The muscular system and respiratory system are intertwined in a number of ways not commonly thought of. They are directly related in that an increase in muscular function is accompanied by an increase in respiratory rate and vice versa. Additionally, they rely on each other to perform their functions, which most people take for granted.
Picking up a glass of milk, walking a dog, or even brushing teeth are all actions most people don't consciously think about while performing them. There are a number of complex physiological mechanisms involved in performing these seemingly basic tasks, however. Two key players in physical motion are the muscular system and the respiratory system. The muscular system is involved primarily in movement, whereas the respiratory system allows for the gas exchange necessary to acquire nutrients from and release toxins to the environment.
Gas exchange occurs through inhalation and exhalation, a motion dependent upon pressure changes in the lungs. This pressure alters with expansion and contraction controlled by the diaphragm muscle, located at the base of the lungs. The diaphragm is part of the muscular system, and the faster it contracts and relaxes, the greater the rate of respiration. There are a number of regulatory mechanisms involved in the involuntary changes in breathing frequency.
The muscles of the body may require different nutrient levels based on their individual needs. If a person is sprinting, for example, more oxygen and energy is needed than if that same person is sleeping. The body acquires many of these nutrients through the respiratory system's actions. If there is an increase in need for nutrients, a feedback loop eventually causes an increase in respiratory rate appropriate for the need.
The muscular system and respiratory system could not function without one another. The muscular system relies on the gas exchange made possible by the respiratory system, while the respiratory system could not move gases without the action of the diaphragm, a component of the muscular system. Their interconnectedness is therefore necessary for sustaining life. Most creatures of the universe that live off oxygen see a correlation between these two vital systems.