- "receives" oxygen from the lungs and transports it to all living human cells (because oxygen combines with respiratory dye - hemoglobin, which is in the blood)
- transports CO2 (carbon dioxide) to the lungs
otherwise:
thanks to blood, there is gas exchange:
a) external (occurs in the lungs) - blood donates CO2 and draws oxygen
b) internal (occurs in tissues and cells - blood donates oxygen and takes away CO2