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.
Capillaries are the smallest of blood vessels. Their walls consist of a single layer of endothelial cells and the smallest have a single endothelial cell wrapped around to join with itself. These permit a single red blood cell to pass through them but only by deforming itself. Capillary endothelial cells vary in structure depending upon the tissue type in which they are found. Continuous capillaries are the most common (i.e.muscle, fat, nervous tissue) have no transcellular perforations and the cells are joined by tight nonpermeable junctions.