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Case Study: Cardiorespiratory System
Noah Mayeda
Liberty University
EXSC 510: Advanced Exercise Physiology
Dr. Kilian
September 4, 2022
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Part of what makes athletes great is their ability to produce energy. Specifically, the way
they produce their energy. Apart from focusing on the basics, improving their form, techniques,
and strategies, athletes must also push past fatigue. Part of pushing past fatigue is training their
anaerobic system and/or their aerobic system, depending on their sport. Many sports utilize only
one type of energy system, such as, football, weightlifting, and high intensity interval training all
build the anaerobic system. Track and Field is one sport that is different because it brings athletes
who specialize in all energy systems together, and typically the athlete who can create the most
energy can generate more power or speed to win the competition.
Our bodies use a molecule known as, adenosine triphosphate (ATP), as energy. When
ATP is used, a phosphate molecule is broken off to form adenosine diphosphate (ADP). This new
molecule, ADP, is useless until a new phosphate bonds to reform ATP. While ATP is stored in
muscle cells, they can only store limited amounts, “Therefore, because muscular exercise
requires a constant supply of ATP to provide the energy needed for contraction, metabolic
pathways must exist in the cell with the capability to produce ATP rapidly” (Powers, 57). Muscle
cells create ATP with a combination of three pathways. The first one breaks down
phosphocreatine (PC), next is the utilization of glucose or glycogen, and lastly is the oxidative
formation of ATP in the absence of oxygen.
The first energy system that is called upon is the ATP-PC system, also known as the
phosphagen system. This is the easiest and quickest way of creating energy. It only lasts for
about 5 seconds, which would be a 50-meter sprint in track and field. The ATP-PC system works
by speeding up the replacement of a phosphate group to create ADP into ATP. This is done by
phosphocreatine donating a phosphate to ADP. The ATP-PC system is short lived because
“muscle cells store only small amounts of PC, and thus the total amount of ATP that can be
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formed via this reaction is limited” (Powers, 58). It is believed that this system can be slightly
prolonged by supplementing creatine to fill muscle stores.
For a sprinter who finishes the 100-meter race in exactly 10 seconds, they would use the
ATP-PC system for the first 5 seconds then transfer to the second pathway called “glycolysis”.
Comparing this to a longer race, the 800-meter, glycolysis would be the primary method of ATP
production. Glycolysis supports high intensity activity for one to three minutes. Glycolysis
begins with glucose and two ATP molecules and ends with four ATP molecules, two NADH
molecules, and two pyruvate molecules. As a result, glycolysis has a net gain of two or three ATP
molecules. Glycolysis begins with the energy investment stage. This is where 2 ATP molecules
are used in the phosphorylation of glucose to produce an activated glucose molecule. The next
stage of glycolysis is the “energy generation” stage. This stage has a series of chemical reactions
ending with two or three molecules of ATP. The amount of ATP produced depends on if glucose
or glycogen is used. “If glycolysis begins with glycogen as the substrate, the addition of only one
ATP is required. That is, glycogen does not require phosphorylation by ATP” (Powers, 60). This
has to do with the energy investment stage where glucose requires two ATP to be primed in the
beginning of glycolysis. With glycogen, inorganic phosphates phosphorylate the molecule which
saves one ATP in the process and creating an end product of three ATP molecules.
While those two races create ATP through anaerobic processes, longer races such as the
10k will trigger aerobic production of ATP. After the ATP-PC cycle has exhausted itself, and after
glycolysis has been consumed, the aerobic metabolic pathways of the citric acid cycle and the
electron transport chain will be the primary producers of ATP. These processes are slower, but
more efficient than anaerobic pathways, as well as being able to use carbohydrates, fats, and
protein as fuel. These two pathways take place in the mitochondria and work together to create
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32 molecules of ATP per glucose. The citric acid cycle uses two 3 carbon pyruvic acid molecules
that was formed from glycolysis. They both enter the mitochondria and lose a carbon and forms a
2-carbon acetyl coenzyme A. It then combines with 4-carbon oxaloacetic acid to corm 6 carbon
citric acid. Each citric acid undergoes a series of reactions creating 1 ATP and high energy
electrons. The citric acid cycle also “completes the oxidation of carbohydrates, fats, or proteins;
produces CO2; and supplies electrons to be passed through the electron transport chain to
provide the energy for the aerobic production of ATP” (Powers, 65). The electron transport chain,
also called oxidative phosphorylation is the last step to aerobic production of ATP. The transport
chain works by removing electrons from hydrogen atoms and are “passed down a series of
electron carriers known as cytochromes. During this passage of electrons… enough energy is
released to rephosphorylate ADP to form ATP” (Powers, 66). Oxygen is required in this process
to accept electrons at the end of the chain. As a result, the biproducts of aerobic metabolism are
32 ATP molecules, heat, carbon dioxide, and water.
Our bodies progress through each pathway as needed and will continue to produce energy
through that pathway as well as proceeding to the next pathway. For example, the 800-meter
runner will almost entirely rely on the ATP-PC system for the first 5 seconds. After that, they will
transition to primarily glycolysis, but the ATP-PC system will still contribute in small amounts.
As with a 10k runner, after going through ATP-PC then glycolysis, their body finds a need for
more ATP and relies on aerobic pathways. While they are still receiving energy from anaerobic
pathways, majority of their ATP will come from aerobic metabolism.
-Noah Mayeda
