Why is it necessary for the respiratory and circulatory systems to work together?

The respiratory and circulatory systems must work together to provide oxygen to the cells.
In the cells a process called cellular respiration takes place. Cellular respiration uses the Kreb’s cycle to produce energy in the form ATP needed for the cell to function. Oxygen is necessary for the Kreb’s cycle to work.
The carries Oxygen to the cells by binding the Oxygen to the hemoglobin on the red blood cells. When the blood reaches the cells Oxygen is released into the cells that are depleted of Oxygen. Carbon Dioxide is taken up by the blood in return.
The blood then returns to the lungs where the Oxygen rich content of the lungs causes Oxygen to flow into the blood.
The is essentially the lungs and diaphragm. The function of the respiratory system is to get Oxygen to the lungs where it can be absorbed by the blood. ( Also to get the Carbon Dioxide out of the blood)
The respiratory system provide the Oxygen that the Circulatory system provides to the cells.