1. The Rib Cage Is a Bony Cage, Not a Solid Plate
The thoracic cage, or rib cage, is made of 12 pairs of ribs (24 in all), the breastbone (sternum) at the front, and 12 thoracic vertebrae of the backbone behind. The ribs are anchored to those vertebrae at the back, and the cage protects the heart and lungs [4].
Notice the word "pairs." Each side of the body has 12 ribs, which is why the total is 24 and not 12.
If protection were the only job, a single solid plate of bone would seem sturdier. Yet the chest is built as a cage of separate curved bars. As the next section shows, the cage has a second job: it changes shape every time you breathe.
2. Why Can't the Lungs Stretch Themselves?
Here is the puzzle. When you breathe in, your chest swells and air rushes into the lungs. But the MSD Manual states plainly that the lungs have no skeletal muscles (the kind that move your body) of their own. The work of breathing is done by the muscles around them: the diaphragm, the muscles between the ribs, and muscles in the neck and abdomen [1].
The diaphragm is a dome-shaped sheet of muscle that separates the chest from the abdomen. It is the most important muscle for breathing in. When it contracts, it moves down and makes the chest cavity longer and wider, which expands the lungs. The muscles between the ribs and in the neck help move the rib cage [1].
A simple way to picture it: the lungs are like a balloon inside a space that gets bigger when the ribs move outward and the diaphragm moves down. The balloon does not blow itself up; the space gets bigger, and air flows in to fill the extra room. This is only a picture to help you think about it, since real lungs are not empty balloons.
3. Does Breathing Out Take Work Too?
Not always. The MSD Manual notes that abdominal muscles are only sometimes involved in breathing out [1]. For quiet breathing at rest, the usual textbook account is that the stretched lungs and chest wall spring back on their own, so the effort is mostly in breathing in. When you run or blow out a candle, extra muscles do join in, so "breathing out is effortless" is true only for calm breathing.
You do not have to think about any of this. Breathing is usually automatic, controlled without thought by the respiratory center at the base of the brain, and it continues during sleep. You can still take control when you want, for example to speak, sing or hold your breath. For adults at rest, the usual range is about 12 to 20 breaths a minute [1].
4. What Is the Heart Doing Between the Lungs?
The heart is a hollow muscular organ in the center and left side of the chest. It has a right side and a left side, and each side has an upper chamber (atrium) and a lower chamber (ventricle). The heart pumps blood to the lungs to pick up oxygen, then pumps the oxygen-rich blood out to the rest of the body [2].
Valves keep the blood moving one way. Each ventricle has an inlet valve and an outlet valve, and their flaps open and close like one-way swinging doors [2]. Without the lungs the heart would have nothing to refill with oxygen. Without the heart the lungs could not send that oxygen anywhere. The two organs need each other.
5. Where Do Air and Blood Actually Meet?
Deep in the lungs, inhaled air reaches tiny air sacs called alveoli. The cells lining the alveoli and the thin blood vessels around them (capillaries) are each only one cell thick and sit in very close contact. The barrier between air and blood averages about 1 micron, which is 1/1000 of a millimeter [3].
Oxygen passes quickly through that barrier into the blood. In the other direction, carbon dioxide, a waste gas, passes from the blood into the alveoli and leaves when you breathe out [3]. Blood that picked up oxygen travels to the left side of the heart, which sends it around the body. Blood low in oxygen returns to the right side of the heart, which pumps it back to the lungs [3].
The amounts are bigger than you might guess. Even at rest, about 5 to 8 liters of air move in and out of the lungs each minute, and about 0.3 liter of oxygen moves into the blood. During hard exercise, a person can move more than 100 liters of air per minute [3].
6. How Do the Three Share the Work in One Small Space?
Putting it together: the rib cage and its muscles change the size of the chest to move air in and out; the alveoli let oxygen and carbon dioxide cross between air and blood; and the heart keeps the blood circulating through the lungs and the body. Protecting the organs, moving air and moving blood are three separate jobs done in one space. This is the reason the chest is built as a cage that can move, not a plate that cannot.
7. Try Feeling the Cage Move With Your Own Hands
Place both hands on the sides of your lower ribs. Breathe in slowly. Your hands should move outward. Then breathe out gently without pushing, and notice how the cage settles back. Next, find your pulse with two fingers on your wrist and count the beats for one minute. Pulse changes after running or when you feel nervous, so one count cannot tell you how healthy you are. This is only a game for noticing your body.
If you want to read more, search for the MSD Manual Consumer Version pages "Control of Breathing" and "Exchanging Oxygen and Carbon Dioxide," or the OpenStax chapter on the thoracic cage.