Atelectasis (/ËÃ¦táµ»ËlÉktÉsáµ»s/ uses combining forms of atel- + ectasis, from Greek: á¼ÏÎµÎ»Î®Ï, incomplete + á¼ÎºÏÎ±ÏÎ¹Ï, extension) is the collapse or closure of a lung resulting in reduced or absent gas exchange.
A single capillary is so small that it allows only one blood cell to flow through it at a time. The capillary walls are also very small, only one cell thick. These thin walls easily allow water, oxygen, carbon dioxide, and other nutrient and waste substances to exchange between blood cells and the surrounding tissue.
Arteries carry oxygenated blood under high pressures - so they have thickened muscular walls and small centres (lumens) and no valves. Veins carry slower moving deoxygenated blood so they have thinner walls and contain valves to prevent back flow of blood. Capillaries are designed to deposit the oxygen in the remotest parts of the body. They have thin walls - up to only 1 cell thick to allow diffusion of oxygen into tissues and waste carbon dioxide back into the blood.