Capillary: One of the tiny blood vessels that connect the arterioles (the smallest divisions of the arteries) and the venules (the smallest divisions of the veins). The capillaries form a fine network in many parts of the body.
Although minute, the capillaries are a site where much action takes place in the circulatory system. The walls of the capillaries act as semipermeable membranes permitting the exchange of various substances between the blood stream and the tissues of the body. The substances that are interchanged through the capillary walls include fluids and the key gases oxygen and carbon dioxide.
The word “capillary” originally had more to do with hair than blood vessels. It comes from the Latin “capillaris” = hair-like, which was derived from “capillus” = a hair of the head and “caput” = head.
The first person to apply the word “capillary” to the most minute of blood vessels is thought to have been the architect, engineer, scientist, inventor, poet, sculptor, painter and anatomic artist Leonardo da Vinci. Leonardo (1452-1519) first became interested in anatomic art when he was asked by a Veronese anatomist named Marc Antonia Della Torre to do the illustrations for a text of anatomy. Della Torre was to do the dissecting and Leonardo the drawings. But Della Torre died unexpectedly. Not to be deterred by Della Torre's demise, Leonardo assumed both tasks. He dissected and drew more than 10 human bodies in the cathedral cellar of the mortuary of Santa Spirito under the secrecy of candlelight, necessitated by the Church's belief in the sanctity of the human body and a papal decree that forbade human dissection.
Leonardo recognized that a scientific knowledge of the human anatomy could only be gained by dissecting the human body. This was in striking contrast to the pronouncements of Galen and other anatomists. Da Vinci injected the blood vessels with wax for preservation, an anatomical technique still used today, and in so doing he discovered and named the capillaries (although he did not understand the role they played connecting the arterioles and venules).