Let's start with the main reason we exhale. When we exhale the air we breathe out contains carbon dioxide that we are getting rid of. Carbon dioxide is a waste product of cellular respiration, the process by which our cells convert energy from nutrients into usable energy in the form of ATP. The carbon dioxide that is produce during cellular respiration leaves the cells and enters the blood stream by diffusion. In the lungs, the carbon dioxide leaves the blood stream and enters the alveoli by diffusion. Once in the alveoli, the carbon dioxide moves through airways that get progressively larger until we breathe it out during exhalation.
The process of exhalation occurs mainly from the action of the diaphragm. The diaphragm is a dome-shaped muscle that separates our thorax from our abdomen. Because it is a muscle, it can contract and relax. When it contracts, the muscle gets shorter which makes it flatter. This provides more space in the thorax and causes the lungs to expand for inhalation. For exhalation, the diaphragm relaxes and gets longer. This takes up more space in the thorax and puts pressure on the lungs causing the air to leave. For forced exhalation, such as blowing out birthday candles, other muscles such as abdominals and intercostal muscles between the ribs get involved to provide more pressure on the lungs and more exhalation.
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