Respiratory organs of human body are nose, larynx, alveoli, bronchi etc. In human beings, the air is taken into the body through a pair of external nostrils. This air is filtered by fine hairs and mucus that are lining the nasal cavity. From here the air enters into the throat through larynx. Larynx is enlarged part of trachea. It is also called as voice box. Throat consists rings of cartilage, these ensure that the air passage does not collapse.
The trachea or windpipe divides into two bronchi. One bronchi enters into the right lung, another into the left lung. Bronchi divides the bronchioles which are further divided into alveolar ducts that are opened into alveoli.
Alveoli is the exact site of gaseous exchange. It has large surface area. If the surface area of alveoli are spread out, it would cover about 80m2. This large surface area fulfils the requirement of oxygen. The alveoli walls contains an extensive network of blood vessels. During inspiration process, the concentration of O2 in the alveoli is maximum when compared with the concentration of CO2. So that haemoglobin [Hb] binds with O2 and forms oxyhaemoglobin that is carried to the organs.
In the tissues CO2 concentration is maximum when it is compared with O2. So that Hb binds with CO2 and forms carboxyhaemoglobin which is carried to the lungs by the blood.
The body size of the animals is large, so that the diffusion pressure alone cannot take care of oxygen delivery to all parts of the body. Respiratory pigments [such as Hb] helps in the gaseous exchange from the lungs to organs and vice versa.
If O2 moves through diffusion, it will take about 3 years for a molecule of oxygen to reach our toes from the lungs.