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Cori cycle process
Did You Know?
The Cori cycle is named after physicians Carl and Gerty Cori (married couple), who first mapped it in 1929.
To understand how the human body functions, it is essential to analyze the numerous smaller processes that take place within it. These dependent and independent processes work together in tandem, allowing us to live and perform all our daily activities.

The Cori cycle is one such important process that helps the human body produce the energy required by our muscles when performing a strenuous activity. The following is a description of the working and significance of the Cori cycle, starting with a discussion on how the energy required by our muscles is produced.
Energy Production for Muscle Activity
Running muscle anatomy man
The muscles in our body enable us to perform all our daily activities, including walking, standing, running, lifting weights, etc. They produce an amount of force which is directly proportional to the intensity of the activity that is being performed. For the generation of this force, energy is needed.
For muscular activity, ATP (adenosine triphosphate) is required, which is produced in a process known as glycogenolysis. Glycogenolysis breaks down glycogen, which is stored in the skeletal muscles releasing glucose.
For most of our daily activities, our muscles combine glucose and oxygen aerobically, in a process known as glycolysis, which results in the production of two units of ATP and two units of pyruvate. ATP is directly used for energy generation, and when enough oxygen is available, pyruvate too is further broken down aerobically for generating more energy. Thus, these two metabolic compounds provide energy at the cellular level to the muscles, allowing them to function.
However, when we perform a highly strenuous muscular activity, the amount of oxygen intake becomes disproportional (much less) to the energy requirement of the muscles. In such a scenario, since oxygen is insufficient, glucose is broken down through an anaerobic metabolism process known as fermentation, wherein, the pyruvate is converted to lactate - a soluble milk acid, and then secreted into the bloodstream.

This allows the chemical process responsible for energy generation to continue without the use of oxygen. In this manner, the muscle cells can produce energy anaerobically in this at very high rates, but only for about one to three minutes, after which lactate accumulation in the bloodstream becomes excessive, which leads to fatigue.
What is the Cori Cycle?
Cori cycle
If a strenuous activity continues, the body adopts an alternate metabolic route to get rid of the lactate, and keep producing energy anaerobically. This process of energy production is known as the Cori cycle.
In the Cori cycle, lactate accumulated in the muscle cells is taken up by the liver. The liver performs a chemical process known as gluconeogenesis, to convert lactate back to glucose.

Essentially, gluconeogenesis reverses both the processes of glycolysis and fermentation that the body had performed to produce lactate. This first converts lactate to pyruvate, and then finally into glucose.
This glucose is then introduced into the bloodstream, which carries it to the working muscles, where it is used to feed the additional energy demands of the muscles. The subsequent lactate production by the muscles is again taken up by the liver, and thus the Cori cycle resumes.

In case the muscular activity ceases, the glucose generated in the Cori cycle undergoes glycogenesis to replenish the glycogen stored in the muscles.
Limitations of the Cori Cycle
Using the Cori cycle, the human body is able to convert metabolic by products into a source of energy for the muscles. However, it cannot continue to do so infinitely.
Similar to many other natural cycles, the Cori cycle isn't a completely closed loop. In the muscles, glycolysis results in the production of two units of ATP. However, the liver uses up six units of ATP to carry out the process of gluconeogenesis. The Cori cycle also requires the initial introduction of oxygen, without which it cannot begin. As such, eventually, the muscles are bound to require a new supply of glucose as well as oxygen.

If a physical activity is too strenuous, the energy requirements of the muscles will exceed the capacity of the Cori cycle to regenerate glucose from lactate. This will result in a condition known as lactic acidosis, which is an accumulation of excess lactic acid in the system. Lactic acidosis brings down the pH level of the blood, which can lead to tissue damage. It also induces symptoms associated with panic, such as hyperventilation, abdominal cramps, vomiting, etc., all of which are the body's natural defense mechanisms designed to slow down the rigorous activity, and prevent permanent damage from occurring.