bild

Lung alveoli and an arteriole (left), coloured scanning electron micrograph (SEM). Alveoli are tiny air sacs in the lungs, through whose walls gaseous exchange takes place. Arterioles bring deoxygenated blood to the lungs, where it releases carbon dioxide for exhalation and absorbs oxygen from inhaled air. Magnification: x190 when printed 10cm wide. (KEYSTONE/SCIENCE PHOTO LIBRARY/PHOTO INSOLITE REALITE & V. GREMET)