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Every time you breathe in, oxygen from the air you inhale passes through the thin walls of the alveoli into the surrounding capillaries, where red blood cells pick it up using a protein called hemoglobin .From there, it travels through the pulmonary artery to the lungs, where it flows from the capillaries back into the alveoli in exchange for the incoming oxygen.When you breathe out, or exhale, your diaphragm and rib muscles relax, reducing the space in the chest cavity.At the same time, carbon dioxide, the waste gas carried back to the lungs from the cells of the body, trades places with the oxygen, moving from the blood in the capillaries back into the alveoli.The carbon dioxide, once in the bloodstream, travels back to the heart, where it enters the right side.From the alveoli, the carbon dioxide is breathed back out.As the chest cavity gets smaller, your lungs deflate, similar to how air releases from a balloon.At the same time, carbon dioxide-rich air flows out of your lungs through the windpipe and then out of your nose or mouth.

Every time you breathe in, oxygen from the air you inhale passes through the thin walls of the alveoli into the surrounding capillaries, where red blood cells pick it up using a protein called hemoglobin . At the same time, carbon dioxide, the waste gas carried back to the lungs from the cells of the body, trades places with the oxygen, moving from the blood in the capillaries back into the alveoli.
Blood loaded up with oxygen-rich red blood cells travels to the left side of the heart through the pulmonary veins. The heart then pumps the oxygenated blood to the rest of the body, where it moves from your blood vessels to your cells.

The cells need this oxygen to make the energy your body needs to work. When cells make that energy, they create the waste product carbon dioxide. That carbon dioxide has to be removed from the blood and the body, which is why it is pushed from the cells back to the blood.
The carbon dioxide, once in the bloodstream, travels back to the heart, where it enters the right side. From there, it travels through the pulmonary artery to the lungs, where it flows from the capillaries back into the alveoli in exchange for the incoming oxygen. From the alveoli, the carbon dioxide is breathed back out.
When you breathe out, or exhale, your diaphragm and rib muscles relax, reducing the space in the chest cavity. As the chest cavity gets smaller, your lungs deflate, similar to how air releases from a balloon.
At the same time, carbon dioxide-rich air flows out of your lungs through the windpipe and then out of your nose or mouth.
Breathing out requires no effort from your body unless you have a lung disease or are doing physical activity. When you are physically active, your abdominal muscles contract and push your diaphragm against your lungs even more than usual. This rapidly pushes air out of your lungs.
Your breathing changes depending on how active you are and the condition of the air around you. For example, you need to breathe more often when you do physical activity. At times, you can control your breathing pattern, such as when you hold your breath or sing