The air you breathe into your lungs is 20% oxygen. Your body must have oxygen to produce energy for your cells, and some oxygen combines with carbon to produce carbon dioxide, a waste product that is breathed out. Thus, you need oxygen to nourish and cleanse your cells.
Carbron Dioxide/Oygen Exchange
The oxygen gas molecules enter the alveoli (air sacs) in the lungs. Carbon dioxide leaves the blood in the capillaries and enters the sacs, changing form from solution to gas, while oxygen moves into the capillaries, changing form from gas to solution.
Into the Red Blood Cells
Most of the oxgyen is picked up by the haemoglobin in red blood cells for transport. The blood carries the oxygenated blood through the pulmonary arteries to the heart.
Out to the Body
As the heart beats, the oxygenated blood flows out of the heart through the aorta and into arteries throughout the body.
Into the Tissue
The oxygenated blood flows from large arteries to smaller arterioles and into capillaries that nourish cells of the tissues.
Oxygen/Carbon Dioxide Exchange
Oxygen leaves the capillaries and flows into the cells. At the same time, carbon dioxide leaves the cells and enters the venous capillaries to return to the lungs.
Oxygen provides the fuel that allows the cells to carry out their functions.