When you inhale, it’s as a result of you having contracted and flattened your diaphragm, the dome shaped sheet of muscle attached to your lower ribs, which forms a muscular barrier between your lungs and your abdominal contents and organs. When you flatten your diaphragm, the pressure inside your lungs drops, pulling air in.
Air is made of approximately 78% nitrogen, 21% oxygen and 1% carbon dioxide. It flows in, ideally through your nose, where it gets humidified thanks to your moist nasal passages (an important reason to drink enough water!) and then down through your trachea (windpipe). At the bottom of your trachea, the passage splits into two bronchi, left and right, the tubes that take the air into your left and right lungs — and from there, the passages divide further and further, like branches and twigs on a tree, until the air reaches the alveoli, tiny balloons where you exchange new air for old, or more accurately, oxygen for carbon dioxide.
The membranes that form each alveolus are only one cell thick, and each alveolus is surrounded by a lot of tiny blood vessels called capillaries. When the air reaches the alveolus, the oxygen passes through the membrane and is collected by the red blood cells in the capillaries.
I like to think of the circulatory system as being the body’s bus network. The terminus is at the alveoli, which is where the red blood cells (buses) drop off some passengers (carbon dioxide molecules which the body has used up — effectively, exhausted workers on their way home) and collect others (oxygen molecules fresh and ready for a day’s work forming adenosine triphosphate — ATP — the main energy source needed for cellular activity). The red blood cells then travel around the body via the vessels, with regular stops to drop off the oxygen wherever it’s needed (organs, muscles etc) and to collect the carbon dioxide which is formed when the oxygen has been used up. As long as your heart and lungs keep pumping, your body will work to maintain a useful number of fresh “workers” to help you do whatever activities you want to do.
However, your lungs are large, and not all parts are created equal. The alveoli at the bottom of your lungs (nearest to your diaphragm) are more compliant (stretchy) than the alveoli at the top, so they are better at exchanging gas than the alveoli near the top of your lungs (nearer to your shoulders). There’s also more blood pressure at the bottom of your lungs, due in part to the weight of your lungs resting on the blood vessels, so there’s more blood flow (buses) available at the terminus. This is one reason we encourage you to try to breathe from your diaphragm more: it’s more efficient than breathing from your shoulders.
To work out if you’re breathing more from your diaphragm or from your shoulders, rest one hand on the upper part of your abdomen. As you breathe in, your abdomen should push outwards, and your hand should move forwards. If this isn’t happening, watch yourself breathing in the mirror. If you’re breathing from the diaphragm, your shoulders shouldn’t be moving up and down.
If you need to work on breathing from the diaphragm, placing your hand on your abdomen and trying to ensure it’s being pushed out as you breathe is a good exercise. Bear in mind that this may take some time to get used to if you’re really out of practice though: the diaphragm is a muscle like any other, and building strength and endurance takes time and training.