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USMLE Step 1

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Biochemical Pathways

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Cori Cycle

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Summary

The Cori Cycle, also known as the Lactic Acid Cycle, is a biochemical pathway that is used to manage lactate, which is produced by anaerobic metabolism during muscular activity or in the absence of oxygen (e.g. hypoxemia).

In muscle, glucose is converted into pyruvate through glycolysis, producing ATP in the process. Pyruvate is then converted into lactate by the enzyme lactate dehydrogenase (LDH). Most tissues including muscle have no mechanism to metabolize excess lactate, so they export the lactate to the bloodstream. If left unmanaged, the resulting accumulation can lead to lactic acidosis.

The organ responsible for metabolizing lactate is the liver. The liver contains enzymes necessary for gluconeogenesis, providing a method to convert accumulated lactate and pyruvate into glucose. This glucose can then be exported back out to tissues, which can then use it for additional energy. This full circle of glucose and lactate forms the cycle.

However, the Cori Cycle requires a net loss of 4 ATP to function, so it is not an infinite energy machine. Specifically, 6 ATP are used for gluconeogenesis in the liver, and 2 ATP are yielded by glycolysis in muscle. In other words, the Cori Cycle moves energy from the liver to the muscle, and lactate vice versa, at the expense of 4 ATP.

Key Points

  • Cori Cycle (Lactic Acid Cycle)
    • Enables rapid availability of ATP in muscle (via glycolysis) without lactic acidosis
      • Net loss is 4 molecules of ATP
      • Hypoxic lactic acidosis = massive production of lactate overwhelms liver's ability to compensate
  • In Muscle
    • Glucose → Pyruvate → Lactate
    • Produces lactate and 2 ATP via glycolysis
  • In Liver
    • Lactate → Pyruvate → Glucose
    • Regenerates glucose from lactate, consuming 6 ATP


Find Cori Cycle and more Biochemical Pathways among Pixorize's visual mnemonics for the USMLE Step 1 and NBME Shelf Exams.