The smallest blood or lymph vessel, formed of single layers of interconnected endothelial cells, sometimes with loosely attached connective tissue basement cells for added support. Capillaries allow the transport across their membranes and between their crevices of diffusible nutrients and waste products. Blood capillaries expand and contract, depending upon how much blood is needed in a given tissue and how much is piped into them by the small feeder arteries upstream. They further maintain a strong repelling charge that keeps blood proteins and red blood cells pushed into the centre of the flow. Lymph capillaries have many open crypts, allowing free absorption of interstitial fluid forced out of the blood; these capillaries tend to maintain a charge that attracts bits of cellular garbage too large to return through the membranes of exiting venous capillaries.