order to meet this increased energy requirement, respiration is accelerated, so that a greater amount of oxygen (O2) is inhaled. Oxygen is needed in high concentration for the aerobic respiration within the muscle cells. During aerobic respiration, the mitochondria use oxygen to break down glucose into ATP, which is the source of energy. For each molecule of glucose 37 ATP molecules are produced. The waste product produced by this process is six molecules of water and six molecules of carbon dioxide[1]. ATP production occurs in several steps, glycolysis, link reaction and Krebs cycle. The majority of the 37 molecules are produced in Krebs cycle, where citric acid is broken down into oxaloacetate, to be turned back to citrate with the addition of acetyl CoA[2]. Breathing out eliminates the carbon dioxide. If not enough oxygen is provided to the muscle cells during exercise, anaerobic respiration takes place. Anaerobic respiration does not use oxygen; glucose molecules are stored in the muscle tissues as lactic acid. The gaseous exchange takes place in the lungs, through diffusion, which is a passive movement of gas through a one cell thick alveolar-capillary membrane, from a high partial pressure to a low partial pressure, until it stabilizes[3]. Both alveoli and capillaries have one cell thick lining to enable gases and nutrients to diffuse easily. Oxygen molecules diffuse across the membrane into the blood and carbon dioxide goes the opposite direction, diffusing into the alveoli[4].


  1. Des Jardins, T. R. 2013. Cardiopulmonary anatomy and physiology. Clifton Park, NY: Delmar Cengage Learning.
  2. Berg J, Tymoczko J, Stryer L. Biochemistry. New York: W.H. Freeman and Co.; 2002
  3. Evans, J. D. W. and Sutton, P. 2012. Cardiovascular system. Edinburgh: Mosby/Elsevier.
  4. Bbc.co.uk. 2014. BBC - GCSE Bitesize: Effect of exercise on breathing. [online] Available at: http://www.bbc.co.uk/schools/gcsebitesize/science/add_edexcel/organism_energy/respirationrev3.shtml [Accessed: 24 November 2014].
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