When you breathe in and out with your lungs throughout the day and night, everything is controlled by a respiratory control center located in your brain stem. This control center constantly monitors the levels of oxygen and carbon dioxide in your bloodstream and adjust your breathing rate to maintain balance and homeostasis in the body.
One of the most important parts of breathing is taking in oxygen from the air surrounding your body and delivering it to your bloodstream. Once the oxygen reaches the bloodstream, the cells of your body begin to absorb it. Oxygen is one of the most important elements in your body because it is crucial for creating energy in the cells. Without the energy obtained from the use of oxygen, your muscles couldn't move, your cells couldn't take in nutrients or get rid of waste and your nervous system would be unable to send impulses throughout your body.
In exchange for the use of oxygen by the cells, a waste product is created from the energy-making process called carbon dioxide. The cells dump this waste product back into the bloodstream so that it can be eliminated from the body. The carbon dioxide is sent back to the lungs, where it is exhaled from the body.
When your body is at rest, your breathing rate remains steady. This allows just the right amount of oxygen to be placed in the blood when you inhale, and just the right amount of carbon dioxide to exit the blood when you exhale. When you perform activities that work your muscles, or other activities that require additional oxygen, the oxygen levels in your blood decrease, and the carbon dioxide levels increase. The respiratory control center of the brain senses that the levels are incorrect and increases both the heart rate and breathing rate to make up the difference. As you stop the activity, the respiratory control center slows the heart and breathing rate back down to maintain homeostasis in the bloodstream.
Breathing by Choice
In most cases, your body controls your breathing automatically, without you having to think about it. In some cases, you do have the ability to control your breathing rate. For example, if you are swimming, you can hold your breath for extended periods of time. However, the brain will only allow the oxygen levels to dip below a certain point before it will override your system and force you to breathe again. This is why those who try to hold their breath for too long pass out.