Capillarie
Gas exchange is the delivery of oxygen from the lungs to the bloodstream, and the elimination of carbon dioxide from the bloodstream to the lungs. It occurs in the lungs between the alveoli and a network of tiny blood vessels called capillaries, which are located in the walls of the alveoli. The walls of the alveoli actually share a membrane with the capillaries in which oxygen and carbon dioxide move freely between the respiratory system and the bloodstream. The alveoli in the lungs are coated with a moist film that dissolves air in the lungs. Oxygen diffuses across the thin epithelium of the alveoli sacs into the blood within the surrounding capillaries. Carbon dioxide also diffuses from the blood in the capillaries to the alveoli air sacs. The now oxygen rich blood is returned to the heart via the pulmonary veins. 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. Within the lungs, your bronchi branch into thousands of smaller, thinner tubes called bronchioles. These tubes end in bunches of tiny round air sacs called alveoli (al-VEE-uhl-eye). Each of these air sacs is covered in a mesh of tiny blood vessels called capillaries. The carbon dioxide in the blood is exchanged for oxygen in the alveoli. These tiny air sacs in the lungs are only one cell thick and they are surrounded by capillaries that are also only one cell thick. Blood from the heart flows through these capillaries and collects oxygen from Figure A shows the location of the respiratory structures in the body. Figure B is an enlarged view of the airways, alveoli (air sacs), and capillaries (tiny blood vessels). Figure C is a closeup view of gas exchange between the capillaries and alveoli. CO2 is carbon dioxide, and O2 is oxygen.
https://www.nlm.nih.gov/medlineplus/ency/anatomyvideos/000059.htm http://biology.about.com/od/anatomy/ss/the-lungs.htm http://lsa.colorado.edu/essence/texts/lungs.html http://www.nhlbi.nih.gov/health/health-topics/topics/hlw/system http://lsa.colorado.edu/essence/texts/lungs.html http://www.nhlbi.nih.gov/health/health-topics/topics/hlw/system
what are the tiny blood vessels that surround alveoli