What passes between alveoli and capillaries?

Capillaries are tiny thin-walled blood vessels that connect the smallest arteries and veins and allow the exchange of nutrients and waste between the blood and tissues. Oxygen and carbon dioxide are exchanged between the circulatory and respiratory systems; oxygen diffuses from the lungs into the bloodstream and is transported by erythrocytes (red blood cells) throughout the body. Carbon dioxide diffuses out of the blood into the alveoli and goes the reverse direction through the respiratory system and out the nose.