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During inspiration, contraction of the diaphragm muscle, cause the normal dome-shaped to flatten increasing the chest cavity volume, this increase in volume creates negative pressure between the air in the lungs and that in the atmosphere. This is very much like a vacuum effect and the negative pressure literally sucks air into the lungs until the two pressures are balanced.

During expiration, diaphragm muscles relax returning upwards to a dome shape decreasing the chest cavity volume this creates a positive pressure which pushes some of the air out of the lungs.

Costal breathing is a shallow pattern of breathing through the chest and involves the contraction of the external intercostal muscles.

Diaphragmatic breathing is a deeper method of breathing through the outward distension of the abdomen and involving the contraction and lowering of the diaphragm. Diaphragmatic breathing is promoted to aid relaxation in activities like yoga and is linked to improved health. Expansion of the rib cage provides an additional increase in chest cavity size, this should only be required during times of laboured breathing such as moderate or high-intensity aerobic exercise.

Oxygen is pulled down the Bronchi and bronchioles into the alveoli by negative pressure, but also because it flows down a concentration gradient.

Diffusion is the movement of gas, from an area of high concentration to an area of low concentration. The concentration of oxygen decreases between the mouth and the lungs, therefore the gas flows in this direction.

Carbon dioxide flows in the opposite direction for the same reason. Once the oxygen gets into the alveoli, also called the air sacs, it will continue to follow this concentration gradient and will diffuse into the bloodstream. The alveoli have minute capillaries running over and around them. Both the alveolar walls and capillary walls are so thin that they allow gases to pass through them, oxygen passes into the bloodstream at the same time as carbon dioxide passes back into the lungs to be exhaled.

The oxygen binds to the haemoglobin in the red blood cells at the same time carbon dioxide disassociates from the haemoglobin and defuses from the blood into the lungs. The red blood cells are then pumped into the blood via the pulmonary vein towards the heart.