Pulmonary capillaries stem from the pulmonary artery. The pulmonary capillaries are tiny blood vessels that are found within the walls the alveoli of the lungs.
Gas exchange occurs between the alveoli sacs and these small capillaries. It is here that oxygen is delivered from the lungs to the bloodstream. The oxygen attaches to the hemoglobin of red blood cells. The red blood cells then transport the oxygen to cells. The oxygen is used by the mitochondria of the vertebrate eukaryotic cells in the process that is known as cellular respiration. During cellular respiration, oxygen gas and the sugar glucose are converted into water, carbon dioxide gas, and an energy source called adenosine triphosphate (ATP). The blood that circulates within the bloodstream picks up the carbon dioxide generated via cellular respiration. The carbon dioxide from the bloodstream is then delivered back to the alveoli of the respiratory system in order to be eliminated from the body.