Intravital microscopic observation of human skin reveals some characteristics of blood flow in the microcirculation during health and disease. In healthy humans, the capillary diameter in the skin of the nailfold is about 8 µm. Blood flow varies considerably with time and between the various capillaries. Usually, individual red blood cells (RBCs) cannot be seen. In patients with secondary Raynaud’s disease, the capillary diameter is dramatically increased (to about 30–60 µm) and blood flow has become so sluggish that aggregates of RBCs can be seen. In some patients with occlusive arterial disease, blood flow has become slow, while the capillary diameter has not changed. 9