What's the process of gas transport in the human body?

Gas transport in humans commences with the process of respiration. This process has two phases, the inspiratory phase in which air is inhaled into the lungs and an expiratory phase during which gas is expelled from the lungs.

Inspiration starts with the expansion of the lungs brought about by the contraction of the diaphragm and the intercostal muscles. This creates a negative pressure in the lungs which causes air from the atmosphere to enter the lungs through the nose.

The air passes into the wind pipe (trachea) through the larynx. At its lower end, the trachea divides into two pipes called the bronchi, each bronchus supplying its air content to each of the two lobes of the lungs where exchange of gases actually takes place.

To facilitate gaseous exchange in the lungs, each bronchus further divides into thinner tubes called bronchioles. The bronchioles empty their gas content into grape-shaped air sacs called alveoli which are covered by blood capillaries. This is the point at which exchange of gases actually takes place in the lungs.

Oxygen is extracted from the air in the alveoli and passes into the blood in the capillaries while carbon dioxide extracted from the blood in the capillaries passes and mixes with alveoli air for expulsion through the second phase of respiration called expiration.

At expiration, the diaphragm and the intercostal muscles relax, decreasing the volume of the chest cavity and increasing the air pressure in the lungs. This pushes air in the lungs into the two bronchi, through the trachea and finally taking its exit through the nose.

In the human body, only about 1.5% of the oxygen which crosses the alveolar membrane in the lungs gets transported through the body by mixing directly with blood. The remaining 98.5% of the oxygen is transported bound to a protein in the blood called hemoglobin.

On the other hand, carbon dioxide is transported by three methods. As carbon dioxide is more soluble in water, about 5 to 7% of it dissolves directly in blood. About 10% of the carbon dioxide is transported bound to hemoglobin but the majority of the carbon dioxide is transported through the bicarbonate buffer system in which an enzyme, carbonic anhydrase, converts the carbon dioxide into carbonic acid inside the red blood cells.