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.
Arterioles are the smallest arteries and regulate blood flow into capillary beds through vasoconstriction and vasodilation. Capillaries are the smallest vessels and allow for exchange of substances between the blood and interstitial fluid.Continuous capillaries are most common and allow passage of fluids and small solutes. Fenestrated capillaries are more permeable to fluids and solutes than continuous capillaries.enous System. Venules are formed where capillaries converge and allow fluid and white blood cells to move easily between the blood and tissues. Venules join to form veins, which are relatively thin-walled vessels with large lumens containing about 65% of the total blood volume.