The Journey of a Breath of Air

When you breath in, oxygen is drawn in through the nose or mouth and down into the lungs. The oxygen then passes across the thin lining of the capillaries and into the blood. The oxygen molecules are carried to the body cells by the blood. Carbon dioxide from the body cells is carried by the blood to the lungs where it is released into the air.
Breathing is only part of the process of delivering oxygen to where it is needed in the body. Gas exchange occurs in the alveoli by passive diffusion of gases between the alveoli and the blood in the capillaries of the lungs.

Diffusion is the movement of substances from an area of higher concentration to an area of lower concentration. The concentration of O2 in the alveoli is at a higher level than in the blood and the concentration of CO2 in the alveoli is at a lower lever than in the blood. O2 molecules diffuse across the thin walls of the alveoli and capillaries and into the blood. Carbon dioxide (CO2) moves out of the blood and into the alveoli in a similar way
The pulmonary artery carries the blood traveling to the lungs and alveoli. Upon reaching the alveoli the blood picks up the inhaled oxygen and at the same time releases the carbon dioxide that needs to be expelled. The pulmonary veins then carry the oxygenated blood to the heart to be pumped through the aorta and around the body. The oxygenated blood travels to the smaller arteries and finally to the capillaries where gas exchange occurs.
The oxygen molecules move out of the capillaries and into the body cells. It is used by the cells to Cellular respiration, the process of breaking down glucose to release energy .
C6H12O6 + 6O2 → 6H2O + 6 CO2 + ENERGY
The waste products of cellular respiration include carbon dioxide and water. The carbon dioxide molecules move out of the cells and into the capillaries that surround the cells. The carbon dioxide is removed from the body by the lungs.