What is internal respiration?

The last stage of the journey is for the oxygen to get from the haemoglobin in the red cells in your blood to all the other cells in your body.

The left side of the heart pumps the blood through your aorta, and then through your arteries and arterioles, and finally through the systemic capillaries. Capillaries are the smallest blood vessels in your body, some are in the lungs where oxygen is picked up, the others (the systemic capillaries) are in the rest of your body where oxygen is delivered. There is a capillary within a short distance of almost every cell in your body.

In the capillaries oxygen detaches from the haemoglobin and diffuses through the capillary wall to the cells around the capillary.

Oxygen diffusing from the blood into the cells and carbon dioxide diffusing from the cells into the blood are together known as 'internal respiration'.