Human Breathe

As we inhale, the diaphragm contracts and flattens downwards. The intercostal muscles also contract and this lifts the ribs upwards and outwards. This results in a larger chest volume. As the volume increases, the pressure becomes less so that the surrounding air is drawn in through the nose since the outside pressure is higher.

The oxygen in the air then passes through the membrane of the alveoli into the blood. Carbon dioxide from the blood passes into the alveoli. The diaphragm relaxes and moves up. The rib muscles also relax. These actions create a smaller chest volume. The higher pressure resulted from these movements forces the air containing carbon dioxide out of the lungs into the bronchioles, trachea and nose.