शरीर की सबसे पतली रक्त-वाहिनियाँ केशिका (Capillaries) कहलातीं हैं। ये सूक्ष्मपरिसंचरण (microcirculation) का भाग हैं। वे केवल एक कोशिका के बराबर मोटी होती हैं।

A capillary is a small blood vessel, from 5 to 10 micrometres in diameter, and is part of the microcirculation system. Capillaries are microvessels and the smallest blood vessels in the body. They are composed of only the tunica intima, consisting of a thin wall of simple squamous endothelial cells. They are the site of the exchange of many substances from the surrounding interstitial fluid, and they convey blood from the smallest branches of the arteries (arterioles) to those of the veins (venules). Other substances which cross capillaries include water, oxygen, carbon dioxide, urea, glucose, uric acid, lactic acid and creatinine. Lymph capillaries connect with larger lymph vessels to drain lymphatic fluid collected in microcirculation.