Oxygen has low solubility in body fluids. So, it cannot move to different parts of the body by diffusion.
So humans and many other animals have developed a specific mechanism for this. Here oxygen diffuses quickly in lungs and then transported to every cell by blood.
Lungs contain a large number of a tiny sac-like structure called alveoli and rice supply of blood. Alveoli increase the surface area, and oxygen diffuses from the alveoli to the nearby blood capillaries.
RBC’s of blood contains an iron-containing pigment called Haemoglobin.
Haemoglobin has a high affinity for oxygen. They absorb oxygen from the lungs quickly and carry them to every cell.
Carbon dioxide transportation:
Carbon dioxide is more soluble in water, so it dissolves in blood plasma and exported out at alveoli in the lungs. Some percentage of carbon dioxide forms bicarbonate in RBC’s and are exchanged at the lungs. Haemoglobin also transports it but the percentage is negligible.