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.
The function of veins, arteries and capillaries are reflected in their structure. The walls of the capillaries are relatively thin and made of flat endothelial cells to facilitate exchange of gases, nutrients, and waste between the blood and the fluid surrounding the cells.Arteries, by contrast, have thick, strong walls to withstand the high speed and pressure of blood pumped from the heart.owever, for larger animals, the exchange of gases and nutrients is accomplished through specialized systems for circulation and gas exchange. Humans and other vertebrates have a closed circulatory system where blood is confined to the heart and blood vessels.