Gas exchange in lungs takes place through the little sacs of air called alveoli (singular is alveolus). There are about 480 million alveoli located at the end of the windpipes. When we breathe in, alveoli expand to take in oxygen and when we breathe out, alveoli contract to expel out carbondioxide.
How do alveoli work:
The three-step-process of breathing at the level of alveoli involve:
The millions of tiny alveoli provides increased surface area for the absorption of oxygen and release of carbondioxide, as the blood passes through them. The blood moves like a conveyer belt in the capillaries located in the walls of the alveoli. Oxygen is taken in and carbondioxide is released as the blood moves. The total surface area provided by the tiny alveoli for this exchange amounts to a whooping 100 square meters.
But this amount of surface area is the minimal requirement for taking in huge amounts of air, from which oxygen is selectively taken in. The lungs take in about 5 to 8 liters of air per minute, but alveoli is able to absorb only 0.3 liters of oxygen.
The muscles of the chest and abdomen contract to create pressure to aid inhalation or exhalation. The diaphragm also plays an important role in breathing. During inhalation, the diaphragm and the muscles help create a negative pressure inside the chest so that air can be sucked in. During exhalation, the same muscles recoil along with the lungs to return to their normal size.
The respiratory system:
Where does the blood come from:
Blood comes from various parts of the body to the heart and is pumped into the lungs via the pulmonary artery. The blood that comes to the heart is devoid of oxygen and rich in carbondioxide. Once the oxygen is taken by the blood, it flows through the pulmonary vein to the right side of the artery. From here the blood is pumped to all parts of the body.
The oxygen in blood is taken away by the various tissues and oxygen for making energy and other purposes. In return, carbondioxide is transferred into the blood. This carbondioxide-rich blood then comes to the left side of the artery from where it is pumped to the lungs for replenishing oxygen and getting rid of carbondioxide.