The alveoli are surrounded by tiny blood vessels, called capillaries. The alveoli and capillaries both have very thin walls, which allow the oxygen to pass from the alveoli to the blood. The capillaries then connect to larger blood vessels, called veins, which bring the oxygenated blood from the lungs to the heart. The largest veins that do this work are called the pulmonary veins , and they connect directly to the heart. Sometimes we use the terms breathing and respiration to mean the same thing, but they actually are distinct processes.
Breathing is the process of moving oxygen-rich air into and out of the lungs.
Respiration refers to how the cells of the body use oxygen to create energy and how they exhale carbon dioxide that is a waste product of this process. The lungs have to work continuously because the body cells are constantly using up oxygen and producing carbon dioxide. Unlike the heart, the lungs have no muscle tissue. Instead, muscles in the rib cage and the diaphragm do all the work of lifting the ribs upward and outward to let the air in, and then relaxing to force the air out. All cells of the body need energy to do their work.