TEM of a mouse lung, showing the thin capillary wall. The lungs function in the respiratory system is to extract oxygen from the atmosphere and transfer it into the bloodstream, and to release carbon dioxide from the bloodstream into the atmosphere, in a process of gas exchange. Respiration is driven by different muscular systems in different species. Capillaries are the smallest of a body's blood vessels (and lymph vessels) that make up the microcirculation. The capillary wall performs an important function by allowing nutrients and waste substances to pass across it. Magnification unknown.